Better support for $ref to schema's properties (#18233)

* use once

* add support for reference to schema properties

modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

@@ -2464,6 +2464,14 @@ public class DefaultCodegen implements CodegenConfig {
     protected String getSingleSchemaType(Schema schema) {
         Schema unaliasSchema = unaliasSchema(schema);
 
+        if (ModelUtils.isRefToSchemaWithProperties(unaliasSchema.get$ref())) {
+            // ref to schema's properties, e.g. #/components/schemas/Pet/properties/category
+            Schema refSchema = ModelUtils.getReferencedSchema(openAPI, unaliasSchema);
+            if (refSchema != null) {
+                return getSingleSchemaType(refSchema);
+            }
+        }
+
         if (StringUtils.isNotBlank(unaliasSchema.get$ref())) { // reference to another definition/schema
             // get the schema/model name from $ref
             String schemaName = ModelUtils.getSimpleRef(unaliasSchema.get$ref());
@@ -3354,7 +3362,7 @@ public class DefaultCodegen implements CodegenConfig {
                     String modelName = ModelUtils.getSimpleRef(((Schema) oneOf).get$ref());
                     CodegenProperty thisCp = discriminatorFound(composedSchemaName, (Schema) oneOf, discPropName, visitedSchemas);
                     if (thisCp == null) {
-                        LOGGER.warn(
+                        once(LOGGER).warn(
                                 "'{}' defines discriminator '{}', but the referenced OneOf schema '{}' is missing {}",
                                 composedSchemaName, discPropName, modelName, discPropName);
                     }
@@ -3363,7 +3371,7 @@ public class DefaultCodegen implements CodegenConfig {
                         continue;
                     }
                     if (cp != thisCp) {
-                        LOGGER.warn(
+                        once(LOGGER).warn(
                                 "'{}' defines discriminator '{}', but the OneOf schema '{}' has a different {} definition than the prior OneOf schema's. Make sure the {} type and required values are the same",
                                 composedSchemaName, discPropName, modelName, discPropName, discPropName);
                     }
@@ -3377,7 +3385,7 @@ public class DefaultCodegen implements CodegenConfig {
                     String modelName = ModelUtils.getSimpleRef(((Schema) anyOf).get$ref());
                     CodegenProperty thisCp = discriminatorFound(composedSchemaName, (Schema) anyOf, discPropName, visitedSchemas);
                     if (thisCp == null) {
-                        LOGGER.warn(
+                        once(LOGGER).warn(
                                 "'{}' defines discriminator '{}', but the referenced AnyOf schema '{}' is missing {}",
                                 composedSchemaName, discPropName, modelName, discPropName);
                     }
@@ -3386,7 +3394,7 @@ public class DefaultCodegen implements CodegenConfig {
                         continue;
                     }
                     if (cp != thisCp) {
-                        LOGGER.warn(
+                        once(LOGGER).warn(
                                 "'{}' defines discriminator '{}', but the AnyOf schema '{}' has a different {} definition than the prior AnyOf schema's. Make sure the {} type and required values are the same",
                                 composedSchemaName, discPropName, modelName, discPropName, discPropName);
                     }
@@ -3454,7 +3462,7 @@ public class DefaultCodegen implements CodegenConfig {
                     }
                 }
                 if (discriminatorsPropNames.size() > 1) {
-                    LOGGER.warn("The oneOf schemas have conflicting discriminator property names. " +
+                    once(LOGGER).warn("The oneOf schemas have conflicting discriminator property names. " +
                             "oneOf schemas must have the same property name, but found " + String.join(", ", discriminatorsPropNames));
                 }
                 if (foundDisc != null && (hasDiscriminatorCnt + hasNullTypeCnt) == composedSchema.getOneOf().size() && discriminatorsPropNames.size() == 1) {
@@ -3483,7 +3491,7 @@ public class DefaultCodegen implements CodegenConfig {
                     }
                 }
                 if (discriminatorsPropNames.size() > 1) {
-                    LOGGER.warn("The anyOf schemas have conflicting discriminator property names. " +
+                    once(LOGGER).warn("The anyOf schemas have conflicting discriminator property names. " +
                             "anyOf schemas must have the same property name, but found " + String.join(", ", discriminatorsPropNames));
                 }
                 if (foundDisc != null && (hasDiscriminatorCnt + hasNullTypeCnt) == composedSchema.getAnyOf().size() && discriminatorsPropNames.size() == 1) {
@@ -3532,7 +3540,7 @@ public class DefaultCodegen implements CodegenConfig {
                     // schemas also has inline composed schemas
                     // Note: if it is only inline one level, then the inline model resolver will move it into its own
                     // schema and make it a $ref schema in the oneOf/anyOf location
-                    LOGGER.warn(
+                    once(LOGGER).warn(
                             "Invalid inline schema defined in oneOf/anyOf in '{}'. Per the OpenApi spec, for this case when a composed schema defines a discriminator, the oneOf/anyOf schemas must use $ref. Change this inline definition to a $ref definition",
                             composedSchemaName);
                 }
@@ -3554,14 +3562,14 @@ public class DefaultCodegen implements CodegenConfig {
                             msgSuffix += spacer + "invalid optional definition of " + discPropName + ", include it in required";
                         }
                     }
-                    LOGGER.warn("'{}' defines discriminator '{}', but the referenced schema '{}' is incorrect. {}",
+                    once(LOGGER).warn("'{}' defines discriminator '{}', but the referenced schema '{}' is incorrect. {}",
                             composedSchemaName, discPropName, modelName, msgSuffix);
                 }
                 MappedModel mm = new MappedModel(modelName, toModelName(modelName));
                 descendentSchemas.add(mm);
                 Schema cs = ModelUtils.getSchema(openAPI, modelName);
                 if (cs == null) { // cannot lookup the model based on the name
-                    LOGGER.error("Failed to lookup the schema '{}' when processing oneOf/anyOf. Please check to ensure it's defined properly.", modelName);
+                    once(LOGGER).error("Failed to lookup the schema '{}' when processing oneOf/anyOf. Please check to ensure it's defined properly.", modelName);
                 } else {
                     Map<String, Object> vendorExtensions = cs.getExtensions();
                     if (vendorExtensions != null && !vendorExtensions.isEmpty() && vendorExtensions.containsKey("x-discriminator-value")) {
@@ -3674,7 +3682,7 @@ public class DefaultCodegen implements CodegenConfig {
                 if (e.getValue().indexOf('/') >= 0) {
                     name = ModelUtils.getSimpleRef(e.getValue());
                     if (ModelUtils.getSchema(openAPI, name) == null) {
-                        LOGGER.error("Failed to lookup the schema '{}' when processing the discriminator mapping of oneOf/anyOf. Please check to ensure it's defined properly.", name);
+                        once(LOGGER).error("Failed to lookup the schema '{}' when processing the discriminator mapping of oneOf/anyOf. Please check to ensure it's defined properly.", name);
                     }
                 } else {
                     name = e.getValue();
@@ -3983,6 +3991,13 @@ public class DefaultCodegen implements CodegenConfig {
             return cpc;
         }
 
