Add Kotlin Coroutines Support

[artembilan]

Discussed in #3894

^{Originally posted by tomvandenberge September 25, 2022}
Hi,
I'm using Spring Integration in various Kotlin (previously Java) projects. Some of these projects are using coroutines. So far, I haven't been able to find a way to use coroutines together with Spring Integration. I'd love to be able e.g. to have a suspending function as a service activator. Currently this is not possible.

I realise that this probably requires fundamental changes to Spring Integration, but I wanted to check with you if this is something that is already on your roadmap?

Thanks,
Tom

[artembilan]

@tomvandenberge,

So, I've created an issue from a discussion we had before since this request is really valid.
And those Kotlin Coroutines look for me as another Reactive Streams library.
Turns out that Spring Framework provides for already a CoroutinesUtils with its API around Reactor's Mono which we already support in Spring Integration.
So, suspended functions should not be too hard to implement as far as I see.

Thank you!

[tomvandenberge]

Thanks for looking into this. I would say coroutines is a bit more than just another Reactive Streams library -- it's a feature that's built into the kotlin language itself. Other parts of the Spring framework already provide support for coroutines, e.g. WebFlux, which allows you to use suspending @controller functions. In my view, Spring Integration could provide similar support, meaning that it's possible to use Spring Integration components using suspending functions. I don't see how you can do this very easily though; I think it means making all Spring Integration components work with suspending functions.

[artembilan]

Well, I look at that from the point of view of this Java framework.
It is really out of my scope if that is high-level language feature or some extra library. For me me it is just a set of respective types I can deal with in the framework Java code.
And that's probably not a surprise why Spring Framework asks for the kotlinx-coroutines-reactor dependency...
But that's already this framework responsibility how to deal with those types.

From end-user perspective it indeed would look similar to that @RequestMapping for suspend fun:

    @ServiceActivator(inputChannel = "input")
    suspend fun render(payload: String): String {

The change (I believe) is going to happen only in one place - MessagingMethodInvokerHelper. So, all other messaging annotations would be able to be used with suspend functions as well.

Another part of the puzzle indeed going to be a @MessagingGateway for those suspend method definitions on the interface.

Since it is still a Reactive Streams library to make the whole integration flow fully suspendable we have to use a FluxMessageChannel in between endpoints as we recommend for Flux and Mono: https://docs.spring.io/spring-integration/docs/current/reference/html/reactive-streams.html#reactive-streams.

[artembilan]

@tomvandenberge ,

here you are PR: #3905 !