Spring-framework: Add support for Kotlin coroutines [SPR-15413]

[spring-projects-issues]

Konrad Kamiński opened SPR-15413 and commented

Since 1.1 Kotlin supports coroutines. Its library support includes utility functions for converting _suspending functions/lambdas_ to/from CompletableFuture / Deferred. It would be nice to have such support ListenableFuture as well.

The API could look as follows:

// a function for creating ListenableFuture from suspending lambda
fun <T> listenableFuture(context: CoroutineContext = CommonPool, block: suspend () -> T): ListenableFuture<T>

// a function for creating ListenableFuture from Deferred
fun <T> Deferred<T>.asListenableFuture(): ListenableFuture<T>

// an extension suspending function which awaits for ListenableFuture completion
suspend fun <T> ListenableFuture<T>.await(): T

---

Affects: 5.0 M5

Issue Links:

• #21058 Incorrect return type of Kotlin suspending functions

Referenced from: pull request https://github.com/spring-projects/spring-framework/pull/1375

18 votes, 26 watchers

[spring-projects-issues]

Konrad Kamiński commented

I created a pull request.

[spring-projects-issues]

Sébastien Deleuze commented

Thanks for contributing this pull request.

Kotlin coroutines seems indeed a good fit for ListenableFuture, but since coroutines are still experimental, I prefer to wait before integrating such support in Spring Framework codebase.

I would suggest submitting a similar pull request on kotlinx.coroutines in order to provide such support like it has been done for Reactor Flux and Mono types. Any chance you could do that?

[spring-projects-issues]

Sébastien Deleuze commented

Konrad Kamiński I turn this issue into a global coroutine support one, since there may be other areas than ListenableFuture impacted as shown by your spring-kotlin-coroutine project.

Did you experiment with coroutines + Spring WebFlux? After reading the guide to reactive streams with coroutines, it seems to me this is something worth to explore if we can leverage the existing Reactor based infrastructure of Spring WebFlux (via annotation or functional API) to provide the ability to use imperative style code for those who are not confortable with functional APIs.

That said we really need to experiment in order to see how this work with backpressure, see if it can enable easily to implement custom operator as advertised, etc.
I am quite confident about the ListenableFuture / Mono use case but the Flux use case is less obvious to me.

Any sample WebFlux + coroutine application example could help to move this topic forward.

[spring-projects-issues]

Konrad Kamiński commented

I created a proof-of-concept implementation of mixit application with coroutines. I have not yet touched either WebClient or WebSocket API. The basic approach to introducing coroutines with Spring WebFlux is:

1. Create a parallel interface/implementation to the existing one, e.g. there is CoroutineServerRequest/DefaultCoroutineServerRequest for ServerRequest which on the one hand is a wrapper around the Reactor based API and on the other provides a coroutines-based API.
2. For every method which returns a Mono<T> provide instead a _suspending function_, which returns a T?.
3. For every method which returns a Flux<T> provide instead a _suspending_ or _regular_ function, which returns ReceivableChannel<T>.

In this poc I only covered whatever was needed for the mixit application to work, though I may have missed something since I have not checked the application thoroughly. I left the tests untouched and they seem to work fine. :)

[spring-projects-issues]

Sébastien Deleuze commented

That's super useful Konrad Kamiński thank you! I will have a deeper look to this branch and send you my feedback.

[spring-projects-issues]

Sébastien Deleuze commented

I have submitted a pull-request on kotlinx.coroutines to upgrade Reactor coroutine support from Aluminium to Bismuth release train.

[spring-projects-issues]

Sébastien Deleuze commented

Reactor Bismuth and Spring Framework 5 support is now available as part of kotlinx.coroutines 0.19.1.

[spring-projects-issues]

Sébastien Deleuze commented

Konrad Kamiński Now that Spring Framework 5 and Reactor Core 3.1 are GA + are supported in kotlinx.coroutines, do you have any plan to add WebFlux support to your great project https://github.com/konrad-kaminski/spring-kotlin-coroutine ? Based on what you did on MiXiT application, and now that our APIs are stable, I guess it could be useful for Spring + Kotlin developers to have CoroutineFoo variants of the ReactiveFoo main existing ones, + provide some extensions to have a bridge between the 2 worlds ? I would be interested to have your thoughts on this.

[spring-projects-issues]

Sébastien Deleuze commented

Some additional thoughts about bridging Reactor-based APIs to Coroutines based on your previous experiment.

For streaming use case, Flux<T> -> suspending function which returns ReceivableChannel<T> should be fine. On that topic, it seems that channel operators are coming on Coroutine side.

For non streaming use cases, maybe most of the time translating Flux<T> -> suspending function which returns List<T> via a preliminary call to Flux.collectList() will be more efficient and more familiar with the programming model people have in non-Reactive world and in line with the goal of Coroutines to provide a simplified imperative like programming model.

Mono<T> could translate to suspending function which returns T (most of the time) or T? depending on the context (should be specified in the Javadoc, if not please raise an issue).

Mono<Void> could translate to suspending function which returns Unit.

[spring-projects-issues]

Sébastien Deleuze commented

See this PR comment.

[spring-projects-issues]

Sébastien Deleuze commented

A quick update:

• Coroutines support for is currently incubating in spring-fu.
• Interop between Reactor and Coroutines context needs to be implemented.
• Cold stream abstraction and non-experiental channel API (not yet the case for Kotlin 1.3) are mandatory for Spring support
• Konrad Kamiński offered help and is likely to work with me on this feature

[sdeleuze]

Coroutines support for WebClient and WebFlux.fn has been merged in master branch via this commit which is the first part of a more complete Coroutines support coming in Spring Framework 5.2. It introduces suspendable Kotlin extensions for Mono based methods in WebFlux classes like WebClient, ServerRequest, ServerResponse as well as a Coroutines router usable via coRouter { }. See more details in the Coroutines section of the reference documentation.

Coroutines extensions use await prefix or AndAwait suffix, and most are using names close to their Reactive counterparts, except exchange in WebClient.RequestHeadersSpec which translates to awaitResponse.

Upcoming expected changes are:

• Leverage Dispatchers.Unconfined (Kotlin/kotlinx.coroutines#972)
• Expose extensions for Flux based API (Kotlin/kotlinx.coroutines#254)
• Introduce interop with CoroutineContext (Kotlin/kotlinx.coroutines#284)
• Support Coroutines in ReactiveAdapterRegistry
• Support Coroutines for WebFlux annotated controllers
• Fix return type of Kotlin suspending functions (gh-21058)

I am also preparing pull-requests for Spring Data MongoDB and Spring Data R2DBC.

A demo project is available in this spring-boot-coroutines-demo repository.

[sdeleuze]

Support for WebFlux annotated controllers, including view resolution and Deferred, has been merged in master branch. See this sample Coroutines controller for a concrete example.

[sdeleuze]

With Coroutines Flow support (see this commit), Spring Framework Coroutines support is now complete. See https://github.com/sdeleuze/spring-boot-coroutines-demo for concrete examples of Flow usage and also see :

• Flow documentation
• Preview status
• The related GitHub issue for feedback since Flow API is not frozen yet

A detailed blog post is coming next week to present an overview of Spring Coroutines support.