+        // if it's ref to schema's properties, get the actual schema defined in the properties
+        Schema refToPropertiesSchema = ModelUtils.getSchemaFromRefToSchemaWithProperties(openAPI, p.get$ref());
+        if (refToPropertiesSchema != null) {
+            p = refToPropertiesSchema;
+            return fromProperty(name, refToPropertiesSchema, required, schemaIsFromAdditionalProperties);
+        }
+
         Schema original = null;
         // check if it's allOf (only 1 sub schema) with or without default/nullable/etc set in the top level
         if (ModelUtils.isAllOf(p) && p.getAllOf().size() == 1) {

modules/openapi-generator/src/main/java/org/openapitools/codegen/utils/ModelUtils.java

@@ -929,16 +929,76 @@ public class ModelUtils {
      * @return schema without '$ref'
      */
     public static Schema getReferencedSchema(OpenAPI openAPI, Schema schema) {
-        if (schema != null && StringUtils.isNotEmpty(schema.get$ref())) {
-            String name = getSimpleRef(schema.get$ref());
-            Schema referencedSchema = getSchema(openAPI, name);
-            if (referencedSchema != null) {
-                return referencedSchema;
-            }
+        if (schema == null) {
+            return null;
         }
+
+        if (StringUtils.isEmpty(schema.get$ref())) {
+            return schema;
+        }
+
+        try {
+            Schema refSchema = getSchemaFromRefToSchemaWithProperties(openAPI, schema.get$ref());
+            if (refSchema != null) {
+                // it's ref to schema's properties, #/components/schemas/Pet/properties/category for example
+                return refSchema;
+            }
+        } catch (Exception e) {
+            LOGGER.warn("Failed to parse $ref {}. Please report the issue to openapi-generator GitHub repo.", schema.get$ref());
+        }
+
+        // a simple ref, e.g. #/components/schemas/Pet
+        String name = getSimpleRef(schema.get$ref());
+        Schema referencedSchema = getSchema(openAPI, name);
+        if (referencedSchema != null) {
+            return referencedSchema;
+        }
+
         return schema;
     }
 
