public class RegisterBeansListener implements IPlatformListener {
  @Override
  public void stateChanged(PlatformEvent event) {
    if (event.getState() == IPlatform.State.BeanManagerPrepared) {
      register(ApiClient.class, ApiClientProducer.class, false);
    }
  }

  private void register(Class<?> clazz, Class<?> producerClazz, boolean isSingleton) {
    IBeanInstanceProducer<?> producer = (IBeanInstanceProducer<?>) BEANS.get(producerClazz);
    BeanMetaData beanData = new BeanMetaData(clazz)
      .withProducer(producer)
      .withApplicationScoped(isSingleton);
    BEANS.getBeanManager().registerBean(beanData);
  }
}

@ApplicationScoped
public class ApiClientProducer extends NonSingeltonBeanInstanceProducer<ApiClient> {

  private static final Logger log = LoggerFactory.getLogger(ApiClientProducer.class);

  private final ApiClientConfig apiClientConfig;

  @InjectBean
  public ApiClientProducer(ApiClientConfig apiClientConfig) {
    this.apiClientConfig = apiClientConfig;
  }

  @Override
  public ApiClient produce(IBean<ApiClient> bean) {
    ApiClient apiClient = super.produce(bean);
    apiClient.setBasePath(apiClientConfig.getBasePath());
    apiClient.setUsername(apiClientConfig.getUsername());
    apiClient.setPassword(apiClientConfig.getPassword());

    var currentUserId = Sessions.getCurrentUserId();
    apiClient.addDefaultHeader("x-frontend-username", currentUserId);

    var cid = CorrelationId.CURRENT.get();
    apiClient.addDefaultHeader("X-Request-ID", cid);

    log.info("created API client for user: {} -> {}", currentUserId, apiClient);

    return apiClient;
  }
}

@ApplicationScoped
public class PriceGroupRestControllerApiAdapter implements RestControllerApiAdapter<PriceGroup> {
  private final PriceGroupRestControllerApi api;

  @InjectBean
  public PriceGroupRestControllerApiAdapter(ApiClient apiClient) {
    this.api = new PriceGroupRestControllerApi(apiClient);
  }

  @Override
  public PriceGroup create(PriceGroup entity) throws ApiException {
    return api.createPriceGroup(entity);
  }
}

public class RegisterBeansListener implements IPlatformListener {
  @Override
  public void stateChanged(PlatformEvent event) {
    if (event.getState() == IPlatform.State.BeanManagerPrepared) {
	  // changed isSingleton => true
      register(ApiClient.class, ApiClientProducer.class, true);
    }
  }

  private void register(Class<?> clazz, Class<?> producerClazz, boolean isSingleton) {
    IBeanInstanceProducer<?> producer = (IBeanInstanceProducer<?>) BEANS.get(producerClazz);
    BeanMetaData beanData = new BeanMetaData(clazz)
      .withProducer(producer)
      .withApplicationScoped(isSingleton);
    BEANS.getBeanManager().registerBean(beanData);
  }
}

@ApplicationScoped
// implement necessary Interface instead of extending the provided ProducerClasses
public class ApiClientProducer implements IBeanInstanceProducer<ApiClient> {

  private static final Logger log = LoggerFactory.getLogger(ApiClientProducer.class);

  private final ApiClientConfig apiClientConfig;
  private final String userAgent;


  @InjectBean
  public ApiClientProducer(
    ApiClientConfig apiClientConfig,
    UserAgentFactory userAgentFactory
  ) {
    this.apiClientConfig = apiClientConfig;
    this.userAgent = userAgentFactory.create();
  }

  @Override
  public ApiClient produce(IBean<ApiClient> bean) {
    // created a new anonymous subclass of ApiClient with the additional methods
    var apiClient = new ApiClient() {
      // HashMap of the functions, which create the header values
      private final Map<String, Supplier<String>> lazyHeaderSuppliers = new HashMap<>();

      public void addLazyDefaultHeader(String header, Supplier<String> lazyHeaderValueSupplier) {
        lazyHeaderSuppliers.put(header, lazyHeaderValueSupplier);
      }

      @Override
      public <T> ApiResponse<T> invokeAPI(String operation,
                                          String path,
                                          String method,
                                          List<Pair> queryParams,
                                          Object body,
                                          Map<String, String> headerParams,
                                          Map<String, String> cookieParams,
                                          Map<String, Object> formParams,
                                          String accept,
                                          String contentType,
                                          String[] authNames,
                                          GenericType<T> returnType,
                                          boolean isBodyNullable) throws ApiException {

        // call the functions to create the header values and put it into the provided hashmap
        lazyHeaderSuppliers.keySet().forEach(header -> {
          var headerValueSupplier = lazyHeaderSuppliers.get(header);
          headerParams.put(header, headerValueSupplier.get());
        });
		// call super with the enriched HashMap for the header information
        return super.invokeAPI(operation, path, method, queryParams, body, headerParams, cookieParams,
          formParams, accept, contentType, authNames, returnType, isBodyNullable);
      }
    };
    apiClient.setBasePath(apiClientConfig.getBasePath());
    apiClient.setUsername(apiClientConfig.getUsername());
    apiClient.setPassword(apiClientConfig.getPassword());

	// use the method reference Sessions::getCurrentUserId as a Supplier<String> for the header
	// note that the method is executed in the context of the call to ApiClient.invokeApi and therefore returns the correct userId
    apiClient.addLazyDefaultHeader("x-frontend-username", Sessions::getCurrentUserId);
    apiClient.addLazyDefaultHeader("X-Request-ID", CorrelationId.CURRENT::get);

    apiClient.setUserAgent(userAgent);
    return apiClient;
  }
}

// unchanged
@ApplicationScoped
public class PriceGroupRestControllerApiAdapter implements RestControllerApiAdapter<PriceGroup> {
  private final PriceGroupRestControllerApi api;

  @InjectBean
  public PriceGroupRestControllerApiAdapter(ApiClient apiClient) {
    this.api = new PriceGroupRestControllerApi(apiClient);
  }

  @Override
  public PriceGroup create(PriceGroup entity) throws ApiException {
    return api.createPriceGroup(entity);
  }
}