Demonstrate @Publisher with an @EventListener

[artembilan]

The @EventListener from Spring Framework is a good tool for POJO configuration to subscribe to events from an ApplicationContext. The @Publisher is an AOP tool to publish a Message from POJO method call.

• Add a test and docs to demonstrate how the @Publisher can be used together with an @EventListener

[garyrussell]

The @RabbitListener one looks a bit awkward too.

What drove you to create this PR?

[garyrussell]

While this would work, do we really want to promote it? An ApplicationEventListeningMessageProducer is a much simpler approach.

[artembilan]

What drove you to create this PR?

Yeah... When I asked end-user to catch an application event and publish it into a channel it turned out to not be so smooth to digest.
Their first question was: why do we need extra dependency for events?
Another one that ApplicationEventListeningMessageProducer is a bit long name.
Plus eventTypes is a setter, so it leads to more code lines, then just couple annotations on a POJO method.
Plus method signature dictates type of the event to consume and body result is naturally a message to produce.

Right, with @RabbitListener it is a bit awkward, but at least we it covered in docs if someone comes to us with the question that they know this annotation, but would like to produce to the channel without code, but rather annotation style.

Another thought (but that's for other day contribution) is to have something like IntegrationFlowBuilder fromQueue() in the Amqp factory to avoid extra codding for IntegrationFlow.from(Amqp.).
Something similar for events - fromEvent(Class<?>... eventTypes).

Plus it might be great to look into sensible default beans for something like ConnectionFactory like we do for container factory in the @RabbitListener.
There idea is to come up with some hook which would let end-users to end up with a solution with Spring Integration faster than it is now like they claim to have it with just two annotations for @RabbitListener.