+    /**
+     * Get the schema referenced by $ref to schema's properties, e.g. #/components/schemas/Pet/properties/category.
+     *
+     * @param openAPI    specification being checked
+     * @param refString  schema reference
+     * @return schema
+     */
+    public static Schema getSchemaFromRefToSchemaWithProperties(OpenAPI openAPI, String refString) {
+        if (refString == null) {
+            return null;
+        }
+
+        String[] parts = refString.split("/");
+        // #/components/schemas/Pet/properties/category
+        if (parts.length == 6 && "properties".equals(parts[4])) {
+            Schema referencedSchema = getSchema(openAPI, parts[3]); // parts[3] is Pet
+            return (Schema) referencedSchema.getProperties().get(parts[5]); // parts[5] is category
+        } else {
+            return null;
+        }
+    }
+
+    /**
+     * Returns true if $ref to a reference to schema's properties, e.g. #/components/schemas/Pet/properties/category.
+     *
+     * @param refString  schema reference
+     * @return true if $ref to a reference to schema's properties
+     */
+    public static boolean isRefToSchemaWithProperties(String refString) {
+        if (refString == null) {
+            return false;
+        }
+
+        String[] parts = refString.split("/");
+        // #/components/schemas/Pet/properties/category
+        if (parts.length == 6 && "properties".equals(parts[4])) {
+            return true;
+        } else {
+            return false;
+        }
+    }
+
     public static Schema getSchema(OpenAPI openAPI, String name) {
         if (name == null) {
             return null;
@@ -1272,7 +1332,9 @@ public class ModelUtils {
             }
             Schema ref = allSchemas.get(simpleRef);
             if (ref == null) {
-                once(LOGGER).warn("{} is not defined", schema.get$ref());
+                if (!isRefToSchemaWithProperties(schema.get$ref())) {
+                    once(LOGGER).warn("{} is not defined", schema.get$ref());
+                }
                 return schema;
             } else if (ref.getEnum() != null && !ref.getEnum().isEmpty()) {
                 // top-level enum class

modules/openapi-generator/src/test/java/org/openapitools/codegen/utils/ModelUtilsTest.java

@@ -295,6 +295,25 @@ public class ModelUtilsTest {
         Assert.assertEquals(decoded, "~1 Hallo/Welt");
     }
 
+    @Test
+    public void testRefToSchemaProperties() {
+        final OpenAPI openAPI = TestUtils.parseFlattenSpec("src/test/resources/3_0/petstore.yaml");
+
+        Schema category = ModelUtils.getSchemaFromRefToSchemaWithProperties(openAPI, "#/components/schemas/Pet/properties/category");
+        Assert.assertEquals(category.get$ref(), "#/components/schemas/Category");
+
+        Schema name = ModelUtils.getSchemaFromRefToSchemaWithProperties(openAPI, "#/components/schemas/Pet/properties/name");
+        Assert.assertEquals(name.getType(), "string");
+
+        Schema id = ModelUtils.getSchemaFromRefToSchemaWithProperties(openAPI, "#/components/schemas/Pet/properties/id");
+        Assert.assertEquals(id.getType(), "integer");
+        Assert.assertEquals(id.getFormat(), "int64");
+
+        Assert.assertEquals(null, ModelUtils.getSchemaFromRefToSchemaWithProperties(openAPI, "#/components/schemas/Pet/prop/category"));
+        Assert.assertEquals(null, ModelUtils.getSchemaFromRefToSchemaWithProperties(openAPI, "#/components/schemas/Pet/properties/categoryyyy"));
+        Assert.assertEquals(null, ModelUtils.getSchemaFromRefToSchemaWithProperties(openAPI, "#/components/schemas/Pet"));
+    }
+
 
     // 3.0 spec tests
     @Test

modules/openapi-generator/src/test/resources/3_0/java/petstore-with-fake-endpoints-models-for-testing-okhttp-gson.yaml

