Asynchronous messaging necessitates the need for asynchronous error handling. Luckily, support for handling errors is built directly into the MessageBuilder API, utilizing the ErrorCallback interface. In the examples shown in previous exceptions, error handing has been glossed over with aubiquitous usage of the noErrorHandling() method while building messaging. We chose to require the explicit use of such a method to remind developers of the fact that they are responsible for their own error handling, requiring you to explicitly make the decision to forego handling potential errors.

As a general rule, you should always handle your errors. It will lead to faster and quicker identification of problems with your applications if you have error handlers, and generally help you build more robust code.

MessageBuilder.createMessage()
    .toSubject("HelloWorldService")
    .signalling()
    .with("msg", "Hi there!")
    .errorsHandledBy(new ErrorCallback() {
      public boolean error(Message message, Throwable throwable) {
        throwable.printStackTrace();
          return true;
      }
    })
    .sendNowWith(dispatcher);

The addition of error handling at first may put off developers as it makes code more verbose and less-readable. This is nothing that some good practice can't fix. In fact, you may find cases where the same error handler can appropriately be shared between multiple different calls.

ErrorCallback error = new ErrorCallback() {
  public boolean error(Message message, Throwable throwable) {
    throwable.printStackTrace();
    return true;
  }
}

MessageBuilder.createMessage()
  .toSubject("HelloWorldService")
  .signalling()
  .with("msg", "Hi there!")
  .errorsHandledBy(error)
  .sendNowWith(dispatcher);

The error handler is required to return a boolean value. This is to indicate whether or not Errai should perform the default error handling actions it would normally take during a failure. You will almost always want to return true here, unless you are trying to explicitly surpress some undesirably activity by Errai, such as automatic subject-termination in conversations. But this is almost never the case.

messageBus.addTransportErrorHandler(new TransportErrorHandler() {
  public void onError(TransportError error) {
    // error handling code.
  }
});