Kotlin: Fix kotlinx_serialization code generation (#9576)

And update to Kotlin 1.5.0 and kotlinx.serialization 1.2.1. Fix nested
enum annotation '@Serializable' instead of '@KSerializable' when
'kotlinx_serialization' is used. Fix missing JsonMediaType in
ApiClient.kt (#9242). Add 'kotlinx_serialization' serialization library
to documentation. Use explicity type in RequestConfig to keep type
information for JSON serialization.

Resolves #9242

modules/openapi-generator/src/main/resources/kotlin-client/build.gradle.mustache

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
     {{#jvm-retrofit2}}
     ext.retrofitVersion = '2.7.2'
     {{/jvm-retrofit2}}
@@ -72,7 +72,7 @@ dependencies {
     compile "com.fasterxml.jackson.datatype:jackson-datatype-jsr310:2.12.1"
     {{/jackson}}
     {{#kotlinx_serialization}}
-    compile "org.jetbrains.kotlinx:kotlinx-serialization-json:1.1.0"
+    compile "org.jetbrains.kotlinx:kotlinx-serialization-json:1.2.1"
     {{/kotlinx_serialization}}
     {{#jvm-okhttp3}}
     compile "com.squareup.okhttp3:okhttp:3.12.13"

modules/openapi-generator/src/main/resources/kotlin-client/data_class.mustache

@@ -70,7 +70,7 @@ import java.io.Serializable
      * Values: {{#allowableValues}}{{#enumVars}}{{&name}}{{^-last}},{{/-last}}{{/enumVars}}{{/allowableValues}}
      */
     {{#kotlinx_serialization}}
-    @KSerializable
+    {{#serializableModel}}@KSerializable{{/serializableModel}}{{^serializableModel}}@Serializable{{/serializableModel}}
     {{/kotlinx_serialization}}
     {{#nonPublicApi}}internal {{/nonPublicApi}}enum class {{{nameInCamelCase}}}(val value: {{^isContainer}}{{dataType}}{{/isContainer}}{{#isContainer}}kotlin.String{{/isContainer}}) {
     {{#allowableValues}}

modules/openapi-generator/src/main/resources/kotlin-client/enum_class.mustache

@@ -22,7 +22,7 @@ import kotlinx.serialization.internal.CommonEnumSerializer
 * {{{description}}}
 * Values: {{#allowableValues}}{{#enumVars}}{{&name}}{{^-last}},{{/-last}}{{/enumVars}}{{/allowableValues}}
 */
-{{#multiplatform}}@Serializable(with = {{classname}}.Serializer::class){{/multiplatform}}{{#kotlinx_serialization}}@Serializable(with = {{classname}}.Serializer::class){{/kotlinx_serialization}}
+{{#multiplatform}}@Serializable(with = {{classname}}.Serializer::class){{/multiplatform}}{{#kotlinx_serialization}}@Serializable{{/kotlinx_serialization}}
 {{#nonPublicApi}}internal {{/nonPublicApi}}enum class {{classname}}(val value: {{{dataType}}}) {
 
 {{#allowableValues}}{{#enumVars}}

modules/openapi-generator/src/main/resources/kotlin-client/infrastructure/RequestConfig.kt.mustache

@@ -8,10 +8,10 @@ package {{packageName}}.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-{{#nonPublicApi}}internal {{/nonPublicApi}}data class RequestConfig(
+{{#nonPublicApi}}internal {{/nonPublicApi}}data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-okhttp/api.mustache

@@ -43,7 +43,7 @@ import {{packageName}}.infrastructure.toMultiValue
     {{^doNotUseRxAndCoroutines}}{{#useCoroutines}}suspend {{/useCoroutines}}{{/doNotUseRxAndCoroutines}}fun {{operationId}}({{#allParams}}{{{paramName}}}: {{{dataType}}}{{^required}}?{{/required}}{{^-last}}, {{/-last}}{{/allParams}}) : {{#returnType}}{{{returnType}}}{{#nullableReturnType}}?{{/nullableReturnType}}{{/returnType}}{{^returnType}}Unit{{/returnType}} {
         val localVariableConfig = {{operationId}}RequestConfig({{#allParams}}{{{paramName}}} = {{{paramName}}}{{^-last}}, {{/-last}}{{/allParams}})
 
-        val localVarResponse = request<{{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}Any?{{/returnType}}>(
+        val localVarResponse = request<{{#hasBodyParam}}{{#bodyParams}}{{{dataType}}}{{/bodyParams}}{{/hasBodyParam}}{{^hasBodyParam}}{{^hasFormParams}}Unit{{/hasFormParams}}{{#hasFormParams}}Map<String, Any?>{{/hasFormParams}}{{/hasBodyParam}}, {{#returnType}}{{{returnType}}}{{/returnType}}{{^returnType}}Unit{{/returnType}}>(
             localVariableConfig
         )
 
@@ -71,8 +71,8 @@ import {{packageName}}.infrastructure.toMultiValue
     {{#isDeprecated}}
     @Deprecated(message = "This operation is deprecated.")
     {{/isDeprecated}}
-    fun {{operationId}}RequestConfig({{#allParams}}{{{paramName}}}: {{{dataType}}}{{^required}}?{{/required}}{{^-last}}, {{/-last}}{{/allParams}}) : RequestConfig {
+    fun {{operationId}}RequestConfig({{#allParams}}{{{paramName}}}: {{{dataType}}}{{^required}}?{{/required}}{{^-last}}, {{/-last}}{{/allParams}}) : RequestConfig<{{#hasBodyParam}}{{#bodyParams}}{{{dataType}}}{{/bodyParams}}{{/hasBodyParam}}{{^hasBodyParam}}{{^hasFormParams}}Unit{{/hasFormParams}}{{#hasFormParams}}Map<String, Any?>{{/hasFormParams}}{{/hasBodyParam}}> {
-        val localVariableBody: kotlin.Any? = {{#hasBodyParam}}{{#bodyParams}}{{{paramName}}}{{/bodyParams}}{{/hasBodyParam}}{{^hasBodyParam}}{{^hasFormParams}}null{{/hasFormParams}}{{#hasFormParams}}mapOf({{#formParams}}"{{{baseName}}}" to {{{paramName}}}{{^-last}}, {{/-last}}{{/formParams}}){{/hasFormParams}}{{/hasBodyParam}}
+        val localVariableBody = {{#hasBodyParam}}{{#bodyParams}}{{{paramName}}}{{/bodyParams}}{{/hasBodyParam}}{{^hasBodyParam}}{{^hasFormParams}}null{{/hasFormParams}}{{#hasFormParams}}mapOf({{#formParams}}"{{{baseName}}}" to {{{paramName}}}{{^-last}}, {{/-last}}{{/formParams}}){{/hasFormParams}}{{/hasBodyParam}}
         val localVariableQuery: MultiValueMap = {{^hasQueryParams}}mutableMapOf()
 {{/hasQueryParams}}{{#hasQueryParams}}mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
@@ -99,16 +99,14 @@ import {{packageName}}.infrastructure.toMultiValue
         {{#headerParams}}
         {{{paramName}}}{{^required}}?{{/required}}.apply { localVariableHeaders["{{baseName}}"] = {{#isContainer}}this.joinToString(separator = collectionDelimiter("{{collectionFormat}}")){{/isContainer}}{{^isContainer}}this.toString(){{/isContainer}} }
         {{/headerParams}}
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.{{httpMethod}},
             path = "{{path}}"{{#pathParams}}.replace("{"+"{{baseName}}"+"}", "${{{paramName}}}"){{/pathParams}},
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     {{/operation}}

modules/openapi-generator/src/main/resources/kotlin-client/libraries/jvm-okhttp/infrastructure/ApiClient.kt.mustache

@@ -27,11 +27,10 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 {{^threetenbp}}
 import java.time.LocalDate
 import java.time.LocalDateTime
@@ -39,6 +38,12 @@ import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
 {{/threetenbp}}
+import java.util.Date
+import java.util.Locale
+{{#kotlinx_serialization}}
+import kotlinx.serialization.decodeFromString
+import kotlinx.serialization.encodeToString
+{{/kotlinx_serialization}}
 {{#threetenbp}}
 import org.threeten.bp.LocalDate
 import org.threeten.bp.LocalDateTime
@@ -86,14 +91,10 @@ import org.threeten.bp.OffsetTime
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
             {{#jvm-okhttp3}}
-            content is File -> RequestBody.create(
+            content is File -> RequestBody.create(MediaType.parse(mediaType), content)
-                MediaType.parse(mediaType), content
-            )
             {{/jvm-okhttp3}}
             {{#jvm-okhttp4}}
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             {{/jvm-okhttp4}}
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
@@ -154,12 +155,12 @@ import org.threeten.bp.OffsetTime
                 MediaType.parse(mediaType), Serializer.jacksonObjectMapper.writeValueAsString(content)
                 {{/jackson}}
                 {{#kotlinx_serialization}}
-                MediaType.parse(mediaType), Serializer.jvmJson.decodeFromString<T>(content)
+                MediaType.parse(mediaType), Serializer.jvmJson.encodeToString(content)
                 {{/kotlinx_serialization}}
             )
             {{/jvm-okhttp3}}
             {{#jvm-okhttp4}}
-            mediaType == JsonMediaType -> {{#moshi}}Serializer.moshi.adapter(T::class.java).toJson(content){{/moshi}}{{#gson}}Serializer.gson.toJson(content, T::class.java){{/gson}}{{#jackson}}Serializer.jacksonObjectMapper.writeValueAsString(content){{/jackson}}{{#kotlinx_serialization}}Serializer.jvmJson.decodeFromString<T>(content){{/kotlinx_serialization}}.toRequestBody(
+            mediaType == JsonMediaType -> {{#moshi}}Serializer.moshi.adapter(T::class.java).toJson(content){{/moshi}}{{#gson}}Serializer.gson.toJson(content, T::class.java){{/gson}}{{#jackson}}Serializer.jacksonObjectMapper.writeValueAsString(content){{/jackson}}{{#kotlinx_serialization}}Serializer.jvmJson.encodeToString(content){{/kotlinx_serialization}}.toRequestBody(
                 mediaType.toMediaTypeOrNull()
             )
             {{/jvm-okhttp4}}
@@ -196,13 +197,13 @@ import org.threeten.bp.OffsetTime
             return f as T
         }
         return when(mediaType) {
-            JsonMediaType -> {{#moshi}}Serializer.moshi.adapter(T::class.java).fromJson(bodyContent){{/moshi}}{{#gson}}Serializer.gson.fromJson(bodyContent, T::class.java){{/gson}}{{#jackson}}Serializer.jacksonObjectMapper.readValue(bodyContent, T::class.java){{/jackson}}
+            JsonMediaType -> {{#moshi}}Serializer.moshi.adapter(T::class.java).fromJson(bodyContent){{/moshi}}{{#gson}}Serializer.gson.fromJson(bodyContent, T::class.java){{/gson}}{{#jackson}}Serializer.jacksonObjectMapper.readValue(bodyContent, T::class.java){{/jackson}}{{#kotlinx_serialization}}Serializer.jvmJson.decodeFromString<T>(bodyContent){{/kotlinx_serialization}}
             else ->  throw UnsupportedOperationException("responseBody currently only supports JSON body.")
         }
     }
 