@@ -2655,3 +2655,23 @@ components:
     PetComposition:
       allOf:
         - $ref: '#/components/schemas/Pet'
+    PetRef:
+      type: object
+      required:
+        - name
+        - photoUrls
+      properties:
+        id:
+          $ref: '#/components/schemas/Pet/properties/id'
+        category:
+          $ref: '#/components/schemas/Pet/properties/category'
+        name:
+          $ref: '#/components/schemas/Pet/properties/name'
+        photoUrls:
+          $ref: '#/components/schemas/Pet/properties/photoUrls'
+        tags:
+          $ref: '#/components/schemas/Pet/properties/tags'
+        status:
+          $ref: '#/components/schemas/Pet/properties/status'
+      xml:
+        name: Pet

samples/client/petstore/java/okhttp-gson/.openapi-generator/FILES

@@ -79,6 +79,7 @@ docs/ParentPet.md
 docs/Pet.md
 docs/PetApi.md
 docs/PetComposition.md
+docs/PetRef.md
 docs/PetUsingAllOf.md
 docs/PetWithRequiredTags.md
 docs/Pig.md
@@ -213,6 +214,7 @@ src/main/java/org/openapitools/client/model/OuterEnumIntegerDefaultValue.java
 src/main/java/org/openapitools/client/model/ParentPet.java
 src/main/java/org/openapitools/client/model/Pet.java
 src/main/java/org/openapitools/client/model/PetComposition.java
+src/main/java/org/openapitools/client/model/PetRef.java
 src/main/java/org/openapitools/client/model/PetUsingAllOf.java
 src/main/java/org/openapitools/client/model/PetWithRequiredTags.java
 src/main/java/org/openapitools/client/model/Pig.java

samples/client/petstore/java/okhttp-gson/README.md

@@ -235,6 +235,7 @@ Class | Method | HTTP request | Description
  - [ParentPet](docs/ParentPet.md)
  - [Pet](docs/Pet.md)
  - [PetComposition](docs/PetComposition.md)
+ - [PetRef](docs/PetRef.md)
  - [PetUsingAllOf](docs/PetUsingAllOf.md)
  - [PetWithRequiredTags](docs/PetWithRequiredTags.md)
  - [Pig](docs/Pig.md)

samples/client/petstore/java/okhttp-gson/api/openapi.yaml

@@ -2691,6 +2691,26 @@ components:
     PetComposition:
       allOf:
       - $ref: '#/components/schemas/Pet'
+    PetRef:
+      properties:
+        id:
+          $ref: '#/components/schemas/Pet/properties/id'
+        category:
+          $ref: '#/components/schemas/Pet/properties/category'
+        name:
+          $ref: '#/components/schemas/Pet/properties/name'
+        photoUrls:
+          $ref: '#/components/schemas/Pet/properties/photoUrls'
+        tags:
+          $ref: '#/components/schemas/Pet/properties/tags'
+        status:
+          $ref: '#/components/schemas/Pet/properties/status'
+      required:
+      - name
+      - photoUrls
+      type: object
+      xml:
+        name: Pet
     _foo_get_default_response:
       example:
         string:

samples/client/petstore/java/okhttp-gson/docs/PetRef.md

@@ -0,0 +1,28 @@
+
+
+# PetRef
+
+
+## Properties
+
+| Name | Type | Description | Notes |
+|------------ | ------------- | ------------- | -------------|
+|**id** | **Long** |  |  [optional] |
+|**category** | [**Category**](Category.md) |  |  [optional] |
+|**name** | **String** |  |  |
+|**photoUrls** | **List<String>** |  |  |
+|**tags** | [**List<Tag>**](Tag.md) |  |  [optional] |
+|**status** | [**StatusEnum**](#StatusEnum) | pet status in the store |  [optional] |
+
+
+
+## Enum: StatusEnum
+
+| Name | Value |
+|---- | -----|
+| AVAILABLE | "available" |
+| PENDING | "pending" |
+| SOLD | "sold" |
+
+
+

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

@@ -298,6 +298,7 @@ public class JSON {
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.ParentPet.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Pet.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.PetComposition.CustomTypeAdapterFactory());
+        gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.PetRef.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.PetUsingAllOf.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.PetWithRequiredTags.CustomTypeAdapterFactory());
         gsonBuilder.registerTypeAdapterFactory(new org.openapitools.client.model.Pig.CustomTypeAdapterFactory());

