RxJavaFiberInterop

Library for interoperation between RxJava 3 and JDK 21's Virtual Threads (aka Fibers, Project Loom).

dependencies {
    implementation "com.github.akarnokd:rxjava3-fiber-interop:0.0.18"
}

Requires a JDK that has Virtual Threads as standard feature (i.e., not preview), such as https://jdk.java.net/21/.

Components

FiberInterop

Schedulers

Currently, the Virtual Thread API does not offer public means to specify the carrier thread(pool) thus it is not possible to use RxJava Schedulers as such.

You can use the Schedulers.from though to convert the Fork-Join-pool backed standard Virtual Thread Executor into an RxJava Scheduler:

var vte = Executors.newVirtualThreadExecutor();
Scheduler vtScheduler = Schedulers.from(vte);

// sometime later
vte.close();

You can then use vtScheduler from the example with subscribeOn and observeOn to let traditional functional callbacks to block virtually:

Observable.fromCallable(() -> someBlockingNetworkCall())
.subscribeOn(vtScheduler)
.observeOn(vtScheduler)
.map(v -> someOtherBlockingCall(v))
.observeOn(uiThread)
.subscribe(v -> label.setText(v), e -> label.setText(e.toString()));

ℹ️ You need the special operators below to make RxJava's non-blocking backpressure into virtually blocked backpressure.

create

Creates a Flowable from a generator callback, that can emit via FiberEmitter, run on an ExecutorService provided by the user and is suspended automatically upon backpressure. The callback is executed inside the virtual thread thus you can call the usual blocking APIs and get suspensions the same way.

The created Flowable will complete once the callback returns normally or with an error if the callback throws an exception.

try (var scope = Executors.newVirtualThreadExecutor()) {
    FiberInterop.create(emitter -> {
        for (int i = 1; i <= 5; i++) {
             emitter.emit(1);
        }
    }, scope)
    .test()
    .awaitDone(5, TimeUnit.SECONDS)
    .assertResult(1, 2, 3, 4, 5);
}

transform

Transforms each upstream value via a callback that can emit zero or more values for each of those upstream values, run on an ExecutorService provided by the user and is suspended automatically upon backpressure. The callback is executed inside the virtual thread thus you can call the usual blocking APIs and get suspensions the same way.

try (var scope = Executors.newVirtualThreadExecutor()) {
    Flowable.range(1, 5)
    .compose(FiberInterop.transform((value, emitter) -> {
        emitter.emit(value);
        emitter.emit(value + 1);
    }, scope))
    .test()
    .awaitDone(5, TimeUnit.SECONDS)
    .assertResult(1, 2, 2, 3, 3, 4, 4, 5, 5, 6);
}

blockingXXX

RxJava uses java.util.concurrent locks and CountDownLatches via its blockingXXX which will automatically work within a virtual thread. Therefore, there is no need for a separate interop operator. Just block.

try (var scope = Executors.newVirtualThreadExecutor()) {
    scope.submit(() -> {
        var v = Flowable.just(1)
        .delay(1, TimeUnit.SECONDS)
        .blockingLast();
        System.out.println(v);
    });
}