     {{#hasAuthMethods}}
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         {{#authMethods}}
         {{#isApiKey}}
         {{#isKeyInHeader}}
@@ -259,7 +260,7 @@ import org.threeten.bp.OffsetTime
     }
     {{/hasAuthMethods}}
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         {{#jvm-okhttp3}}
         val httpUrl = HttpUrl.parse(baseUrl) ?: throw IllegalStateException("baseUrl is invalid.")
         {{/jvm-okhttp3}}
@@ -300,7 +301,7 @@ import org.threeten.bp.OffsetTime
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -315,57 +316,46 @@ import org.threeten.bp.OffsetTime
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
+                response.code{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
-                    response.headers{{#jvm-okhttp3}}(){{/jvm-okhttp3}}.toMultimap()
+                response.headers{{#jvm-okhttp3}}(){{/jvm-okhttp3}}.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
+                response.message{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
-                    response.code{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
+                response.code{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
-                    response.headers{{#jvm-okhttp3}}(){{/jvm-okhttp3}}.toMultimap()
+                response.headers{{#jvm-okhttp3}}(){{/jvm-okhttp3}}.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body{{#jvm-okhttp3}}(){{/jvm-okhttp3}}, accept),
+                responseBody(response.body{{#jvm-okhttp3}}(){{/jvm-okhttp3}}, accept),
-                    response.code{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
+                response.code{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
-                    response.headers{{#jvm-okhttp3}}(){{/jvm-okhttp3}}.toMultimap()
+                response.headers{{#jvm-okhttp3}}(){{/jvm-okhttp3}}.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
+                response.message{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
-                    response.body{{#jvm-okhttp3}}(){{/jvm-okhttp3}}?.string(),
+                response.body{{#jvm-okhttp3}}(){{/jvm-okhttp3}}?.string(),
-                    response.code{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
+                response.code{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
-                    response.headers{{#jvm-okhttp3}}(){{/jvm-okhttp3}}.toMultimap()
+                response.headers{{#jvm-okhttp3}}(){{/jvm-okhttp3}}.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
+                response.message{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
-                    response.body{{#jvm-okhttp3}}(){{/jvm-okhttp3}}?.string(),
+                response.body{{#jvm-okhttp3}}(){{/jvm-okhttp3}}?.string(),
-                    response.code{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
+                response.code{{#jvm-okhttp3}}(){{/jvm-okhttp3}},
-                    response.headers{{#jvm-okhttp3}}(){{/jvm-okhttp3}}.toMultimap()
+                response.headers{{#jvm-okhttp3}}(){{/jvm-okhttp3}}.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

modules/openapi-generator/src/main/resources/kotlin-client/libraries/multiplatform/infrastructure/ApiClient.kt.mustache

@@ -155,7 +155,7 @@ import {{packageName}}.auth.*
         else request(requestConfig, authNames = authNames)
     }
 
-    protected suspend fun request(requestConfig: RequestConfig, body: OutgoingContent = EmptyContent, authNames: kotlin.collections.List<String>): HttpResponse {
+    protected suspend inline fun <reified T: Any?> request(requestConfig: RequestConfig<T>, body: OutgoingContent = EmptyContent, authNames: kotlin.collections.List<String>): HttpResponse {
         requestConfig.updateForAuth(authNames)
         val headers = requestConfig.headers
 

samples/client/petstore/kotlin-gson/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin-gson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -47,7 +47,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun addPet(body: Pet) : Unit {
         val localVariableConfig = addPetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -72,20 +72,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun addPetRequestConfig(body: Pet) : RequestConfig {
+    fun addPetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -102,7 +100,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableConfig = deletePetRequestConfig(petId = petId, apiKey = apiKey)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -128,21 +126,19 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param apiKey  (optional)
     * @return RequestConfig
     */
-    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig {
+    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -159,7 +155,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByStatus(status: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByStatusRequestConfig(status = status)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -184,23 +180,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Status values that need to be considered for filter 
     * @return RequestConfig
     */
-    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("status", toMultiValue(status.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByStatus",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -218,7 +212,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByTags(tags: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByTagsRequestConfig(tags = tags)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -244,23 +238,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @return RequestConfig
     */
     @Deprecated(message = "This operation is deprecated.")
-    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("tags", toMultiValue(tags.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByTags",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -277,7 +269,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getPetById(petId: kotlin.Long) : Pet {
         val localVariableConfig = getPetByIdRequestConfig(petId = petId)
 
-        val localVarResponse = request<Pet>(
+        val localVarResponse = request<Unit, Pet>(
             localVariableConfig
         )
 
@@ -302,20 +294,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param petId ID of pet to return 
     * @return RequestConfig
     */
-    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig {
+    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -331,7 +321,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePet(body: Pet) : Unit {
         val localVariableConfig = updatePetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -356,20 +346,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun updatePetRequestConfig(body: Pet) : RequestConfig {
+    fun updatePetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -387,7 +375,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableConfig = updatePetWithFormRequestConfig(petId = petId, name = name, status = status)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Map<String, Any?>, Unit>(
             localVariableConfig
         )
 
@@ -414,20 +402,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Updated status of the pet (optional)
     * @return RequestConfig
     */
-    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig {
+    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
+        val localVariableBody = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -446,7 +432,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableConfig = uploadFileRequestConfig(petId = petId, additionalMetadata = additionalMetadata, file = file)
 
-        val localVarResponse = request<ApiResponse>(
+        val localVarResponse = request<Map<String, Any?>, ApiResponse>(
             localVariableConfig
         )
 
@@ -473,20 +459,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param file file to upload (optional)
     * @return RequestConfig
     */
-    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig {
+    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
+        val localVariableBody = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-gson/src/main/kotlin/org/openapitools/client/apis/StoreApi.kt

@@ -46,7 +46,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun deleteOrder(orderId: kotlin.String) : Unit {
         val localVariableConfig = deleteOrderRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of the order that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig {
+    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getInventory() : kotlin.collections.Map<kotlin.String, kotlin.Int> {
         val localVariableConfig = getInventoryRequestConfig()
 
-        val localVarResponse = request<kotlin.collections.Map<kotlin.String, kotlin.Int>>(
+        val localVarResponse = request<Unit, kotlin.collections.Map<kotlin.String, kotlin.Int>>(
             localVariableConfig
         )
 
@@ -124,20 +122,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     *
     * @return RequestConfig
     */
-    fun getInventoryRequestConfig() : RequestConfig {
+    fun getInventoryRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/inventory",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getOrderById(orderId: kotlin.Long) : Order {
         val localVariableConfig = getOrderByIdRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Unit, Order>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of pet that needs to be fetched 
     * @return RequestConfig
     */
-    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig {
+    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -209,7 +203,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun placeOrder(body: Order) : Order {
         val localVariableConfig = placeOrderRequestConfig(body = body)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Order, Order>(
             localVariableConfig
         )
 
@@ -234,20 +228,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param body order placed for purchasing the pet 
     * @return RequestConfig
     */
-    fun placeOrderRequestConfig(body: Order) : RequestConfig {
+    fun placeOrderRequestConfig(body: Order) : RequestConfig<Order> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/store/order",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-gson/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -46,7 +46,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUser(body: User) : Unit {
         val localVariableConfig = createUserRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Created user object 
     * @return RequestConfig
     */
-    fun createUserRequestConfig(body: User) : RequestConfig {
+    fun createUserRequestConfig(body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithArrayInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithArrayInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -125,20 +123,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithArray",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithListInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithListInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithList",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -208,7 +202,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deleteUser(username: kotlin.String) : Unit {
         val localVariableConfig = deleteUserRequestConfig(username = username)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -233,20 +227,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig {
+    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -263,7 +255,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getUserByName(username: kotlin.String) : User {
         val localVariableConfig = getUserByNameRequestConfig(username = username)
 
-        val localVarResponse = request<User>(
+        val localVarResponse = request<Unit, User>(
             localVariableConfig
         )
 
@@ -288,20 +280,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be fetched. Use user1 for testing. 
     * @return RequestConfig
     */
-    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig {
+    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -319,7 +309,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableConfig = loginUserRequestConfig(username = username, password = password)
 
-        val localVarResponse = request<kotlin.String>(
+        val localVarResponse = request<Unit, kotlin.String>(
             localVariableConfig
         )
 
@@ -345,24 +335,22 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param password The password for login in clear text 
     * @return RequestConfig
     */
-    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig {
+    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("username", listOf(username.toString()))
                 put("password", listOf(password.toString()))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/login",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -377,7 +365,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun logoutUser() : Unit {
         val localVariableConfig = logoutUserRequestConfig()
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -401,20 +389,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     *
     * @return RequestConfig
     */
-    fun logoutUserRequestConfig() : RequestConfig {
+    fun logoutUserRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/logout",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -431,7 +417,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updateUser(username: kotlin.String, body: User) : Unit {
         val localVariableConfig = updateUserRequestConfig(username = username, body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -457,20 +443,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Updated user object 
     * @return RequestConfig
     */
-    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig {
+    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-gson/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -12,16 +12,17 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
+import java.util.Date
+import java.util.Locale
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -61,9 +62,7 @@ open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -131,7 +130,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         if (requestConfig.headers["api_key"].isNullOrEmpty()) {
             if (apiKey["api_key"] != null) {
                 if (apiKeyPrefix["api_key"] != null) {
@@ -148,7 +147,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = baseUrl.toHttpUrlOrNull() ?: throw IllegalStateException("baseUrl is invalid.")
 
         // take authMethod from operation
@@ -182,7 +181,7 @@ open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -197,57 +196,46 @@ open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message,
+                response.message,
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body, accept),
+                responseBody(response.body, accept),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin-gson/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin-jackson/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin-jackson/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -47,7 +47,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun addPet(body: Pet) : Unit {
         val localVariableConfig = addPetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -72,20 +72,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun addPetRequestConfig(body: Pet) : RequestConfig {
+    fun addPetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -102,7 +100,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableConfig = deletePetRequestConfig(petId = petId, apiKey = apiKey)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -128,21 +126,19 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param apiKey  (optional)
     * @return RequestConfig
     */
-    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig {
+    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -159,7 +155,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByStatus(status: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByStatusRequestConfig(status = status)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -184,23 +180,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Status values that need to be considered for filter 
     * @return RequestConfig
     */
-    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("status", toMultiValue(status.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByStatus",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -218,7 +212,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByTags(tags: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByTagsRequestConfig(tags = tags)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -244,23 +238,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @return RequestConfig
     */
     @Deprecated(message = "This operation is deprecated.")
-    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("tags", toMultiValue(tags.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByTags",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -277,7 +269,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getPetById(petId: kotlin.Long) : Pet {
         val localVariableConfig = getPetByIdRequestConfig(petId = petId)
 
-        val localVarResponse = request<Pet>(
+        val localVarResponse = request<Unit, Pet>(
             localVariableConfig
         )
 
@@ -302,20 +294,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param petId ID of pet to return 
     * @return RequestConfig
     */
-    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig {
+    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -331,7 +321,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePet(body: Pet) : Unit {
         val localVariableConfig = updatePetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -356,20 +346,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun updatePetRequestConfig(body: Pet) : RequestConfig {
+    fun updatePetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -387,7 +375,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableConfig = updatePetWithFormRequestConfig(petId = petId, name = name, status = status)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Map<String, Any?>, Unit>(
             localVariableConfig
         )
 
@@ -414,20 +402,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Updated status of the pet (optional)
     * @return RequestConfig
     */
-    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig {
+    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
+        val localVariableBody = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -446,7 +432,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableConfig = uploadFileRequestConfig(petId = petId, additionalMetadata = additionalMetadata, file = file)
 
-        val localVarResponse = request<ApiResponse>(
+        val localVarResponse = request<Map<String, Any?>, ApiResponse>(
             localVariableConfig
         )
 
@@ -473,20 +459,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param file file to upload (optional)
     * @return RequestConfig
     */
-    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig {
+    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
+        val localVariableBody = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-jackson/src/main/kotlin/org/openapitools/client/apis/StoreApi.kt

@@ -46,7 +46,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun deleteOrder(orderId: kotlin.String) : Unit {
         val localVariableConfig = deleteOrderRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of the order that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig {
+    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getInventory() : kotlin.collections.Map<kotlin.String, kotlin.Int> {
         val localVariableConfig = getInventoryRequestConfig()
 
-        val localVarResponse = request<kotlin.collections.Map<kotlin.String, kotlin.Int>>(
+        val localVarResponse = request<Unit, kotlin.collections.Map<kotlin.String, kotlin.Int>>(
             localVariableConfig
         )
 
@@ -124,20 +122,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     *
     * @return RequestConfig
     */
-    fun getInventoryRequestConfig() : RequestConfig {
+    fun getInventoryRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/inventory",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getOrderById(orderId: kotlin.Long) : Order {
         val localVariableConfig = getOrderByIdRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Unit, Order>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of pet that needs to be fetched 
     * @return RequestConfig
     */
-    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig {
+    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -209,7 +203,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun placeOrder(body: Order) : Order {
         val localVariableConfig = placeOrderRequestConfig(body = body)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Order, Order>(
             localVariableConfig
         )
 