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

@@ -0,0 +1,531 @@
+/*
+ * OpenAPI Petstore
+ * This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import java.util.Objects;
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+import org.openapitools.client.model.Category;
+import org.openapitools.client.model.Tag;
+
+import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
+import com.google.gson.JsonArray;
+import com.google.gson.JsonDeserializationContext;
+import com.google.gson.JsonDeserializer;
+import com.google.gson.JsonElement;
+import com.google.gson.JsonObject;
+import com.google.gson.JsonParseException;
+import com.google.gson.TypeAdapterFactory;
+import com.google.gson.reflect.TypeToken;
+import com.google.gson.TypeAdapter;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+
+import java.lang.reflect.Type;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Set;
+
+import org.openapitools.client.JSON;
+
+/**
+ * PetRef
+ */
+@javax.annotation.Generated(value = "org.openapitools.codegen.languages.JavaClientCodegen", comments = "Generator version: 7.5.0-SNAPSHOT")
+public class PetRef {
+  public static final String SERIALIZED_NAME_ID = "id";
+  @SerializedName(SERIALIZED_NAME_ID)
+  private Long id;
+
+  public static final String SERIALIZED_NAME_CATEGORY = "category";
+  @SerializedName(SERIALIZED_NAME_CATEGORY)
+  private Category category;
+
+  public static final String SERIALIZED_NAME_NAME = "name";
+  @SerializedName(SERIALIZED_NAME_NAME)
+  private String name;
+
+  public static final String SERIALIZED_NAME_PHOTO_URLS = "photoUrls";
+  @SerializedName(SERIALIZED_NAME_PHOTO_URLS)
+  private List<String> photoUrls = new ArrayList<>();
+
+  public static final String SERIALIZED_NAME_TAGS = "tags";
+  @SerializedName(SERIALIZED_NAME_TAGS)
+  private List<Tag> tags = new ArrayList<>();
+
+  /**
+   * pet status in the store
+   */
+  @JsonAdapter(StatusEnum.Adapter.class)
+  public enum StatusEnum {
+    AVAILABLE("available"),
+    
+    PENDING("pending"),
+    
+    SOLD("sold");
+
+    private String value;
+
+    StatusEnum(String value) {
+      this.value = value;
+    }
+
+    public String getValue() {
+      return value;
+    }
+
+    @Override
+    public String toString() {
+      return String.valueOf(value);
+    }
+
+    public static StatusEnum fromValue(String value) {
+      for (StatusEnum b : StatusEnum.values()) {
+        if (b.value.equals(value)) {
+          return b;
+        }
+      }
+      throw new IllegalArgumentException("Unexpected value '" + value + "'");
+    }
+
+    public static class Adapter extends TypeAdapter<StatusEnum> {
+      @Override
+      public void write(final JsonWriter jsonWriter, final StatusEnum enumeration) throws IOException {
+        jsonWriter.value(enumeration.getValue());
+      }
+
+      @Override
+      public StatusEnum read(final JsonReader jsonReader) throws IOException {
+        String value =  jsonReader.nextString();
+        return StatusEnum.fromValue(value);
+      }
+    }
+
+    public static void validateJsonElement(JsonElement jsonElement) throws IOException {
+      String value = jsonElement.getAsString();
+      StatusEnum.fromValue(value);
+    }
+  }
+
+  public static final String SERIALIZED_NAME_STATUS = "status";
+  @SerializedName(SERIALIZED_NAME_STATUS)
+  private StatusEnum status;
+
+  public PetRef() {
+  }
+
+  public PetRef id(Long id) {
+    this.id = id;
+    return this;
+  }
+
+   /**
+   * Get id
+   * @return id
+  **/
+  @javax.annotation.Nullable
+  public Long getId() {
+    return id;
+  }
+
+  public void setId(Long id) {
+    this.id = id;
+  }
+
+
+  public PetRef category(Category category) {
+    this.category = category;
+    return this;
+  }
+
+   /**
+   * Get category
+   * @return category
+  **/
+  @javax.annotation.Nullable
+  public Category getCategory() {
+    return category;
+  }
+
+  public void setCategory(Category category) {
+    this.category = category;
+  }
+
+
+  public PetRef name(String name) {
+    this.name = name;
+    return this;
+  }
+
+   /**
+   * Get name
+   * @return name
+  **/
+  @javax.annotation.Nonnull
+  public String getName() {
+    return name;
+  }
+
+  public void setName(String name) {
+    this.name = name;
+  }
+
+
+  public PetRef photoUrls(List<String> photoUrls) {
+    this.photoUrls = photoUrls;
+    return this;
+  }
+
+  public PetRef addPhotoUrlsItem(String photoUrlsItem) {
+    if (this.photoUrls == null) {
+      this.photoUrls = new ArrayList<>();
+    }
+    this.photoUrls.add(photoUrlsItem);
