PublishSubscribeChannel having multiple subscribers and return value - spring

Question

I would like to understand how returning values work for PublishSubscribeChannel having multiple subscribers.
#Bean
public PublishSubscribeChannel channel(){
return new PublishSubscribeChannel();
}
#Bean
#ServiceActivator(inputChannel = "channel")
public MessageHandler handler1() {
//...
return handler1;
}
#Bean
#ServiceActivator(inputChannel = "channel")
public MessageHandler handler2() {
//...
return handler2;
}
#Bean
#ServiceActivator(inputChannel = "channel")
public MessageHandler handler3() {
//...
return handler3;
}
#MessagingGateway
public interface TestGateway{
#Gateway(requestChannel = "channel")
String method(String payload);
}
method expects some String as a return type. If a message is sent to all three handlers via channel, the value coming from which handler would be returned? From what I understand, messages are sent to each subscriber one by one, so would it be the value returned by the last handler?
Also, would it be possible to have handlers returning type different than the method return type, also if it wouldn't necessarily expect String?
When it comes to a scenario where any Exception occurs, I believe if setIgnoreFailures = false, the processing would stop on it and not process to the next handler. Otherwise, the last exception would be thrown.
Thanks in advance

Answer 1

I'm sure there is a specific business task behind your question.
But i you really are about an academic knowledge to see how Spring Integration works internally, then here is some answer for you.
Since your PublishSubscribeChannel is not configure with an Executor, then all your subscribers are called one by one, and only when the previous has done its job. And the part of that job is really a reply producing. So, if your first MessageHandler produced some reply, then exactly this one fulfills CountDownLatch in the TempraryReplyChannel for a gateway request-reply functionality.
The replies from the rest of handlers are going to be ignored and they may throw a late reply error.
Yes, you can return any type as long as it can be converted to the expected return type. See more info about ConversionService: https://docs.spring.io/spring-integration/reference/html/messaging-endpoints.html#payload-type-conversion
About ignoreFailures I'd suggest to look into a PublishSubscribeChannel source code and how it is propagated down to BroadcastingDispatcher:
private boolean invokeHandler(MessageHandler handler, Message<?> message) {
try {
handler.handleMessage(message);
return true;
}
catch (RuntimeException e) {
if (!this.ignoreFailures) {
if (e instanceof MessagingException && ((MessagingException) e).getFailedMessage() == null) { // NOSONAR
throw new MessagingException(message, "Failed to handle Message", e);
}
throw e;
}
else if (this.logger.isWarnEnabled()) {
logger.warn("Suppressing Exception since 'ignoreFailures' is set to TRUE.", e);
}
return false;
}
}
And no: otherwise none exception will be thrown. See that code again.