@@ -234,20 +228,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param body order placed for purchasing the pet 
     * @return RequestConfig
     */
-    fun placeOrderRequestConfig(body: Order) : RequestConfig {
+    fun placeOrderRequestConfig(body: Order) : RequestConfig<Order> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/store/order",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-jackson/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -46,7 +46,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUser(body: User) : Unit {
         val localVariableConfig = createUserRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Created user object 
     * @return RequestConfig
     */
-    fun createUserRequestConfig(body: User) : RequestConfig {
+    fun createUserRequestConfig(body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithArrayInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithArrayInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -125,20 +123,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithArray",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithListInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithListInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithList",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -208,7 +202,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deleteUser(username: kotlin.String) : Unit {
         val localVariableConfig = deleteUserRequestConfig(username = username)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -233,20 +227,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig {
+    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -263,7 +255,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getUserByName(username: kotlin.String) : User {
         val localVariableConfig = getUserByNameRequestConfig(username = username)
 
-        val localVarResponse = request<User>(
+        val localVarResponse = request<Unit, User>(
             localVariableConfig
         )
 
@@ -288,20 +280,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be fetched. Use user1 for testing. 
     * @return RequestConfig
     */
-    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig {
+    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -319,7 +309,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableConfig = loginUserRequestConfig(username = username, password = password)
 
-        val localVarResponse = request<kotlin.String>(
+        val localVarResponse = request<Unit, kotlin.String>(
             localVariableConfig
         )
 
@@ -345,24 +335,22 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param password The password for login in clear text 
     * @return RequestConfig
     */
-    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig {
+    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("username", listOf(username.toString()))
                 put("password", listOf(password.toString()))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/login",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -377,7 +365,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun logoutUser() : Unit {
         val localVariableConfig = logoutUserRequestConfig()
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -401,20 +389,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     *
     * @return RequestConfig
     */
-    fun logoutUserRequestConfig() : RequestConfig {
+    fun logoutUserRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/logout",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -431,7 +417,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updateUser(username: kotlin.String, body: User) : Unit {
         val localVariableConfig = updateUserRequestConfig(username = username, body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -457,20 +443,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Updated user object 
     * @return RequestConfig
     */
-    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig {
+    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-jackson/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -12,16 +12,17 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
+import java.util.Date
+import java.util.Locale
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -61,9 +62,7 @@ open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -131,7 +130,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         if (requestConfig.headers["api_key"].isNullOrEmpty()) {
             if (apiKey["api_key"] != null) {
                 if (apiKeyPrefix["api_key"] != null) {
@@ -148,7 +147,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = baseUrl.toHttpUrlOrNull() ?: throw IllegalStateException("baseUrl is invalid.")
 
         // take authMethod from operation
@@ -182,7 +181,7 @@ open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -197,57 +196,46 @@ open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message,
+                response.message,
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body, accept),
+                responseBody(response.body, accept),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin-jackson/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin-json-request-string/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -47,7 +47,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun addPet(body: Pet) : Unit {
         val localVariableConfig = addPetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -72,20 +72,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun addPetRequestConfig(body: Pet) : RequestConfig {
+    fun addPetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -102,7 +100,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableConfig = deletePetRequestConfig(petId = petId, apiKey = apiKey)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -128,21 +126,19 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param apiKey  (optional)
     * @return RequestConfig
     */
-    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig {
+    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -160,7 +156,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByTags(tags: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByTagsRequestConfig(tags = tags)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -186,23 +182,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @return RequestConfig
     */
     @Deprecated(message = "This operation is deprecated.")
-    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("tags", toMultiValue(tags.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByTags",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -219,7 +213,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getAllPets(lastUpdated: java.time.OffsetDateTime?) : kotlin.collections.List<Pet> {
         val localVariableConfig = getAllPetsRequestConfig(lastUpdated = lastUpdated)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -244,8 +238,8 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param lastUpdated When this endpoint was hit last to help indentify if the client already has the latest copy. (optional)
     * @return RequestConfig
     */
-    fun getAllPetsRequestConfig(lastUpdated: java.time.OffsetDateTime?) : RequestConfig {
+    fun getAllPetsRequestConfig(lastUpdated: java.time.OffsetDateTime?) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 if (lastUpdated != null) {
@@ -253,16 +247,14 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
                 }
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/getAll",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -279,7 +271,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getPetById(petId: kotlin.Long) : Pet {
         val localVariableConfig = getPetByIdRequestConfig(petId = petId)
 
-        val localVarResponse = request<Pet>(
+        val localVarResponse = request<Unit, Pet>(
             localVariableConfig
         )
 
@@ -304,20 +296,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param petId ID of pet to return 
     * @return RequestConfig
     */
-    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig {
+    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -333,7 +323,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePet(body: Pet) : Unit {
         val localVariableConfig = updatePetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -358,20 +348,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun updatePetRequestConfig(body: Pet) : RequestConfig {
+    fun updatePetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -389,7 +377,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableConfig = updatePetWithFormRequestConfig(petId = petId, name = name, status = status)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Map<String, Any?>, Unit>(
             localVariableConfig
         )
 
@@ -416,20 +404,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Updated status of the pet (optional)
     * @return RequestConfig
     */
-    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig {
+    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
+        val localVariableBody = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -448,7 +434,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableConfig = uploadFileRequestConfig(petId = petId, additionalMetadata = additionalMetadata, file = file)
 
-        val localVarResponse = request<ApiResponse>(
+        val localVarResponse = request<Map<String, Any?>, ApiResponse>(
             localVariableConfig
         )
 
@@ -475,20 +461,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param file file to upload (optional)
     * @return RequestConfig
     */
-    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig {
+    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
+        val localVariableBody = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/apis/StoreApi.kt

@@ -46,7 +46,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun deleteOrder(orderId: kotlin.String) : Unit {
         val localVariableConfig = deleteOrderRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of the order that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig {
+    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getInventory() : kotlin.collections.Map<kotlin.String, kotlin.Int> {
         val localVariableConfig = getInventoryRequestConfig()
 
-        val localVarResponse = request<kotlin.collections.Map<kotlin.String, kotlin.Int>>(
+        val localVarResponse = request<Unit, kotlin.collections.Map<kotlin.String, kotlin.Int>>(
             localVariableConfig
         )
 
@@ -124,20 +122,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     *
     * @return RequestConfig
     */
-    fun getInventoryRequestConfig() : RequestConfig {
+    fun getInventoryRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/inventory",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getOrderById(orderId: kotlin.Long) : Order {
         val localVariableConfig = getOrderByIdRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Unit, Order>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of pet that needs to be fetched 
     * @return RequestConfig
     */
-    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig {
+    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -209,7 +203,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun placeOrder(body: Order) : Order {
         val localVariableConfig = placeOrderRequestConfig(body = body)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Order, Order>(
             localVariableConfig
         )
 
@@ -234,20 +228,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param body order placed for purchasing the pet 
     * @return RequestConfig
     */
-    fun placeOrderRequestConfig(body: Order) : RequestConfig {
+    fun placeOrderRequestConfig(body: Order) : RequestConfig<Order> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/store/order",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -46,7 +46,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUser(body: User) : Unit {
         val localVariableConfig = createUserRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Created user object 
     * @return RequestConfig
     */
-    fun createUserRequestConfig(body: User) : RequestConfig {
+    fun createUserRequestConfig(body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithArrayInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithArrayInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -125,20 +123,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithArray",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithListInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithListInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithList",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -208,7 +202,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deleteUser(username: kotlin.String) : Unit {
         val localVariableConfig = deleteUserRequestConfig(username = username)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -233,20 +227,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig {
+    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -263,7 +255,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getUserByName(username: kotlin.String) : User {
         val localVariableConfig = getUserByNameRequestConfig(username = username)
 
-        val localVarResponse = request<User>(
+        val localVarResponse = request<Unit, User>(
             localVariableConfig
         )
 
@@ -288,20 +280,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be fetched. Use user1 for testing. 
     * @return RequestConfig
     */
-    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig {
+    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -319,7 +309,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableConfig = loginUserRequestConfig(username = username, password = password)
 
-        val localVarResponse = request<kotlin.String>(
+        val localVarResponse = request<Unit, kotlin.String>(
             localVariableConfig
         )
 
@@ -345,24 +335,22 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param password The password for login in clear text 
     * @return RequestConfig
     */
-    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig {
+    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("username", listOf(username.toString()))
                 put("password", listOf(password.toString()))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/login",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -377,7 +365,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun logoutUser() : Unit {
         val localVariableConfig = logoutUserRequestConfig()
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -401,20 +389,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     *
     * @return RequestConfig
     */
-    fun logoutUserRequestConfig() : RequestConfig {
+    fun logoutUserRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/logout",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -431,7 +417,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updateUser(username: kotlin.String, body: User) : Unit {
         val localVariableConfig = updateUserRequestConfig(username = username, body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -457,20 +443,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Updated user object 
     * @return RequestConfig
     */
-    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig {
+    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -13,16 +13,17 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
+import java.util.Date
+import java.util.Locale
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -62,9 +63,7 @@ open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -137,7 +136,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         if (requestConfig.headers["api_key"].isNullOrEmpty()) {
             if (apiKey["api_key"] != null) {
                 if (apiKeyPrefix["api_key"] != null) {
@@ -154,7 +153,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = baseUrl.toHttpUrlOrNull() ?: throw IllegalStateException("baseUrl is invalid.")
 
         // take authMethod from operation
@@ -188,7 +187,7 @@ open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -203,57 +202,46 @@ open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message,
+                response.message,
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body, accept),
+                responseBody(response.body, accept),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin-json-request-string/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin-jvm-okhttp4-coroutines/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -47,7 +47,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun addPet(body: Pet) : Unit {
         val localVariableConfig = addPetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -72,20 +72,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun addPetRequestConfig(body: Pet) : RequestConfig {
+    fun addPetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -102,7 +100,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableConfig = deletePetRequestConfig(petId = petId, apiKey = apiKey)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -128,21 +126,19 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param apiKey  (optional)
     * @return RequestConfig
     */
-    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig {
+    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -159,7 +155,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun findPetsByStatus(status: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByStatusRequestConfig(status = status)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -184,23 +180,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Status values that need to be considered for filter 
     * @return RequestConfig
     */
-    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("status", toMultiValue(status.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByStatus",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -218,7 +212,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun findPetsByTags(tags: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByTagsRequestConfig(tags = tags)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -244,23 +238,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @return RequestConfig
     */
     @Deprecated(message = "This operation is deprecated.")
-    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("tags", toMultiValue(tags.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByTags",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -277,7 +269,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun getPetById(petId: kotlin.Long) : Pet {
         val localVariableConfig = getPetByIdRequestConfig(petId = petId)
 
-        val localVarResponse = request<Pet>(
+        val localVarResponse = request<Unit, Pet>(
             localVariableConfig
         )
 
@@ -302,20 +294,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param petId ID of pet to return 
     * @return RequestConfig
     */
-    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig {
+    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -331,7 +321,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun updatePet(body: Pet) : Unit {
         val localVariableConfig = updatePetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -356,20 +346,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun updatePetRequestConfig(body: Pet) : RequestConfig {
+    fun updatePetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -387,7 +375,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableConfig = updatePetWithFormRequestConfig(petId = petId, name = name, status = status)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Map<String, Any?>, Unit>(
             localVariableConfig
         )
 
@@ -414,20 +402,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Updated status of the pet (optional)
     * @return RequestConfig
     */
-    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig {
+    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
+        val localVariableBody = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -446,7 +432,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableConfig = uploadFileRequestConfig(petId = petId, additionalMetadata = additionalMetadata, file = file)
 
-        val localVarResponse = request<ApiResponse>(
+        val localVarResponse = request<Map<String, Any?>, ApiResponse>(
             localVariableConfig
         )
 
@@ -473,20 +459,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param file file to upload (optional)
     * @return RequestConfig
     */
-    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig {
+    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
+        val localVariableBody = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/apis/StoreApi.kt

@@ -46,7 +46,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     suspend fun deleteOrder(orderId: kotlin.String) : Unit {
         val localVariableConfig = deleteOrderRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of the order that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig {
+    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     suspend fun getInventory() : kotlin.collections.Map<kotlin.String, kotlin.Int> {
         val localVariableConfig = getInventoryRequestConfig()
 
-        val localVarResponse = request<kotlin.collections.Map<kotlin.String, kotlin.Int>>(
+        val localVarResponse = request<Unit, kotlin.collections.Map<kotlin.String, kotlin.Int>>(
             localVariableConfig
         )
 