+    return this;
+  }
+
+   /**
+   * Get photoUrls
+   * @return photoUrls
+  **/
+  @javax.annotation.Nonnull
+  public List<String> getPhotoUrls() {
+    return photoUrls;
+  }
+
+  public void setPhotoUrls(List<String> photoUrls) {
+    this.photoUrls = photoUrls;
+  }
+
+
+  public PetRef tags(List<Tag> tags) {
+    this.tags = tags;
+    return this;
+  }
+
+  public PetRef addTagsItem(Tag tagsItem) {
+    if (this.tags == null) {
+      this.tags = new ArrayList<>();
+    }
+    this.tags.add(tagsItem);
+    return this;
+  }
+
+   /**
+   * Get tags
+   * @return tags
+  **/
+  @javax.annotation.Nullable
+  public List<Tag> getTags() {
+    return tags;
+  }
+
+  public void setTags(List<Tag> tags) {
+    this.tags = tags;
+  }
+
+
+  public PetRef status(StatusEnum status) {
+    this.status = status;
+    return this;
+  }
+
+   /**
+   * pet status in the store
+   * @return status
+  **/
+  @javax.annotation.Nullable
+  public StatusEnum getStatus() {
+    return status;
+  }
+
+  public void setStatus(StatusEnum status) {
+    this.status = status;
+  }
+
+  /**
+   * A container for additional, undeclared properties.
+   * This is a holder for any undeclared properties as specified with
+   * the 'additionalProperties' keyword in the OAS document.
+   */
+  private Map<String, Object> additionalProperties;
+
+  /**
+   * Set the additional (undeclared) property with the specified name and value.
+   * If the property does not already exist, create it otherwise replace it.
+   *
+   * @param key name of the property
+   * @param value value of the property
+   * @return the PetRef instance itself
+   */
+  public PetRef putAdditionalProperty(String key, Object value) {
+    if (this.additionalProperties == null) {
+        this.additionalProperties = new HashMap<String, Object>();
+    }
+    this.additionalProperties.put(key, value);
+    return this;
+  }
+
+  /**
+   * Return the additional (undeclared) property.
+   *
+   * @return a map of objects
+   */
+  public Map<String, Object> getAdditionalProperties() {
+    return additionalProperties;
+  }
+
+  /**
+   * Return the additional (undeclared) property with the specified name.
+   *
+   * @param key name of the property
+   * @return an object
+   */
+  public Object getAdditionalProperty(String key) {
+    if (this.additionalProperties == null) {
+        return null;
+    }
+    return this.additionalProperties.get(key);
+  }
+
+
+  @Override
+  public boolean equals(Object o) {
+    if (this == o) {
+      return true;
+    }
+    if (o == null || getClass() != o.getClass()) {
+      return false;
+    }
+    PetRef petRef = (PetRef) o;
+    return Objects.equals(this.id, petRef.id) &&
+        Objects.equals(this.category, petRef.category) &&
+        Objects.equals(this.name, petRef.name) &&
+        Objects.equals(this.photoUrls, petRef.photoUrls) &&
+        Objects.equals(this.tags, petRef.tags) &&
+        Objects.equals(this.status, petRef.status)&&
+        Objects.equals(this.additionalProperties, petRef.additionalProperties);
+  }
+
+  @Override
+  public int hashCode() {
+    return Objects.hash(id, category, name, photoUrls, tags, status, additionalProperties);
+  }
+
+  @Override
+  public String toString() {
+    StringBuilder sb = new StringBuilder();
+    sb.append("class PetRef {\n");
+    sb.append("    id: ").append(toIndentedString(id)).append("\n");
+    sb.append("    category: ").append(toIndentedString(category)).append("\n");
+    sb.append("    name: ").append(toIndentedString(name)).append("\n");
+    sb.append("    photoUrls: ").append(toIndentedString(photoUrls)).append("\n");
+    sb.append("    tags: ").append(toIndentedString(tags)).append("\n");
+    sb.append("    status: ").append(toIndentedString(status)).append("\n");
+    sb.append("    additionalProperties: ").append(toIndentedString(additionalProperties)).append("\n");
+    sb.append("}");
+    return sb.toString();
+  }
+
+  /**
+   * Convert the given object to string with each line indented by 4 spaces
+   * (except the first line).
+   */
+  private String toIndentedString(Object o) {
+    if (o == null) {
+      return "null";
+    }
+    return o.toString().replace("\n", "\n    ");
+  }
+
+
+  public static HashSet<String> openapiFields;
+  public static HashSet<String> openapiRequiredFields;
+
+  static {
+    // a set of all properties/fields (JSON key names)
+    openapiFields = new HashSet<String>();
+    openapiFields.add("id");
+    openapiFields.add("category");
+    openapiFields.add("name");
