code: public static MessageHandler signallingMessageHandler(final ProcessEngine processEngine) {
          return new MessageHandler() {
              @Override
              public void handleMessage(Message<?> message) throws MessagingException {
                  String executionId = message.getHeaders().containsKey("executionId") ?
                          (String) message.getHeaders().get("executionId") : (String) null;
  
                  if (null != executionId)
                      processEngine.getRuntimeService().trigger(executionId);
              }
          };
      }
docs: Any message that enters this {@link org.springframework.messaging.MessageHandler}
  containing a {@code executionId} parameter will trigger a
  {@link org.activiti.engine.RuntimeService#signalEventReceived(String)}.