@@ -124,20 +122,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     *
     * @return RequestConfig
     */
-    fun getInventoryRequestConfig() : RequestConfig {
+    fun getInventoryRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/inventory",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     suspend fun getOrderById(orderId: kotlin.Long) : Order {
         val localVariableConfig = getOrderByIdRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Unit, Order>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of pet that needs to be fetched 
     * @return RequestConfig
     */
-    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig {
+    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -209,7 +203,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     suspend fun placeOrder(body: Order) : Order {
         val localVariableConfig = placeOrderRequestConfig(body = body)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Order, Order>(
             localVariableConfig
         )
 
@@ -234,20 +228,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param body order placed for purchasing the pet 
     * @return RequestConfig
     */
-    fun placeOrderRequestConfig(body: Order) : RequestConfig {
+    fun placeOrderRequestConfig(body: Order) : RequestConfig<Order> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/store/order",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -46,7 +46,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun createUser(body: User) : Unit {
         val localVariableConfig = createUserRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Created user object 
     * @return RequestConfig
     */
-    fun createUserRequestConfig(body: User) : RequestConfig {
+    fun createUserRequestConfig(body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun createUsersWithArrayInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithArrayInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -125,20 +123,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithArray",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun createUsersWithListInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithListInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithList",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -208,7 +202,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun deleteUser(username: kotlin.String) : Unit {
         val localVariableConfig = deleteUserRequestConfig(username = username)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -233,20 +227,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig {
+    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -263,7 +255,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun getUserByName(username: kotlin.String) : User {
         val localVariableConfig = getUserByNameRequestConfig(username = username)
 
-        val localVarResponse = request<User>(
+        val localVarResponse = request<Unit, User>(
             localVariableConfig
         )
 
@@ -288,20 +280,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be fetched. Use user1 for testing. 
     * @return RequestConfig
     */
-    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig {
+    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -319,7 +309,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableConfig = loginUserRequestConfig(username = username, password = password)
 
-        val localVarResponse = request<kotlin.String>(
+        val localVarResponse = request<Unit, kotlin.String>(
             localVariableConfig
         )
 
@@ -345,24 +335,22 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param password The password for login in clear text 
     * @return RequestConfig
     */
-    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig {
+    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("username", listOf(username.toString()))
                 put("password", listOf(password.toString()))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/login",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -377,7 +365,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun logoutUser() : Unit {
         val localVariableConfig = logoutUserRequestConfig()
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -401,20 +389,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     *
     * @return RequestConfig
     */
-    fun logoutUserRequestConfig() : RequestConfig {
+    fun logoutUserRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/logout",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -431,7 +417,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     suspend fun updateUser(username: kotlin.String, body: User) : Unit {
         val localVariableConfig = updateUserRequestConfig(username = username, body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -457,20 +443,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Updated user object 
     * @return RequestConfig
     */
-    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig {
+    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -12,16 +12,17 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
+import java.util.Date
+import java.util.Locale
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -61,9 +62,7 @@ open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -131,7 +130,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         if (requestConfig.headers["api_key"].isNullOrEmpty()) {
             if (apiKey["api_key"] != null) {
                 if (apiKeyPrefix["api_key"] != null) {
@@ -148,7 +147,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = baseUrl.toHttpUrlOrNull() ?: throw IllegalStateException("baseUrl is invalid.")
 
         // take authMethod from operation
@@ -182,7 +181,7 @@ open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -197,57 +196,46 @@ open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message,
+                response.message,
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body, accept),
+                responseBody(response.body, accept),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin-jvm-okhttp4-coroutines/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin-moshi-codegen/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin-moshi-codegen/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -47,7 +47,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun addPet(body: Pet) : Unit {
         val localVariableConfig = addPetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -72,20 +72,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun addPetRequestConfig(body: Pet) : RequestConfig {
+    fun addPetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -102,7 +100,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableConfig = deletePetRequestConfig(petId = petId, apiKey = apiKey)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -128,21 +126,19 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param apiKey  (optional)
     * @return RequestConfig
     */
-    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig {
+    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -159,7 +155,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByStatus(status: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByStatusRequestConfig(status = status)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -184,23 +180,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Status values that need to be considered for filter 
     * @return RequestConfig
     */
-    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("status", toMultiValue(status.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByStatus",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -218,7 +212,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByTags(tags: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByTagsRequestConfig(tags = tags)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -244,23 +238,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @return RequestConfig
     */
     @Deprecated(message = "This operation is deprecated.")
-    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("tags", toMultiValue(tags.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByTags",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -277,7 +269,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getPetById(petId: kotlin.Long) : Pet {
         val localVariableConfig = getPetByIdRequestConfig(petId = petId)
 
-        val localVarResponse = request<Pet>(
+        val localVarResponse = request<Unit, Pet>(
             localVariableConfig
         )
 
@@ -302,20 +294,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param petId ID of pet to return 
     * @return RequestConfig
     */
-    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig {
+    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -331,7 +321,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePet(body: Pet) : Unit {
         val localVariableConfig = updatePetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -356,20 +346,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun updatePetRequestConfig(body: Pet) : RequestConfig {
+    fun updatePetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -387,7 +375,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableConfig = updatePetWithFormRequestConfig(petId = petId, name = name, status = status)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Map<String, Any?>, Unit>(
             localVariableConfig
         )
 
@@ -414,20 +402,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Updated status of the pet (optional)
     * @return RequestConfig
     */
-    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig {
+    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
+        val localVariableBody = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -446,7 +432,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableConfig = uploadFileRequestConfig(petId = petId, additionalMetadata = additionalMetadata, file = file)
 
-        val localVarResponse = request<ApiResponse>(
+        val localVarResponse = request<Map<String, Any?>, ApiResponse>(
             localVariableConfig
         )
 
@@ -473,20 +459,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param file file to upload (optional)
     * @return RequestConfig
     */
-    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig {
+    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
+        val localVariableBody = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-moshi-codegen/src/main/kotlin/org/openapitools/client/apis/StoreApi.kt

@@ -46,7 +46,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun deleteOrder(orderId: kotlin.String) : Unit {
         val localVariableConfig = deleteOrderRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of the order that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig {
+    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getInventory() : kotlin.collections.Map<kotlin.String, kotlin.Int> {
         val localVariableConfig = getInventoryRequestConfig()
 
-        val localVarResponse = request<kotlin.collections.Map<kotlin.String, kotlin.Int>>(
+        val localVarResponse = request<Unit, kotlin.collections.Map<kotlin.String, kotlin.Int>>(
             localVariableConfig
         )
 
@@ -124,20 +122,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     *
     * @return RequestConfig
     */
-    fun getInventoryRequestConfig() : RequestConfig {
+    fun getInventoryRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/inventory",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getOrderById(orderId: kotlin.Long) : Order {
         val localVariableConfig = getOrderByIdRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Unit, Order>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of pet that needs to be fetched 
     * @return RequestConfig
     */
-    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig {
+    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -209,7 +203,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun placeOrder(body: Order) : Order {
         val localVariableConfig = placeOrderRequestConfig(body = body)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Order, Order>(
             localVariableConfig
         )
 
@@ -234,20 +228,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param body order placed for purchasing the pet 
     * @return RequestConfig
     */
-    fun placeOrderRequestConfig(body: Order) : RequestConfig {
+    fun placeOrderRequestConfig(body: Order) : RequestConfig<Order> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/store/order",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-moshi-codegen/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -46,7 +46,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUser(body: User) : Unit {
         val localVariableConfig = createUserRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Created user object 
     * @return RequestConfig
     */
-    fun createUserRequestConfig(body: User) : RequestConfig {
+    fun createUserRequestConfig(body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithArrayInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithArrayInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -125,20 +123,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithArray",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithListInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithListInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithList",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -208,7 +202,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deleteUser(username: kotlin.String) : Unit {
         val localVariableConfig = deleteUserRequestConfig(username = username)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -233,20 +227,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig {
+    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -263,7 +255,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getUserByName(username: kotlin.String) : User {
         val localVariableConfig = getUserByNameRequestConfig(username = username)
 
-        val localVarResponse = request<User>(
+        val localVarResponse = request<Unit, User>(
             localVariableConfig
         )
 
@@ -288,20 +280,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be fetched. Use user1 for testing. 
     * @return RequestConfig
     */
-    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig {
+    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -319,7 +309,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableConfig = loginUserRequestConfig(username = username, password = password)
 
-        val localVarResponse = request<kotlin.String>(
+        val localVarResponse = request<Unit, kotlin.String>(
             localVariableConfig
         )
 
@@ -345,24 +335,22 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param password The password for login in clear text 
     * @return RequestConfig
     */
-    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig {
+    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("username", listOf(username.toString()))
                 put("password", listOf(password.toString()))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/login",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -377,7 +365,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun logoutUser() : Unit {
         val localVariableConfig = logoutUserRequestConfig()
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -401,20 +389,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     *
     * @return RequestConfig
     */
-    fun logoutUserRequestConfig() : RequestConfig {
+    fun logoutUserRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/logout",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -431,7 +417,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updateUser(username: kotlin.String, body: User) : Unit {
         val localVariableConfig = updateUserRequestConfig(username = username, body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -457,20 +443,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Updated user object 
     * @return RequestConfig
     */
-    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig {
+    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-moshi-codegen/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -12,16 +12,17 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
+import java.util.Date
+import java.util.Locale
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -61,9 +62,7 @@ open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -131,7 +130,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         if (requestConfig.headers["api_key"].isNullOrEmpty()) {
             if (apiKey["api_key"] != null) {
                 if (apiKeyPrefix["api_key"] != null) {
@@ -148,7 +147,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = baseUrl.toHttpUrlOrNull() ?: throw IllegalStateException("baseUrl is invalid.")
 
         // take authMethod from operation
@@ -182,7 +181,7 @@ open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -197,57 +196,46 @@ open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message,
+                response.message,
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body, accept),
+                responseBody(response.body, accept),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin-moshi-codegen/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin-multiplatform/src/commonMain/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -155,7 +155,7 @@ open class ApiClient(
         else request(requestConfig, authNames = authNames)
     }
 
-    protected suspend fun request(requestConfig: RequestConfig, body: OutgoingContent = EmptyContent, authNames: kotlin.collections.List<String>): HttpResponse {
+    protected suspend inline fun <reified T: Any?> request(requestConfig: RequestConfig<T>, body: OutgoingContent = EmptyContent, authNames: kotlin.collections.List<String>): HttpResponse {
         requestConfig.updateForAuth(authNames)
         val headers = requestConfig.headers
 

samples/client/petstore/kotlin-multiplatform/src/commonMain/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin-nonpublic/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -47,7 +47,7 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     fun addPet(body: Pet) : Unit {
         val localVariableConfig = addPetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -72,20 +72,18 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun addPetRequestConfig(body: Pet) : RequestConfig {
+    fun addPetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -102,7 +100,7 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableConfig = deletePetRequestConfig(petId = petId, apiKey = apiKey)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -128,21 +126,19 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     * @param apiKey  (optional)
     * @return RequestConfig
     */
-    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig {
+    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -159,7 +155,7 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     fun findPetsByStatus(status: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByStatusRequestConfig(status = status)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -184,23 +180,21 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     * @param status Status values that need to be considered for filter 
     * @return RequestConfig
     */
-    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("status", toMultiValue(status.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByStatus",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -218,7 +212,7 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     fun findPetsByTags(tags: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByTagsRequestConfig(tags = tags)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -244,23 +238,21 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     * @return RequestConfig
     */
     @Deprecated(message = "This operation is deprecated.")
-    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("tags", toMultiValue(tags.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByTags",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -277,7 +269,7 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     fun getPetById(petId: kotlin.Long) : Pet {
         val localVariableConfig = getPetByIdRequestConfig(petId = petId)
 
-        val localVarResponse = request<Pet>(
+        val localVarResponse = request<Unit, Pet>(
             localVariableConfig
         )
 
@@ -302,20 +294,18 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     * @param petId ID of pet to return 
     * @return RequestConfig
     */
-    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig {
+    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -331,7 +321,7 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     fun updatePet(body: Pet) : Unit {
         val localVariableConfig = updatePetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -356,20 +346,18 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun updatePetRequestConfig(body: Pet) : RequestConfig {
+    fun updatePetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -387,7 +375,7 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableConfig = updatePetWithFormRequestConfig(petId = petId, name = name, status = status)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Map<String, Any?>, Unit>(
             localVariableConfig
         )
 
@@ -414,20 +402,18 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     * @param status Updated status of the pet (optional)
     * @return RequestConfig
     */
-    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig {
+    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
+        val localVariableBody = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -446,7 +432,7 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableConfig = uploadFileRequestConfig(petId = petId, additionalMetadata = additionalMetadata, file = file)
 