+    openapiFields.add("photoUrls");
+    openapiFields.add("tags");
+    openapiFields.add("status");
+
+    // a set of required properties/fields (JSON key names)
+    openapiRequiredFields = new HashSet<String>();
+    openapiRequiredFields.add("name");
+    openapiRequiredFields.add("photoUrls");
+  }
+
+ /**
+  * Validates the JSON Element and throws an exception if issues found
+  *
+  * @param jsonElement JSON Element
+  * @throws IOException if the JSON Element is invalid with respect to PetRef
+  */
+  public static void validateJsonElement(JsonElement jsonElement) throws IOException {
+      if (jsonElement == null) {
+        if (!PetRef.openapiRequiredFields.isEmpty()) { // has required fields but JSON element is null
+          throw new IllegalArgumentException(String.format("The required field(s) %s in PetRef is not found in the empty JSON string", PetRef.openapiRequiredFields.toString()));
+        }
+      }
+
+      // check to make sure all required properties/fields are present in the JSON string
+      for (String requiredField : PetRef.openapiRequiredFields) {
+        if (jsonElement.getAsJsonObject().get(requiredField) == null) {
+          throw new IllegalArgumentException(String.format("The required field `%s` is not found in the JSON string: %s", requiredField, jsonElement.toString()));
+        }
+      }
+        JsonObject jsonObj = jsonElement.getAsJsonObject();
+      // validate the optional field `category`
+      if (jsonObj.get("category") != null && !jsonObj.get("category").isJsonNull()) {
+        Category.validateJsonElement(jsonObj.get("category"));
+      }
+      if (!jsonObj.get("name").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `name` to be a primitive type in the JSON string but got `%s`", jsonObj.get("name").toString()));
+      }
+      // ensure the required json array is present
+      if (jsonObj.get("photoUrls") == null) {
+        throw new IllegalArgumentException("Expected the field `linkedContent` to be an array in the JSON string but got `null`");
+      } else if (!jsonObj.get("photoUrls").isJsonArray()) {
+        throw new IllegalArgumentException(String.format("Expected the field `photoUrls` to be an array in the JSON string but got `%s`", jsonObj.get("photoUrls").toString()));
+      }
+      if (jsonObj.get("tags") != null && !jsonObj.get("tags").isJsonNull()) {
+        JsonArray jsonArraytags = jsonObj.getAsJsonArray("tags");
+        if (jsonArraytags != null) {
+          // ensure the json data is an array
+          if (!jsonObj.get("tags").isJsonArray()) {
+            throw new IllegalArgumentException(String.format("Expected the field `tags` to be an array in the JSON string but got `%s`", jsonObj.get("tags").toString()));
+          }
+
+          // validate the optional field `tags` (array)
+          for (int i = 0; i < jsonArraytags.size(); i++) {
+            Tag.validateJsonElement(jsonArraytags.get(i));
+          };
+        }
+      }
+      if ((jsonObj.get("status") != null && !jsonObj.get("status").isJsonNull()) && !jsonObj.get("status").isJsonPrimitive()) {
+        throw new IllegalArgumentException(String.format("Expected the field `status` to be a primitive type in the JSON string but got `%s`", jsonObj.get("status").toString()));
+      }
+      // validate the optional field `status`
+      if (jsonObj.get("status") != null && !jsonObj.get("status").isJsonNull()) {
+        StatusEnum.validateJsonElement(jsonObj.get("status"));
+      }
+  }
+
+  public static class CustomTypeAdapterFactory implements TypeAdapterFactory {
+    @SuppressWarnings("unchecked")
+    @Override
+    public <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type) {
+       if (!PetRef.class.isAssignableFrom(type.getRawType())) {
+         return null; // this class only serializes 'PetRef' and its subtypes
+       }
+       final TypeAdapter<JsonElement> elementAdapter = gson.getAdapter(JsonElement.class);
+       final TypeAdapter<PetRef> thisAdapter
+                        = gson.getDelegateAdapter(this, TypeToken.get(PetRef.class));
+
+       return (TypeAdapter<T>) new TypeAdapter<PetRef>() {
+           @Override
+           public void write(JsonWriter out, PetRef value) throws IOException {
+             JsonObject obj = thisAdapter.toJsonTree(value).getAsJsonObject();
+             obj.remove("additionalProperties");
+             // serialize additional properties
+             if (value.getAdditionalProperties() != null) {
+               for (Map.Entry<String, Object> entry : value.getAdditionalProperties().entrySet()) {
