Support Java 9 Flow and ReactiveStream

[hantsy]

We have moved to Java SE 11 since Jakarta EE 10, we should consider more new APIs in specs provided in the new Java SE to modernize our Jakarta EE applications.

Such as use Flow( and adapters of ReactiveStreams) as asynchronous method return type.

• https://docs.oracle.com/javase/9/docs/api/java/util/concurrent/Flow.html
• http://www.reactive-streams.org/
• https://github.com/eclipse/microprofile-reactive-streams-operators

[mswatosh]

Hi @hantsy
Thanks for opening this, it's always good to evaluate the new java.util.concurrent APIs. I've been experimenting with the Flow API, and I'm not sure there's anything for the Jakarta Concurrency spec to add here. Since SubmissionPublisher uses an Executor to publish items, it's very easy to provide a ManagedExecutor to access Jakarta Context in the Subscribers. You can see some of my testing here: https://github.com/OpenLiberty/open-liberty/blob/dd23ca159fce78f9f5322936c5da0f02bfb17506/dev/com.ibm.ws.concurrent_fat_reactive/test-applications/reactiveapp/src/reactiveapp/web/ConcurrentReactiveServlet.java

The problem I'm seeing with providing Flow APIs as asynchronous return types is we would need to provide an implementation of Flow.Publisher, and Publisher implementations seem to vary quite a bit depending on how they're used. Maybe I'm missing something here, so let me know if you think there's a way the Concurrency spec could provide value with the Flow APIs

[hantsy]

The problem I'm seeing with providing Flow APIs as asynchronous return types is we would need to provide an implementation of Flow.Publisher, and Publisher implementations seem to vary quite a bit depending on how they're used. Maybe I'm missing something here, so let me know if you think there's a way the Concurrency spec could provide value with the Flow APIs

In the Jakarta EE providers like OpenLIberty, I think it is possible to implement the Flow Api handling internally use Smallrey Mutiny, or other reactive streams libs, such as Reactor, Rxjava etc. which also easily converts between JDK Flow and ReactiveStreams API.

[hantsy]

Spec should allow use Flow.Publishser as return type(in CDI bean, or REST etc.) like the existing CompletableFuture/ComplationStage, and provides built-in mechanism or public API to transfer state between contexts.

[mswatosh]

Support for Flow has been added in #368 and #375
I'll open a separate issue for the TCK updates