How Spring Integration ServiceActivator Return Types work?

Question

When you add a @ServiceActivator annotation on a method, that method can have different return types which seem to have different implications for the service:

@ServiceActivator(inputChannel = "..", outputChannel = "..")
public T messageReceiver() {...}

Where T could be

• void
• Object
• MessageHandler

How does the ServiceActivator differ based on the return type? I am specifically wondering about the line in the docs that says:

Return values from the annotated method may be of any type. If the return value is not a Message, a reply Message will be created with that object as its payload.

But I'm not following this because I've seen people return MessageHandlers from their ServiceActivator-annotated methods, and they don't want their MessageHandlers to be wrapped as a payload right?

Like this:

@Bean
@ServiceActivator(inputChannel = "sendAsyncChannel", autoStartup="false")
public MessageHandler sendAsyncHandler() {
     return // some MessageHandler
}

Answer 1

What you are pointing is a Messaging Annotations on @Bean. This is a bit different story and it implies not related to POJO method invocation aspect.

We use that @ServiceActivator on the MessageHandler @Bean to register an EventDrivenConsumer endpoint for the provided MessageHandler, when the POJO method style, creates a MethodInvokingMessageHandler for the marked with this @ServiceActivator method.

Please, see more info in the Reference Manual:

https://docs.spring.io/spring-integration/docs/5.0.9.RELEASE/reference/html/overview.html#programming-tips and:

https://docs.spring.io/spring-integration/docs/5.0.9.RELEASE/reference/html/configuration.html#annotations_on_beans