openapi-generator-maven-plugin (Java) does not handle allOf properly

With org.openapitools:openapi-generator-maven-plugin, I have noticed that using allOf composed of multiple objects in a response does not generate a class combining these multiple objects. Instead it uses the first class defined in the allOf section.

Here is a minimal example (openapi.yaml) :

openapi: 3.0.0
info:
  title: Test
  version: v1
paths:
  /test:
    get:
      operationId: get
      responses:
        '200':
          description: Get
          content:
            application/json:
              schema:
                allOf:
                  - $ref: '#/components/schemas/A'
                  - $ref: '#/components/schemas/B'
components:
  schemas:
    A:
      type: object
      properties:
        attA:
          type: string
    B:
      type: object
      properties:
        attB:
          type: integer

When generating the classes in Java via :

mvn org.openapitools:openapi-generator-maven-plugin:5.2.0:generate \
    -Dopenapi.generator.maven.plugin.inputSpec=openapi.yaml \
    -Dopenapi.generator.maven.plugin.generatorName=java

It shows a warning:

[WARNING] allOf with multiple schemas defined. Using only the first one: A

As expected, it generates classes A and B. But, when calling get(), the value returned by the call is of type A:

DefaultApi api = new DefaultApi();
A a = api.get();

Instead, I would have expected a composite object containing A and B properties (attA and attB), like this (result from https://editor.swagger.io/):

I have created an issue on GitHub, but hopefully someone here may have had the same issue and managed to fix it.

Also, I can't modify the openapi.yaml file because it's an OpenAPI spec provided by an API I have to call, so modifying it would make no sense and will make it so difficult to manage if their OpenAPI spec change over time.

Version 6.0.0 of openapi-generator-maven-plugin solves the issue by generating a class (Get200Response) composed of the two objects A and B. After generating the classes using:

mvn org.openapitools:openapi-generator-maven-plugin:6.0.0:generate \
    -Dopenapi.generator.maven.plugin.inputSpec=openapi.yaml \
    -Dopenapi.generator.maven.plugin.generatorName=java

I can see that new Get200Response class:

package org.openapitools.client.model;

// ...

public class Get200Response {
  public static final String SERIALIZED_NAME_ATT_A = "attA";
  @SerializedName(SERIALIZED_NAME_ATT_A)
  private String attA;

  public static final String SERIALIZED_NAME_ATT_B = "attB";
  @SerializedName(SERIALIZED_NAME_ATT_B)
  private Integer attB;

  // ...
}

And I was able to make the following code work. In that example, I have a dummy webserver listening on port 5000 and defining a /test endpoint returning {"attA": "hello", "attB": 1}.

package org.example;

import org.openapitools.client.ApiClient;
import org.openapitools.client.ApiException;
import org.openapitools.client.api.DefaultApi;
import org.openapitools.client.model.Get200Response;

public class Main {
  public static void main(String[] args) throws ApiException {
    ApiClient apiClient = new ApiClient();
    apiClient.setBasePath("http://localhost:5000");

    DefaultApi api = new DefaultApi(apiClient);
    Get200Response r = api.get();
    System.out.println(r.getAttA());
    System.out.println(r.getAttB());
  }
}

This successfully prints:

hello
1