-        val localVarResponse = request<ApiResponse>(
+        val localVarResponse = request<Map<String, Any?>, ApiResponse>(
             localVariableConfig
         )
 
@@ -473,20 +459,18 @@ internal class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(bas
     * @param file file to upload (optional)
     * @return RequestConfig
     */
-    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig {
+    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
+        val localVariableBody = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/apis/StoreApi.kt

@@ -46,7 +46,7 @@ internal class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(b
     fun deleteOrder(orderId: kotlin.String) : Unit {
         val localVariableConfig = deleteOrderRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ internal class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(b
     * @param orderId ID of the order that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig {
+    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ internal class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(b
     fun getInventory() : kotlin.collections.Map<kotlin.String, kotlin.Int> {
         val localVariableConfig = getInventoryRequestConfig()
 
-        val localVarResponse = request<kotlin.collections.Map<kotlin.String, kotlin.Int>>(
+        val localVarResponse = request<Unit, kotlin.collections.Map<kotlin.String, kotlin.Int>>(
             localVariableConfig
         )
 
@@ -124,20 +122,18 @@ internal class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(b
     *
     * @return RequestConfig
     */
-    fun getInventoryRequestConfig() : RequestConfig {
+    fun getInventoryRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/inventory",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ internal class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(b
     fun getOrderById(orderId: kotlin.Long) : Order {
         val localVariableConfig = getOrderByIdRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Unit, Order>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ internal class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(b
     * @param orderId ID of pet that needs to be fetched 
     * @return RequestConfig
     */
-    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig {
+    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -209,7 +203,7 @@ internal class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(b
     fun placeOrder(body: Order) : Order {
         val localVariableConfig = placeOrderRequestConfig(body = body)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Order, Order>(
             localVariableConfig
         )
 
@@ -234,20 +228,18 @@ internal class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(b
     * @param body order placed for purchasing the pet 
     * @return RequestConfig
     */
-    fun placeOrderRequestConfig(body: Order) : RequestConfig {
+    fun placeOrderRequestConfig(body: Order) : RequestConfig<Order> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/store/order",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -46,7 +46,7 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     fun createUser(body: User) : Unit {
         val localVariableConfig = createUserRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     * @param body Created user object 
     * @return RequestConfig
     */
-    fun createUserRequestConfig(body: User) : RequestConfig {
+    fun createUserRequestConfig(body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     fun createUsersWithArrayInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithArrayInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -125,20 +123,18 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithArray",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     fun createUsersWithListInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithListInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithList",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -208,7 +202,7 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     fun deleteUser(username: kotlin.String) : Unit {
         val localVariableConfig = deleteUserRequestConfig(username = username)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -233,20 +227,18 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     * @param username The name that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig {
+    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -263,7 +255,7 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     fun getUserByName(username: kotlin.String) : User {
         val localVariableConfig = getUserByNameRequestConfig(username = username)
 
-        val localVarResponse = request<User>(
+        val localVarResponse = request<Unit, User>(
             localVariableConfig
         )
 
@@ -288,20 +280,18 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     * @param username The name that needs to be fetched. Use user1 for testing. 
     * @return RequestConfig
     */
-    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig {
+    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -319,7 +309,7 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableConfig = loginUserRequestConfig(username = username, password = password)
 
-        val localVarResponse = request<kotlin.String>(
+        val localVarResponse = request<Unit, kotlin.String>(
             localVariableConfig
         )
 
@@ -345,24 +335,22 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     * @param password The password for login in clear text 
     * @return RequestConfig
     */
-    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig {
+    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("username", listOf(username.toString()))
                 put("password", listOf(password.toString()))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/login",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -377,7 +365,7 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     fun logoutUser() : Unit {
         val localVariableConfig = logoutUserRequestConfig()
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -401,20 +389,18 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     *
     * @return RequestConfig
     */
-    fun logoutUserRequestConfig() : RequestConfig {
+    fun logoutUserRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/logout",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -431,7 +417,7 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     fun updateUser(username: kotlin.String, body: User) : Unit {
         val localVariableConfig = updateUserRequestConfig(username = username, body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -457,20 +443,18 @@ internal class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(ba
     * @param body Updated user object 
     * @return RequestConfig
     */
-    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig {
+    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -12,16 +12,17 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
+import java.util.Date
+import java.util.Locale
 
 internal open class ApiClient(val baseUrl: String) {
     internal companion object {
@@ -61,9 +62,7 @@ internal open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -131,7 +130,7 @@ internal open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         if (requestConfig.headers["api_key"].isNullOrEmpty()) {
             if (apiKey["api_key"] != null) {
                 if (apiKeyPrefix["api_key"] != null) {
@@ -148,7 +147,7 @@ internal open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = baseUrl.toHttpUrlOrNull() ?: throw IllegalStateException("baseUrl is invalid.")
 
         // take authMethod from operation
@@ -182,7 +181,7 @@ internal open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -197,57 +196,46 @@ internal open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message,
+                response.message,
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body, accept),
+                responseBody(response.body, accept),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin-nonpublic/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-internal data class RequestConfig(
+internal data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin-nullable/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin-nullable/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -47,7 +47,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun addPet(body: Pet) : Unit {
         val localVariableConfig = addPetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -72,20 +72,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun addPetRequestConfig(body: Pet) : RequestConfig {
+    fun addPetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -102,7 +100,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableConfig = deletePetRequestConfig(petId = petId, apiKey = apiKey)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -128,21 +126,19 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param apiKey  (optional)
     * @return RequestConfig
     */
-    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig {
+    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -159,7 +155,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByStatus(status: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet>? {
         val localVariableConfig = findPetsByStatusRequestConfig(status = status)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -184,23 +180,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Status values that need to be considered for filter 
     * @return RequestConfig
     */
-    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("status", toMultiValue(status.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByStatus",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -218,7 +212,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByTags(tags: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet>? {
         val localVariableConfig = findPetsByTagsRequestConfig(tags = tags)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -244,23 +238,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @return RequestConfig
     */
     @Deprecated(message = "This operation is deprecated.")
-    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("tags", toMultiValue(tags.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByTags",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -277,7 +269,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getPetById(petId: kotlin.Long) : Pet? {
         val localVariableConfig = getPetByIdRequestConfig(petId = petId)
 
-        val localVarResponse = request<Pet>(
+        val localVarResponse = request<Unit, Pet>(
             localVariableConfig
         )
 
@@ -302,20 +294,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param petId ID of pet to return 
     * @return RequestConfig
     */
-    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig {
+    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -331,7 +321,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePet(body: Pet) : Unit {
         val localVariableConfig = updatePetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -356,20 +346,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun updatePetRequestConfig(body: Pet) : RequestConfig {
+    fun updatePetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -387,7 +375,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableConfig = updatePetWithFormRequestConfig(petId = petId, name = name, status = status)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Map<String, Any?>, Unit>(
             localVariableConfig
         )
 
@@ -414,20 +402,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Updated status of the pet (optional)
     * @return RequestConfig
     */
-    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig {
+    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
+        val localVariableBody = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -446,7 +432,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse? {
         val localVariableConfig = uploadFileRequestConfig(petId = petId, additionalMetadata = additionalMetadata, file = file)
 
-        val localVarResponse = request<ApiResponse>(
+        val localVarResponse = request<Map<String, Any?>, ApiResponse>(
             localVariableConfig
         )
 
@@ -473,20 +459,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param file file to upload (optional)
     * @return RequestConfig
     */
-    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig {
+    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
+        val localVariableBody = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-nullable/src/main/kotlin/org/openapitools/client/apis/StoreApi.kt

@@ -46,7 +46,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun deleteOrder(orderId: kotlin.String) : Unit {
         val localVariableConfig = deleteOrderRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of the order that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig {
+    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getInventory() : kotlin.collections.Map<kotlin.String, kotlin.Int>? {
         val localVariableConfig = getInventoryRequestConfig()
 
-        val localVarResponse = request<kotlin.collections.Map<kotlin.String, kotlin.Int>>(
+        val localVarResponse = request<Unit, kotlin.collections.Map<kotlin.String, kotlin.Int>>(
             localVariableConfig
         )
 
@@ -124,20 +122,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     *
     * @return RequestConfig
     */
-    fun getInventoryRequestConfig() : RequestConfig {
+    fun getInventoryRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/inventory",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getOrderById(orderId: kotlin.Long) : Order? {
         val localVariableConfig = getOrderByIdRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Unit, Order>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of pet that needs to be fetched 
     * @return RequestConfig
     */
-    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig {
+    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -209,7 +203,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun placeOrder(body: Order) : Order? {
         val localVariableConfig = placeOrderRequestConfig(body = body)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Order, Order>(
             localVariableConfig
         )
 
@@ -234,20 +228,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param body order placed for purchasing the pet 
     * @return RequestConfig
     */
-    fun placeOrderRequestConfig(body: Order) : RequestConfig {
+    fun placeOrderRequestConfig(body: Order) : RequestConfig<Order> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/store/order",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-nullable/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -46,7 +46,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUser(body: User) : Unit {
         val localVariableConfig = createUserRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Created user object 
     * @return RequestConfig
     */
-    fun createUserRequestConfig(body: User) : RequestConfig {
+    fun createUserRequestConfig(body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithArrayInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithArrayInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -125,20 +123,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithArray",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithListInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithListInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithList",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -208,7 +202,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deleteUser(username: kotlin.String) : Unit {
         val localVariableConfig = deleteUserRequestConfig(username = username)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -233,20 +227,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig {
+    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -263,7 +255,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getUserByName(username: kotlin.String) : User? {
         val localVariableConfig = getUserByNameRequestConfig(username = username)
 
-        val localVarResponse = request<User>(
+        val localVarResponse = request<Unit, User>(
             localVariableConfig
         )
 
@@ -288,20 +280,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be fetched. Use user1 for testing. 
     * @return RequestConfig
     */
-    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig {
+    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -319,7 +309,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String? {
         val localVariableConfig = loginUserRequestConfig(username = username, password = password)
 
-        val localVarResponse = request<kotlin.String>(
+        val localVarResponse = request<Unit, kotlin.String>(
             localVariableConfig
         )
 
@@ -345,24 +335,22 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param password The password for login in clear text 
     * @return RequestConfig
     */
-    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig {
+    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("username", listOf(username.toString()))
                 put("password", listOf(password.toString()))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/login",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -377,7 +365,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun logoutUser() : Unit {
         val localVariableConfig = logoutUserRequestConfig()
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -401,20 +389,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     *
     * @return RequestConfig
     */
-    fun logoutUserRequestConfig() : RequestConfig {
+    fun logoutUserRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/logout",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -431,7 +417,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updateUser(username: kotlin.String, body: User) : Unit {
         val localVariableConfig = updateUserRequestConfig(username = username, body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -457,20 +443,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Updated user object 
     * @return RequestConfig
     */
-    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig {
+    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-nullable/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -12,16 +12,17 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
+import java.util.Date
+import java.util.Locale
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -61,9 +62,7 @@ open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -131,7 +130,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         if (requestConfig.headers["api_key"].isNullOrEmpty()) {
             if (apiKey["api_key"] != null) {
                 if (apiKeyPrefix["api_key"] != null) {
@@ -148,7 +147,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = baseUrl.toHttpUrlOrNull() ?: throw IllegalStateException("baseUrl is invalid.")
 