+                 if (entry.getValue() instanceof String)
+                   obj.addProperty(entry.getKey(), (String) entry.getValue());
+                 else if (entry.getValue() instanceof Number)
+                   obj.addProperty(entry.getKey(), (Number) entry.getValue());
+                 else if (entry.getValue() instanceof Boolean)
+                   obj.addProperty(entry.getKey(), (Boolean) entry.getValue());
+                 else if (entry.getValue() instanceof Character)
+                   obj.addProperty(entry.getKey(), (Character) entry.getValue());
+                 else {
+                   JsonElement jsonElement = gson.toJsonTree(entry.getValue());
+                   if (jsonElement.isJsonArray()) {
+                     obj.add(entry.getKey(), jsonElement.getAsJsonArray());
+                   } else {
+                     obj.add(entry.getKey(), jsonElement.getAsJsonObject());
+                   }
+                 }
+               }
+             }
+             elementAdapter.write(out, obj);
+           }
+
+           @Override
+           public PetRef read(JsonReader in) throws IOException {
+             JsonElement jsonElement = elementAdapter.read(in);
+             validateJsonElement(jsonElement);
+             JsonObject jsonObj = jsonElement.getAsJsonObject();
+             // store additional fields in the deserialized instance
+             PetRef instance = thisAdapter.fromJsonTree(jsonObj);
+             for (Map.Entry<String, JsonElement> entry : jsonObj.entrySet()) {
+               if (!openapiFields.contains(entry.getKey())) {
+                 if (entry.getValue().isJsonPrimitive()) { // primitive type
+                   if (entry.getValue().getAsJsonPrimitive().isString())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsString());
+                   else if (entry.getValue().getAsJsonPrimitive().isNumber())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsNumber());
+                   else if (entry.getValue().getAsJsonPrimitive().isBoolean())
+                     instance.putAdditionalProperty(entry.getKey(), entry.getValue().getAsBoolean());
+                   else
+                     throw new IllegalArgumentException(String.format("The field `%s` has unknown primitive type. Value: %s", entry.getKey(), entry.getValue().toString()));
+                 } else if (entry.getValue().isJsonArray()) {
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), List.class));
+                 } else { // JSON object
+                     instance.putAdditionalProperty(entry.getKey(), gson.fromJson(entry.getValue(), HashMap.class));
+                 }
+               }
+             }
+             return instance;
+           }
+
+       }.nullSafe();
+    }
+  }
+
+ /**
+  * Create an instance of PetRef given an JSON string
+  *
+  * @param jsonString JSON string
+  * @return An instance of PetRef
+  * @throws IOException if the JSON string is invalid with respect to PetRef
+  */
+  public static PetRef fromJson(String jsonString) throws IOException {
+    return JSON.getGson().fromJson(jsonString, PetRef.class);
+  }
+
+ /**
+  * Convert an instance of PetRef to an JSON string
+  *
+  * @return JSON string
+  */
+  public String toJson() {
+    return JSON.getGson().toJson(this);
+  }
+}
+

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

@@ -0,0 +1,48 @@
+/*
+ * OpenAPI Petstore
+ * This spec is mainly for testing Petstore server and contains fake endpoints, models. Please do not use this for any other purpose. Special characters: \" \\
+ *
+ * The version of the OpenAPI document: 1.0.0
+ * 
+ *
+ * NOTE: This class is auto generated by OpenAPI Generator (https://openapi-generator.tech).
+ * https://openapi-generator.tech
+ * Do not edit the class manually.
+ */
+
+
+package org.openapitools.client.model;
+
+import com.google.gson.TypeAdapter;
+import com.google.gson.annotations.JsonAdapter;
+import com.google.gson.annotations.SerializedName;
+import com.google.gson.stream.JsonReader;
+import com.google.gson.stream.JsonWriter;
+import java.io.IOException;
+import java.util.Arrays;
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Model tests for PetRef
+ */
+public class PetRefTest {
+    private final PetRef model = new PetRef();
+
+    /**
+     * Model tests for PetRef
+     */
+    @Test
+    public void testPetRef() {
+        // TODO: test PetRef
+    }
+
+    /**
+     * Test the property 'status'
+     */
+    @Test
+    public void statusTest() {
+        // TODO: test status
+    }
+
+}