Sending Message from External Task to Engine (via specific method)

//   public static final String SEND_MESSAGE =  "/message";

  public void sendMessage(ExternalTask externalTask, String message, VariableMap correlationKeys, VariableMap processVariables, Boolean all) throws EngineClientException {
    SendMessageRequestDto payload = new SendMessageRequestDto(externalTask, message, correlationKeys, processVariables, all);
    String resourceUrl = baseUrl + SEND_MESSAGE;
    engineInteraction.postRequest(resourceUrl, payload, Void.class, xsrf);
  }

  @Override
  public void sendMessage(ExternalTask externalTask, String message, VariableMap correlationKeys, VariableMap processVariables, Boolean all) {
    try {
      engineClient.sendMessage(externalTask, message, correlationKeys, processVariables, all);
    } catch (EngineClientException e) {
      throw handleException(e, "send message", true);
    }
  }

  /**
   * Send Message
   *
   * @param message     which message string that you need to send to engine
   * @param correlationKeys indicates the master keys for correlation.
   * @param processVariables provides related variables to the process.
   * @param all      specifies which params should be send (all or not).
   *
   * @throws NotFoundException if the task has been canceled and therefore does not exist anymore
   * @throws NotAcquiredException if the task's most recent lock could not be acquired
   * @throws ConnectionLostException if the connection could not be established
   * @throws ValueMapperException
   * <ul>
   *   <li> if an object cannot be serialized
   *   <li> if no 'objectTypeName' is provided for non-null value
   *   <li> if value is of type abstract
   *   <li> if no suitable serializer could be found
   * </ul>
   */
  void sendMessage(ExternalTask externalTask, String message, VariableMap correlationKeys, VariableMap processVariables, Boolean all);

   public static void main(String[] args) {

        // Personal Test
        ExternalTaskClient client = ExternalTaskClient.create()
                .baseUrl("http://10.1.2.51:8080/engine-rest")
                .asyncResponseTimeout(10000) // long polling timeout
                .build();

        client.subscribe("charge-card")
                .lockDuration(1000)
                .handler((externalTask, externalTaskService) -> {
                    VariableMap correlationKeys = new VariableMapImpl();
                    correlationKeys.putValueTyped("aVariable", Variables.stringValue("aValue"));

                   VariableMap processVariables = new VariableMapImpl();
                   processVariables.putValueTyped("aVariable", Variables.stringValue("aNewValue", true));
                   processVariables.putValueTyped("anotherVariable", Variables.booleanValue(true, true));

                   externalTaskService.sendMessage(externalTask, "aMessage", correlationKeys, processVariables, null);
                   externalTaskService.complete(externalTask);
                })
                .open();
    }