         // take authMethod from operation
@@ -182,7 +181,7 @@ open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -197,57 +196,46 @@ open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message,
+                response.message,
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body, accept),
+                responseBody(response.body, accept),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin-nullable/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin-okhttp3/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin-okhttp3/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -47,7 +47,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun addPet(body: Pet) : Unit {
         val localVariableConfig = addPetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -72,20 +72,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun addPetRequestConfig(body: Pet) : RequestConfig {
+    fun addPetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -102,7 +100,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableConfig = deletePetRequestConfig(petId = petId, apiKey = apiKey)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -128,21 +126,19 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param apiKey  (optional)
     * @return RequestConfig
     */
-    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig {
+    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -159,7 +155,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByStatus(status: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByStatusRequestConfig(status = status)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -184,23 +180,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Status values that need to be considered for filter 
     * @return RequestConfig
     */
-    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("status", toMultiValue(status.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByStatus",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -218,7 +212,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByTags(tags: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByTagsRequestConfig(tags = tags)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -244,23 +238,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @return RequestConfig
     */
     @Deprecated(message = "This operation is deprecated.")
-    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("tags", toMultiValue(tags.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByTags",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -277,7 +269,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getPetById(petId: kotlin.Long) : Pet {
         val localVariableConfig = getPetByIdRequestConfig(petId = petId)
 
-        val localVarResponse = request<Pet>(
+        val localVarResponse = request<Unit, Pet>(
             localVariableConfig
         )
 
@@ -302,20 +294,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param petId ID of pet to return 
     * @return RequestConfig
     */
-    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig {
+    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -331,7 +321,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePet(body: Pet) : Unit {
         val localVariableConfig = updatePetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -356,20 +346,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun updatePetRequestConfig(body: Pet) : RequestConfig {
+    fun updatePetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -387,7 +375,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableConfig = updatePetWithFormRequestConfig(petId = petId, name = name, status = status)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Map<String, Any?>, Unit>(
             localVariableConfig
         )
 
@@ -414,20 +402,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Updated status of the pet (optional)
     * @return RequestConfig
     */
-    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig {
+    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
+        val localVariableBody = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -446,7 +432,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableConfig = uploadFileRequestConfig(petId = petId, additionalMetadata = additionalMetadata, file = file)
 
-        val localVarResponse = request<ApiResponse>(
+        val localVarResponse = request<Map<String, Any?>, ApiResponse>(
             localVariableConfig
         )
 
@@ -473,20 +459,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param file file to upload (optional)
     * @return RequestConfig
     */
-    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig {
+    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
+        val localVariableBody = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-okhttp3/src/main/kotlin/org/openapitools/client/apis/StoreApi.kt

@@ -46,7 +46,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun deleteOrder(orderId: kotlin.String) : Unit {
         val localVariableConfig = deleteOrderRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of the order that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig {
+    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getInventory() : kotlin.collections.Map<kotlin.String, kotlin.Int> {
         val localVariableConfig = getInventoryRequestConfig()
 
-        val localVarResponse = request<kotlin.collections.Map<kotlin.String, kotlin.Int>>(
+        val localVarResponse = request<Unit, kotlin.collections.Map<kotlin.String, kotlin.Int>>(
             localVariableConfig
         )
 
@@ -124,20 +122,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     *
     * @return RequestConfig
     */
-    fun getInventoryRequestConfig() : RequestConfig {
+    fun getInventoryRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/inventory",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getOrderById(orderId: kotlin.Long) : Order {
         val localVariableConfig = getOrderByIdRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Unit, Order>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of pet that needs to be fetched 
     * @return RequestConfig
     */
-    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig {
+    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -209,7 +203,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun placeOrder(body: Order) : Order {
         val localVariableConfig = placeOrderRequestConfig(body = body)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Order, Order>(
             localVariableConfig
         )
 
@@ -234,20 +228,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param body order placed for purchasing the pet 
     * @return RequestConfig
     */
-    fun placeOrderRequestConfig(body: Order) : RequestConfig {
+    fun placeOrderRequestConfig(body: Order) : RequestConfig<Order> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/store/order",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-okhttp3/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -46,7 +46,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUser(body: User) : Unit {
         val localVariableConfig = createUserRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Created user object 
     * @return RequestConfig
     */
-    fun createUserRequestConfig(body: User) : RequestConfig {
+    fun createUserRequestConfig(body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithArrayInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithArrayInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -125,20 +123,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithArray",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithListInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithListInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithList",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -208,7 +202,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deleteUser(username: kotlin.String) : Unit {
         val localVariableConfig = deleteUserRequestConfig(username = username)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -233,20 +227,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig {
+    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -263,7 +255,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getUserByName(username: kotlin.String) : User {
         val localVariableConfig = getUserByNameRequestConfig(username = username)
 
-        val localVarResponse = request<User>(
+        val localVarResponse = request<Unit, User>(
             localVariableConfig
         )
 
@@ -288,20 +280,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be fetched. Use user1 for testing. 
     * @return RequestConfig
     */
-    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig {
+    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -319,7 +309,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableConfig = loginUserRequestConfig(username = username, password = password)
 
-        val localVarResponse = request<kotlin.String>(
+        val localVarResponse = request<Unit, kotlin.String>(
             localVariableConfig
         )
 
@@ -345,24 +335,22 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param password The password for login in clear text 
     * @return RequestConfig
     */
-    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig {
+    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("username", listOf(username.toString()))
                 put("password", listOf(password.toString()))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/login",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -377,7 +365,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun logoutUser() : Unit {
         val localVariableConfig = logoutUserRequestConfig()
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -401,20 +389,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     *
     * @return RequestConfig
     */
-    fun logoutUserRequestConfig() : RequestConfig {
+    fun logoutUserRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/logout",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -431,7 +417,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updateUser(username: kotlin.String, body: User) : Unit {
         val localVariableConfig = updateUserRequestConfig(username = username, body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -457,20 +443,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Updated user object 
     * @return RequestConfig
     */
-    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig {
+    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-okhttp3/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -10,16 +10,17 @@ import okhttp3.ResponseBody
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
+import java.util.Date
+import java.util.Locale
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -59,9 +60,7 @@ open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> RequestBody.create(
+            content is File -> RequestBody.create(MediaType.parse(mediaType), content)
-                MediaType.parse(mediaType), content
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -129,7 +128,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         if (requestConfig.headers["api_key"].isNullOrEmpty()) {
             if (apiKey["api_key"] != null) {
                 if (apiKeyPrefix["api_key"] != null) {
@@ -146,7 +145,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = HttpUrl.parse(baseUrl) ?: throw IllegalStateException("baseUrl is invalid.")
 
         // take authMethod from operation
@@ -180,7 +179,7 @@ open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -195,57 +194,46 @@ open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code(),
+                response.code(),
-                    response.headers().toMultimap()
+                response.headers().toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message(),
+                response.message(),
-                    response.code(),
+                response.code(),
-                    response.headers().toMultimap()
+                response.headers().toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body(), accept),
+                responseBody(response.body(), accept),
-                    response.code(),
+                response.code(),
-                    response.headers().toMultimap()
+                response.headers().toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message(),
+                response.message(),
-                    response.body()?.string(),
+                response.body()?.string(),
-                    response.code(),
+                response.code(),
-                    response.headers().toMultimap()
+                response.headers().toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message(),
+                response.message(),
-                    response.body()?.string(),
+                response.body()?.string(),
-                    response.code(),
+                response.code(),
-                    response.headers().toMultimap()
+                response.headers().toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin-okhttp3/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin-retrofit2-kotlinx_serialization/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
     ext.retrofitVersion = '2.7.2'
 
     repositories {
@@ -32,7 +32,7 @@ test {
 
 dependencies {
     compile "org.jetbrains.kotlin:kotlin-stdlib-jdk8:$kotlin_version"
-    compile "org.jetbrains.kotlinx:kotlinx-serialization-json:1.1.0"
+    compile "org.jetbrains.kotlinx:kotlinx-serialization-json:1.2.1"
     compile "org.apache.oltu.oauth2:org.apache.oltu.oauth2.client:1.0.0"
     compile "com.squareup.okhttp3:logging-interceptor:4.9.0"
     compile "com.squareup.retrofit2:retrofit:$retrofitVersion"

samples/client/petstore/kotlin-retrofit2-rx3/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
     ext.retrofitVersion = '2.7.2'
     ext.rxJava3Version = '3.0.10'
 

samples/client/petstore/kotlin-retrofit2/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
     ext.retrofitVersion = '2.7.2'
 
     repositories {

samples/client/petstore/kotlin-string/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -47,7 +47,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun addPet(body: Pet) : Unit {
         val localVariableConfig = addPetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -72,20 +72,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun addPetRequestConfig(body: Pet) : RequestConfig {
+    fun addPetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -102,7 +100,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(apiKey: kotlin.String?, petId: kotlin.Long) : Unit {
         val localVariableConfig = deletePetRequestConfig(apiKey = apiKey, petId = petId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -128,21 +126,19 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param petId Pet id to delete 
     * @return RequestConfig
     */
-    fun deletePetRequestConfig(apiKey: kotlin.String?, petId: kotlin.Long) : RequestConfig {
+    fun deletePetRequestConfig(apiKey: kotlin.String?, petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -159,7 +155,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByStatus(status: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByStatusRequestConfig(status = status)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -184,23 +180,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Status values that need to be considered for filter 
     * @return RequestConfig
     */
-    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("status", toMultiValue(status.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByStatus",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -218,7 +212,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByTags(tags: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByTagsRequestConfig(tags = tags)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -244,23 +238,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @return RequestConfig
     */
     @Deprecated(message = "This operation is deprecated.")
-    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("tags", toMultiValue(tags.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByTags",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -277,7 +269,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getPetById(petId: kotlin.Long) : Pet {
         val localVariableConfig = getPetByIdRequestConfig(petId = petId)
 
-        val localVarResponse = request<Pet>(
+        val localVarResponse = request<Unit, Pet>(
             localVariableConfig
         )
 
@@ -302,20 +294,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param petId ID of pet to return 
     * @return RequestConfig
     */
-    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig {
+    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -331,7 +321,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePet(body: Pet) : Unit {
         val localVariableConfig = updatePetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -356,20 +346,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun updatePetRequestConfig(body: Pet) : RequestConfig {
+    fun updatePetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -387,7 +375,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableConfig = updatePetWithFormRequestConfig(petId = petId, name = name, status = status)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Map<String, Any?>, Unit>(
             localVariableConfig
         )
 
@@ -414,20 +402,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Updated status of the pet (optional)
     * @return RequestConfig
     */
-    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig {
+    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
+        val localVariableBody = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -446,7 +432,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableConfig = uploadFileRequestConfig(petId = petId, additionalMetadata = additionalMetadata, file = file)
 
-        val localVarResponse = request<ApiResponse>(
+        val localVarResponse = request<Map<String, Any?>, ApiResponse>(
             localVariableConfig
         )
 
@@ -473,20 +459,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param file file to upload (optional)
     * @return RequestConfig
     */
-    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig {
+    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
+        val localVariableBody = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/apis/StoreApi.kt

@@ -46,7 +46,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun deleteOrder(orderId: kotlin.String) : Unit {
         val localVariableConfig = deleteOrderRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of the order that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig {
+    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getInventory() : kotlin.collections.Map<kotlin.String, kotlin.Int> {
         val localVariableConfig = getInventoryRequestConfig()
 
-        val localVarResponse = request<kotlin.collections.Map<kotlin.String, kotlin.Int>>(
+        val localVarResponse = request<Unit, kotlin.collections.Map<kotlin.String, kotlin.Int>>(
             localVariableConfig
         )
 
@@ -124,20 +122,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     *
     * @return RequestConfig
     */
-    fun getInventoryRequestConfig() : RequestConfig {
+    fun getInventoryRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/inventory",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getOrderById(orderId: kotlin.Long) : Order {
         val localVariableConfig = getOrderByIdRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Unit, Order>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of pet that needs to be fetched 
     * @return RequestConfig
     */
-    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig {
+    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -209,7 +203,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun placeOrder(body: Order) : Order {
         val localVariableConfig = placeOrderRequestConfig(body = body)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Order, Order>(
             localVariableConfig
         )
 
@@ -234,20 +228,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param body order placed for purchasing the pet 
     * @return RequestConfig
     */
-    fun placeOrderRequestConfig(body: Order) : RequestConfig {
+    fun placeOrderRequestConfig(body: Order) : RequestConfig<Order> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/store/order",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -46,7 +46,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUser(body: User) : Unit {
         val localVariableConfig = createUserRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Created user object 
     * @return RequestConfig
     */
-    fun createUserRequestConfig(body: User) : RequestConfig {
+    fun createUserRequestConfig(body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithArrayInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithArrayInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -125,20 +123,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithArray",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithListInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithListInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithList",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -208,7 +202,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deleteUser(username: kotlin.String) : Unit {
         val localVariableConfig = deleteUserRequestConfig(username = username)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -233,20 +227,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig {
+    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -263,7 +255,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getUserByName(username: kotlin.String) : User {
         val localVariableConfig = getUserByNameRequestConfig(username = username)
 
-        val localVarResponse = request<User>(
+        val localVarResponse = request<Unit, User>(
             localVariableConfig
         )
 
@@ -288,20 +280,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be fetched. Use user1 for testing. 
     * @return RequestConfig
     */
-    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig {
+    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -319,7 +309,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableConfig = loginUserRequestConfig(username = username, password = password)
 
-        val localVarResponse = request<kotlin.String>(
+        val localVarResponse = request<Unit, kotlin.String>(
             localVariableConfig
         )
 
@@ -345,24 +335,22 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param password The password for login in clear text 
     * @return RequestConfig
     */
-    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig {
+    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("username", listOf(username.toString()))
                 put("password", listOf(password.toString()))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/login",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -377,7 +365,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun logoutUser() : Unit {
         val localVariableConfig = logoutUserRequestConfig()
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -401,20 +389,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     *
     * @return RequestConfig
     */
-    fun logoutUserRequestConfig() : RequestConfig {
+    fun logoutUserRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/logout",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -431,7 +417,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updateUser(username: kotlin.String, body: User) : Unit {
         val localVariableConfig = updateUserRequestConfig(username = username, body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -457,20 +443,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Updated user object 
     * @return RequestConfig
     */
-    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig {
+    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -12,16 +12,17 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
+import java.util.Date
+import java.util.Locale
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -61,9 +62,7 @@ open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -131,7 +130,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         if (requestConfig.headers["api_key"].isNullOrEmpty()) {
             if (apiKey["api_key"] != null) {
                 if (apiKeyPrefix["api_key"] != null) {
@@ -148,7 +147,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = baseUrl.toHttpUrlOrNull() ?: throw IllegalStateException("baseUrl is invalid.")
 
