[JAVA][Jersey2][Regression] Inconsistency between ApiClient deserialisation method for swagger2 and swagger3

Description

During update, we have encountered an issue regarding the generation of the ApiClient.class.

Seems like swagger3 does not have the following issue fixed over from swagger2:
https://github.com/swagger-api/swagger-codegen/issues/6226
and the following PR that fixes it:
https://github.com/swagger-api/swagger-codegen/pull/6977

Due to this, there is the issue of ApiClient failing to deserialise requests with responses without content-type headers.

Swagger3 method:

  /**
   * Deserialize response body to Java object according to the Content-Type.
   * @param <T> Type
   * @param response Response
   * @param returnType Return type
   * @return Deserialize object
   * @throws ApiException API exception
   */
  @SuppressWarnings("unchecked")
  public <T> T deserialize(Response response, GenericType<T> returnType) throws ApiException {
    if (response == null || returnType == null) {
      return null;
    }

    if ("byte[]".equals(returnType.toString())) {
      // Handle binary response (byte array).
      return (T) response.readEntity(byte[].class);
    } else if (returnType.getRawType() == File.class) {
      // Handle file downloading.
      T file = (T) downloadFileFromResponse(response);
      return file;
    }

    String contentType = null;
    List<Object> contentTypes = response.getHeaders().get("Content-Type");
    if (contentTypes != null && !contentTypes.isEmpty())
      contentType = String.valueOf(contentTypes.get(0));
    if (contentType == null)
      throw new ApiException(500, "missing Content-Type in response");

    return response.readEntity(returnType);
  }

Swagger2 method:

/**
   * Deserialize response body to Java object according to the Content-Type.
   * @param <T> Type
   * @param response Response
   * @param returnType Return type
   * @return Deserialize object
   * @throws ApiException API exception
   */
  @SuppressWarnings("unchecked")
  public <T> T deserialize(Response response, GenericType<T> returnType) throws ApiException {
    if (response == null || returnType == null) {
      return null;
    }

    if ("byte[]".equals(returnType.toString())) {
      // Handle binary response (byte array).
      return (T) response.readEntity(byte[].class);
    } else if (returnType.getRawType() == File.class) {
      // Handle file downloading.
      T file = (T) downloadFileFromResponse(response);
      return file;
    }

    String contentType = null;
    List<Object> contentTypes = response.getHeaders().get("Content-Type");
    if (contentTypes != null && !contentTypes.isEmpty())
      contentType = String.valueOf(contentTypes.get(0));

    return response.readEntity(returnType);
  }

Swagger-codegen version

swagger3

Swagger declaration file content or url

Command line used for generation

Steps to reproduce

Generate a sample code both on swagger2 and swagger3. The ApiClient.class will differ.

Related issues/PRs

https://github.com/swagger-api/swagger-codegen/issues/6226
https://github.com/swagger-api/swagger-codegen/pull/6977

Suggest a fix/enhancement

This should be applied to swagger3 generation as well.

Start by tracing how swagger3 generation produces the Java ApiClient.class and inspect its deserialize(Response, GenericType) method alongside the swagger2 version shown in the issue. Generate sample code for both versions and verify that a response without a Content-Type header can be deserialized without the swagger3-specific failure.