[Java][okhttp-gson] Support text/plain body (#10885)

* support serialize RequestBody with contentType text/plain

* add Serialize test

* update test comment

modules/openapi-generator/src/main/resources/Java/libraries/okhttp-gson/ApiClient.mustache

@@ -1081,6 +1081,8 @@ public class ApiClient {
         } else if (obj instanceof File) {
             // File body parameter support.
             return RequestBody.create((File) obj, MediaType.parse(contentType));
+        } else if ("text/plain".equals(contentType) && obj instanceof String) {
+            return RequestBody.create((String) obj, MediaType.parse(contentType));
         } else if (isJsonMime(contentType)) {
             String content;
             if (obj != null) {

samples/client/others/java/okhttp-gson-streaming/src/main/java/org/openapitools/client/ApiClient.java

@@ -871,6 +871,8 @@ public class ApiClient {
         } else if (obj instanceof File) {
             // File body parameter support.
             return RequestBody.create((File) obj, MediaType.parse(contentType));
+        } else if ("text/plain".equals(contentType) && obj instanceof String) {
+            return RequestBody.create((String) obj, MediaType.parse(contentType));
         } else if (isJsonMime(contentType)) {
             String content;
             if (obj != null) {

samples/client/petstore/java/okhttp-gson-dynamicOperations/src/main/java/org/openapitools/client/ApiClient.java

@@ -972,6 +972,8 @@ public class ApiClient {
         } else if (obj instanceof File) {
             // File body parameter support.
             return RequestBody.create((File) obj, MediaType.parse(contentType));
+        } else if ("text/plain".equals(contentType) && obj instanceof String) {
+            return RequestBody.create((String) obj, MediaType.parse(contentType));
         } else if (isJsonMime(contentType)) {
             String content;
             if (obj != null) {

samples/client/petstore/java/okhttp-gson-parcelableModel/src/main/java/org/openapitools/client/ApiClient.java

@@ -973,6 +973,8 @@ public class ApiClient {
         } else if (obj instanceof File) {
             // File body parameter support.
             return RequestBody.create((File) obj, MediaType.parse(contentType));
+        } else if ("text/plain".equals(contentType) && obj instanceof String) {
+            return RequestBody.create((String) obj, MediaType.parse(contentType));
         } else if (isJsonMime(contentType)) {
             String content;
             if (obj != null) {

samples/client/petstore/java/okhttp-gson/src/main/java/org/openapitools/client/ApiClient.java

@@ -973,6 +973,8 @@ public class ApiClient {
         } else if (obj instanceof File) {
             // File body parameter support.
             return RequestBody.create((File) obj, MediaType.parse(contentType));
+        } else if ("text/plain".equals(contentType) && obj instanceof String) {
+            return RequestBody.create((String) obj, MediaType.parse(contentType));
         } else if (isJsonMime(contentType)) {
             String content;
             if (obj != null) {

samples/client/petstore/java/okhttp-gson/src/test/java/org/openapitools/client/ApiClientTest.java

@@ -342,4 +342,22 @@ public class ApiClientTest {
     public void testNullHttpClient() {
         apiClient.setHttpClient(null);
     }
+
+    /**
+     * Tests the ApiClient serialize methods
+     */
+    @Test
+    public void testSerializeRequest() throws ApiException {
+        assertNotNull(apiClient.serialize("test", "text/plain"));
+        assertNotNull(apiClient.serialize("{}", "application/json"));
+    }
+
+    /**
+     * Tests the ApiClient serialize methods with unsupported content-type
+     * should raise ApiException
+     */
+    @Test(expected = ApiException.class)
+    public void testUnsupportedSerializeRequest() throws ApiException {
+        apiClient.serialize("test", "unsupported/type");
+    }
 }