         // take authMethod from operation
@@ -182,7 +181,7 @@ open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -197,57 +196,46 @@ open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message,
+                response.message,
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body, accept),
+                responseBody(response.body, accept),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin-string/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin-threetenbp/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -47,7 +47,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun addPet(body: Pet) : Unit {
         val localVariableConfig = addPetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -72,20 +72,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun addPetRequestConfig(body: Pet) : RequestConfig {
+    fun addPetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -102,7 +100,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableConfig = deletePetRequestConfig(petId = petId, apiKey = apiKey)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -128,21 +126,19 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param apiKey  (optional)
     * @return RequestConfig
     */
-    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig {
+    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -159,7 +155,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByStatus(status: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByStatusRequestConfig(status = status)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -184,23 +180,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Status values that need to be considered for filter 
     * @return RequestConfig
     */
-    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("status", toMultiValue(status.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByStatus",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -218,7 +212,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByTags(tags: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByTagsRequestConfig(tags = tags)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -244,23 +238,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @return RequestConfig
     */
     @Deprecated(message = "This operation is deprecated.")
-    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("tags", toMultiValue(tags.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByTags",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -277,7 +269,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getPetById(petId: kotlin.Long) : Pet {
         val localVariableConfig = getPetByIdRequestConfig(petId = petId)
 
-        val localVarResponse = request<Pet>(
+        val localVarResponse = request<Unit, Pet>(
             localVariableConfig
         )
 
@@ -302,20 +294,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param petId ID of pet to return 
     * @return RequestConfig
     */
-    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig {
+    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -331,7 +321,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePet(body: Pet) : Unit {
         val localVariableConfig = updatePetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -356,20 +346,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun updatePetRequestConfig(body: Pet) : RequestConfig {
+    fun updatePetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -387,7 +375,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableConfig = updatePetWithFormRequestConfig(petId = petId, name = name, status = status)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Map<String, Any?>, Unit>(
             localVariableConfig
         )
 
@@ -414,20 +402,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Updated status of the pet (optional)
     * @return RequestConfig
     */
-    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig {
+    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
+        val localVariableBody = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -446,7 +432,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableConfig = uploadFileRequestConfig(petId = petId, additionalMetadata = additionalMetadata, file = file)
 
-        val localVarResponse = request<ApiResponse>(
+        val localVarResponse = request<Map<String, Any?>, ApiResponse>(
             localVariableConfig
         )
 
@@ -473,20 +459,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param file file to upload (optional)
     * @return RequestConfig
     */
-    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig {
+    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
+        val localVariableBody = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/apis/StoreApi.kt

@@ -46,7 +46,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun deleteOrder(orderId: kotlin.String) : Unit {
         val localVariableConfig = deleteOrderRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of the order that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig {
+    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getInventory() : kotlin.collections.Map<kotlin.String, kotlin.Int> {
         val localVariableConfig = getInventoryRequestConfig()
 
-        val localVarResponse = request<kotlin.collections.Map<kotlin.String, kotlin.Int>>(
+        val localVarResponse = request<Unit, kotlin.collections.Map<kotlin.String, kotlin.Int>>(
             localVariableConfig
         )
 
@@ -124,20 +122,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     *
     * @return RequestConfig
     */
-    fun getInventoryRequestConfig() : RequestConfig {
+    fun getInventoryRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/inventory",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getOrderById(orderId: kotlin.Long) : Order {
         val localVariableConfig = getOrderByIdRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Unit, Order>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of pet that needs to be fetched 
     * @return RequestConfig
     */
-    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig {
+    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -209,7 +203,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun placeOrder(body: Order) : Order {
         val localVariableConfig = placeOrderRequestConfig(body = body)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Order, Order>(
             localVariableConfig
         )
 
@@ -234,20 +228,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param body order placed for purchasing the pet 
     * @return RequestConfig
     */
-    fun placeOrderRequestConfig(body: Order) : RequestConfig {
+    fun placeOrderRequestConfig(body: Order) : RequestConfig<Order> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/store/order",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -46,7 +46,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUser(body: User) : Unit {
         val localVariableConfig = createUserRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Created user object 
     * @return RequestConfig
     */
-    fun createUserRequestConfig(body: User) : RequestConfig {
+    fun createUserRequestConfig(body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithArrayInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithArrayInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -125,20 +123,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithArray",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithListInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithListInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithList",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -208,7 +202,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deleteUser(username: kotlin.String) : Unit {
         val localVariableConfig = deleteUserRequestConfig(username = username)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -233,20 +227,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig {
+    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -263,7 +255,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getUserByName(username: kotlin.String) : User {
         val localVariableConfig = getUserByNameRequestConfig(username = username)
 
-        val localVarResponse = request<User>(
+        val localVarResponse = request<Unit, User>(
             localVariableConfig
         )
 
@@ -288,20 +280,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be fetched. Use user1 for testing. 
     * @return RequestConfig
     */
-    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig {
+    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -319,7 +309,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableConfig = loginUserRequestConfig(username = username, password = password)
 
-        val localVarResponse = request<kotlin.String>(
+        val localVarResponse = request<Unit, kotlin.String>(
             localVariableConfig
         )
 
@@ -345,24 +335,22 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param password The password for login in clear text 
     * @return RequestConfig
     */
-    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig {
+    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("username", listOf(username.toString()))
                 put("password", listOf(password.toString()))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/login",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -377,7 +365,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun logoutUser() : Unit {
         val localVariableConfig = logoutUserRequestConfig()
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -401,20 +389,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     *
     * @return RequestConfig
     */
-    fun logoutUserRequestConfig() : RequestConfig {
+    fun logoutUserRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/logout",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -431,7 +417,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updateUser(username: kotlin.String, body: User) : Unit {
         val localVariableConfig = updateUserRequestConfig(username = username, body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -457,20 +443,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Updated user object 
     * @return RequestConfig
     */
-    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig {
+    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -12,11 +12,12 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
 import java.util.Date
+import java.util.Locale
 import org.threeten.bp.LocalDate
 import org.threeten.bp.LocalDateTime
 import org.threeten.bp.LocalTime
@@ -61,9 +62,7 @@ open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -131,7 +130,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         if (requestConfig.headers["api_key"].isNullOrEmpty()) {
             if (apiKey["api_key"] != null) {
                 if (apiKeyPrefix["api_key"] != null) {
@@ -148,7 +147,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = baseUrl.toHttpUrlOrNull() ?: throw IllegalStateException("baseUrl is invalid.")
 
         // take authMethod from operation
@@ -182,7 +181,7 @@ open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -197,57 +196,46 @@ open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message,
+                response.message,
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body, accept),
+                responseBody(response.body, accept),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin-threetenbp/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin-uppercase-enum/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin-uppercase-enum/src/main/kotlin/org/openapitools/client/apis/EnumApi.kt

@@ -46,7 +46,7 @@ class EnumApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getEnum() : PetEnum {
         val localVariableConfig = getEnumRequestConfig()
 
-        val localVarResponse = request<PetEnum>(
+        val localVarResponse = request<Unit, PetEnum>(
             localVariableConfig
         )
 
@@ -70,20 +70,18 @@ class EnumApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     *
     * @return RequestConfig
     */
-    fun getEnumRequestConfig() : RequestConfig {
+    fun getEnumRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/enum",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin-uppercase-enum/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -12,16 +12,17 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
+import java.util.Date
+import java.util.Locale
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -61,9 +62,7 @@ open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -132,7 +131,7 @@ open class ApiClient(val baseUrl: String) {
     }
 
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = baseUrl.toHttpUrlOrNull() ?: throw IllegalStateException("baseUrl is invalid.")
 
         val url = httpUrl.newBuilder()
@@ -163,7 +162,7 @@ open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -178,57 +177,46 @@ open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message,
+                response.message,
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body, accept),
+                responseBody(response.body, accept),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin-uppercase-enum/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )

samples/client/petstore/kotlin/build.gradle

@@ -7,7 +7,7 @@ wrapper {
 }
 
 buildscript {
-    ext.kotlin_version = '1.4.30'
+    ext.kotlin_version = '1.5.0'
 
     repositories {
         maven { url "https://repo1.maven.org/maven2" }

samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/apis/PetApi.kt

@@ -47,7 +47,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun addPet(body: Pet) : Unit {
         val localVariableConfig = addPetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -72,20 +72,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun addPetRequestConfig(body: Pet) : RequestConfig {
+    fun addPetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -102,7 +100,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deletePet(petId: kotlin.Long, apiKey: kotlin.String?) : Unit {
         val localVariableConfig = deletePetRequestConfig(petId = petId, apiKey = apiKey)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -128,21 +126,19 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param apiKey  (optional)
     * @return RequestConfig
     */
-    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig {
+    fun deletePetRequestConfig(petId: kotlin.Long, apiKey: kotlin.String?) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
         apiKey?.apply { localVariableHeaders["api_key"] = this.toString() }
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -159,7 +155,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByStatus(status: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByStatusRequestConfig(status = status)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -184,23 +180,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Status values that need to be considered for filter 
     * @return RequestConfig
     */
-    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByStatusRequestConfig(status: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("status", toMultiValue(status.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByStatus",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -218,7 +212,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun findPetsByTags(tags: kotlin.collections.List<kotlin.String>) : kotlin.collections.List<Pet> {
         val localVariableConfig = findPetsByTagsRequestConfig(tags = tags)
 
-        val localVarResponse = request<kotlin.collections.List<Pet>>(
+        val localVarResponse = request<Unit, kotlin.collections.List<Pet>>(
             localVariableConfig
         )
 
@@ -244,23 +238,21 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @return RequestConfig
     */
     @Deprecated(message = "This operation is deprecated.")
-    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig {
+    fun findPetsByTagsRequestConfig(tags: kotlin.collections.List<kotlin.String>) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("tags", toMultiValue(tags.toList(), "csv"))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/findByTags",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -277,7 +269,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getPetById(petId: kotlin.Long) : Pet {
         val localVariableConfig = getPetByIdRequestConfig(petId = petId)
 
-        val localVarResponse = request<Pet>(
+        val localVarResponse = request<Unit, Pet>(
             localVariableConfig
         )
 
@@ -302,20 +294,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param petId ID of pet to return 
     * @return RequestConfig
     */
-    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig {
+    fun getPetByIdRequestConfig(petId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -331,7 +321,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePet(body: Pet) : Unit {
         val localVariableConfig = updatePetRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Pet, Unit>(
             localVariableConfig
         )
 
@@ -356,20 +346,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Pet object that needs to be added to the store 
     * @return RequestConfig
     */
-    fun updatePetRequestConfig(body: Pet) : RequestConfig {
+    fun updatePetRequestConfig(body: Pet) : RequestConfig<Pet> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/pet",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -387,7 +375,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updatePetWithForm(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : Unit {
         val localVariableConfig = updatePetWithFormRequestConfig(petId = petId, name = name, status = status)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Map<String, Any?>, Unit>(
             localVariableConfig
         )
 
@@ -414,20 +402,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param status Updated status of the pet (optional)
     * @return RequestConfig
     */
-    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig {
+    fun updatePetWithFormRequestConfig(petId: kotlin.Long, name: kotlin.String?, status: kotlin.String?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("name" to name, "status" to status)
+        val localVariableBody = mapOf("name" to name, "status" to status)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "application/x-www-form-urlencoded")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -446,7 +432,7 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun uploadFile(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : ApiResponse {
         val localVariableConfig = uploadFileRequestConfig(petId = petId, additionalMetadata = additionalMetadata, file = file)
 
-        val localVarResponse = request<ApiResponse>(
+        val localVarResponse = request<Map<String, Any?>, ApiResponse>(
             localVariableConfig
         )
 
@@ -473,20 +459,18 @@ class PetApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param file file to upload (optional)
     * @return RequestConfig
     */
-    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig {
+    fun uploadFileRequestConfig(petId: kotlin.Long, additionalMetadata: kotlin.String?, file: java.io.File?) : RequestConfig<Map<String, Any?>> {
-        val localVariableBody: kotlin.Any? = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
+        val localVariableBody = mapOf("additionalMetadata" to additionalMetadata, "file" to file)
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf("Content-Type" to "multipart/form-data")
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/pet/{petId}/uploadImage".replace("{"+"petId"+"}", "$petId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/apis/StoreApi.kt

@@ -46,7 +46,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun deleteOrder(orderId: kotlin.String) : Unit {
         val localVariableConfig = deleteOrderRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of the order that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig {
+    fun deleteOrderRequestConfig(orderId: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getInventory() : kotlin.collections.Map<kotlin.String, kotlin.Int> {
         val localVariableConfig = getInventoryRequestConfig()
 
-        val localVarResponse = request<kotlin.collections.Map<kotlin.String, kotlin.Int>>(
+        val localVarResponse = request<Unit, kotlin.collections.Map<kotlin.String, kotlin.Int>>(
             localVariableConfig
         )
 
@@ -124,20 +122,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     *
     * @return RequestConfig
     */
-    fun getInventoryRequestConfig() : RequestConfig {
+    fun getInventoryRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/inventory",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun getOrderById(orderId: kotlin.Long) : Order {
         val localVariableConfig = getOrderByIdRequestConfig(orderId = orderId)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Unit, Order>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param orderId ID of pet that needs to be fetched 
     * @return RequestConfig
     */
-    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig {
+    fun getOrderByIdRequestConfig(orderId: kotlin.Long) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/store/order/{orderId}".replace("{"+"orderId"+"}", "$orderId"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -209,7 +203,7 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     fun placeOrder(body: Order) : Order {
         val localVariableConfig = placeOrderRequestConfig(body = body)
 
-        val localVarResponse = request<Order>(
+        val localVarResponse = request<Order, Order>(
             localVariableConfig
         )
 
@@ -234,20 +228,18 @@ class StoreApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath)
     * @param body order placed for purchasing the pet 
     * @return RequestConfig
     */
-    fun placeOrderRequestConfig(body: Order) : RequestConfig {
+    fun placeOrderRequestConfig(body: Order) : RequestConfig<Order> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/store/order",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/apis/UserApi.kt

@@ -46,7 +46,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUser(body: User) : Unit {
         val localVariableConfig = createUserRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -71,20 +71,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Created user object 
     * @return RequestConfig
     */
-    fun createUserRequestConfig(body: User) : RequestConfig {
+    fun createUserRequestConfig(body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -100,7 +98,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithArrayInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithArrayInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -125,20 +123,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithArrayInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithArray",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -154,7 +150,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun createUsersWithListInput(body: kotlin.collections.List<User>) : Unit {
         val localVariableConfig = createUsersWithListInputRequestConfig(body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<kotlin.collections.List<User>, Unit>(
             localVariableConfig
         )
 
@@ -179,20 +175,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body List of user object 
     * @return RequestConfig
     */
-    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig {
+    fun createUsersWithListInputRequestConfig(body: kotlin.collections.List<User>) : RequestConfig<kotlin.collections.List<User>> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.POST,
             path = "/user/createWithList",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -208,7 +202,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun deleteUser(username: kotlin.String) : Unit {
         val localVariableConfig = deleteUserRequestConfig(username = username)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -233,20 +227,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be deleted 
     * @return RequestConfig
     */
-    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig {
+    fun deleteUserRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.DELETE,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -263,7 +255,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun getUserByName(username: kotlin.String) : User {
         val localVariableConfig = getUserByNameRequestConfig(username = username)
 
-        val localVarResponse = request<User>(
+        val localVarResponse = request<Unit, User>(
             localVariableConfig
         )
 
@@ -288,20 +280,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param username The name that needs to be fetched. Use user1 for testing. 
     * @return RequestConfig
     */
-    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig {
+    fun getUserByNameRequestConfig(username: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -319,7 +309,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun loginUser(username: kotlin.String, password: kotlin.String) : kotlin.String {
         val localVariableConfig = loginUserRequestConfig(username = username, password = password)
 
-        val localVarResponse = request<kotlin.String>(
+        val localVarResponse = request<Unit, kotlin.String>(
             localVariableConfig
         )
 
@@ -345,24 +335,22 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param password The password for login in clear text 
     * @return RequestConfig
     */
-    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig {
+    fun loginUserRequestConfig(username: kotlin.String, password: kotlin.String) : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf<kotlin.String, List<kotlin.String>>()
             .apply {
                 put("username", listOf(username.toString()))
                 put("password", listOf(password.toString()))
             }
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/login",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -377,7 +365,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun logoutUser() : Unit {
         val localVariableConfig = logoutUserRequestConfig()
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<Unit, Unit>(
             localVariableConfig
         )
 
@@ -401,20 +389,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     *
     * @return RequestConfig
     */
-    fun logoutUserRequestConfig() : RequestConfig {
+    fun logoutUserRequestConfig() : RequestConfig<Unit> {
-        val localVariableBody: kotlin.Any? = null
+        val localVariableBody = null
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.GET,
             path = "/user/logout",
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
     /**
@@ -431,7 +417,7 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     fun updateUser(username: kotlin.String, body: User) : Unit {
         val localVariableConfig = updateUserRequestConfig(username = username, body = body)
 
-        val localVarResponse = request<Any?>(
+        val localVarResponse = request<User, Unit>(
             localVariableConfig
         )
 
@@ -457,20 +443,18 @@ class UserApi(basePath: kotlin.String = defaultBasePath) : ApiClient(basePath) {
     * @param body Updated user object 
     * @return RequestConfig
     */
-    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig {
+    fun updateUserRequestConfig(username: kotlin.String, body: User) : RequestConfig<User> {
-        val localVariableBody: kotlin.Any? = body
+        val localVariableBody = body
         val localVariableQuery: MultiValueMap = mutableMapOf()
         val localVariableHeaders: MutableMap<String, String> = mutableMapOf()
-        
+
-        val localVariableConfig = RequestConfig(
+        return RequestConfig(
             method = RequestMethod.PUT,
             path = "/user/{username}".replace("{"+"username"+"}", "$username"),
             query = localVariableQuery,
             headers = localVariableHeaders,
             body = localVariableBody
         )
-
-        return localVariableConfig
     }
 
 }

samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/ApiClient.kt

@@ -12,16 +12,17 @@ import okhttp3.MediaType.Companion.toMediaTypeOrNull
 import okhttp3.Request
 import okhttp3.Headers
 import okhttp3.MultipartBody
-import java.io.File
 import java.io.BufferedWriter
+import java.io.File
 import java.io.FileWriter
 import java.net.URLConnection
-import java.util.Date
 import java.time.LocalDate
 import java.time.LocalDateTime
 import java.time.LocalTime
 import java.time.OffsetDateTime
 import java.time.OffsetTime
+import java.util.Date
+import java.util.Locale
 
 open class ApiClient(val baseUrl: String) {
     companion object {
@@ -61,9 +62,7 @@ open class ApiClient(val baseUrl: String) {
 
     protected inline fun <reified T> requestBody(content: T, mediaType: String = JsonMediaType): RequestBody =
         when {
-            content is File -> content.asRequestBody(
+            content is File -> content.asRequestBody(mediaType.toMediaTypeOrNull())
-                mediaType.toMediaTypeOrNull()
-            )
             mediaType == FormDataMediaType -> {
                 MultipartBody.Builder()
                     .setType(MultipartBody.FORM)
@@ -131,7 +130,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected fun updateAuthParams(requestConfig: RequestConfig) {
+    protected fun <T> updateAuthParams(requestConfig: RequestConfig<T>) {
         if (requestConfig.headers["api_key"].isNullOrEmpty()) {
             if (apiKey["api_key"] != null) {
                 if (apiKeyPrefix["api_key"] != null) {
@@ -148,7 +147,7 @@ open class ApiClient(val baseUrl: String) {
         }
     }
 
-    protected inline fun <reified T: Any?> request(requestConfig: RequestConfig): ApiInfrastructureResponse<T?> {
+    protected inline fun <reified I, reified T: Any?> request(requestConfig: RequestConfig<I>): ApiInfrastructureResponse<T?> {
         val httpUrl = baseUrl.toHttpUrlOrNull() ?: throw IllegalStateException("baseUrl is invalid.")
 
         // take authMethod from operation
@@ -182,7 +181,7 @@ open class ApiClient(val baseUrl: String) {
         }
 
         // TODO: support multiple contentType options here.
-        val contentType = (headers[ContentType] as String).substringBefore(";").toLowerCase()
+        val contentType = (headers[ContentType] as String).substringBefore(";").lowercase(Locale.getDefault())
 
         val request = when (requestConfig.method) {
             RequestMethod.DELETE -> Request.Builder().url(url).delete(requestBody(requestConfig.body, contentType))
@@ -197,57 +196,46 @@ open class ApiClient(val baseUrl: String) {
         }.build()
 
         val response = client.newCall(request).execute()
-        val accept = response.header(ContentType)?.substringBefore(";")?.toLowerCase()
+        val accept = response.header(ContentType)?.substringBefore(";")?.lowercase(Locale.getDefault())
 
         // TODO: handle specific mapping types. e.g. Map<int, Class<?>>
-        when {
+        return when {
-            response.isRedirect -> return Redirection(
+            response.isRedirect -> Redirection(
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isInformational -> return Informational(
+            response.isInformational -> Informational(
-                    response.message,
+                response.message,
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isSuccessful -> return Success(
+            response.isSuccessful -> Success(
-                    responseBody(response.body, accept),
+                responseBody(response.body, accept),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            response.isClientError -> return ClientError(
+            response.isClientError -> ClientError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
-            else -> return ServerError(
+            else -> ServerError(
-                    response.message,
+                response.message,
-                    response.body?.string(),
+                response.body?.string(),
-                    response.code,
+                response.code,
-                    response.headers.toMultimap()
+                response.headers.toMultimap()
             )
         }
     }
 
-    protected fun parameterToString(value: Any?): String {
+    protected fun parameterToString(value: Any?): String = when (value) {
-        when (value) {
+        null -> ""
-            null -> {
+        is Array<*> -> toMultiValue(value, "csv").toString()
-                return ""
+        is Iterable<*> -> toMultiValue(value, "csv").toString()
-            }
+        is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date ->
-            is Array<*> -> {
+            parseDateToQueryString(value)
-                return toMultiValue(value, "csv").toString()
+        else -> value.toString()
-            }
-            is Iterable<*> -> {
-                return toMultiValue(value, "csv").toString()
-            }
-            is OffsetDateTime, is OffsetTime, is LocalDateTime, is LocalDate, is LocalTime, is Date -> {
-                return parseDateToQueryString<Any>(value)
-            }
-            else -> {
-                return value.toString()
-            }
-        }
     }
 
     protected inline fun <reified T: Any> parseDateToQueryString(value : T): String {

samples/client/petstore/kotlin/src/main/kotlin/org/openapitools/client/infrastructure/RequestConfig.kt

@@ -8,10 +8,10 @@ package org.openapitools.client.infrastructure
  * NOTE: Headers is a Map<String,String> because rfc2616 defines
  *       multi-valued headers as csv-only.
  */
-data class RequestConfig(
+data class RequestConfig<T>(
     val method: RequestMethod,
     val path: String,
     val headers: MutableMap<String, String> = mutableMapOf(),
     val query: MutableMap<String, List<String>> = mutableMapOf(),
-    val body: kotlin.Any? = null
+    val body: T? = null
 )