Java : Flow.Subscription (Reactive Streams) with Examples

Flow.Subscription (Java SE 20 & JDK 20) with Examples.
You will find code examples on most Flow.Subscription methods.

Summary

Message control linking a Flow.Publisher and Flow.Subscriber. Subscribers receive items only when requested, and may cancel at any time. The methods in this interface are intended to be invoked only by their Subscribers; usages in other contexts have undefined effects.

Class diagram

Please see also : Reactive Streams

class MySubscriber implements Flow.Subscriber<String> {

    private Flow.Subscription subscription;

    @Override
    public void onSubscribe(Flow.Subscription subscription) {
        System.out.printf("  onSubscribe (tid=%d)%n",
                Thread.currentThread().threadId());

        this.subscription = subscription;
        subscription.request(1);
    }

    @Override
    public void onNext(String item) {
        System.out.printf("  onNext (tid=%d) : %s%n",
                Thread.currentThread().threadId(), item);

        subscription.request(1);
    }

    @Override
    public void onError(Throwable throwable) {
        System.out.printf("  onError (tid=%d) : %s%n",
                Thread.currentThread().threadId(), throwable);
    }

    @Override
    public void onComplete() {
        System.out.printf("  onComplete (tid=%d)%n",
                Thread.currentThread().threadId());
    }
}

System.out.printf("main (tid=%d)%n", Thread.currentThread().threadId());

try (final var publisher = new SubmissionPublisher<String>()) {

    System.out.println("-- subscribe --");

    publisher.subscribe(new MySubscriber());

    TimeUnit.SECONDS.sleep(1);
    System.out.println("-- submit --");

    publisher.submit("abc");
    publisher.submit("123");
    publisher.submit("XYZ");

    TimeUnit.SECONDS.sleep(1);
    System.out.println("-- close --");
}

// Result
// ↓
//main (tid=1)
//-- subscribe --
//  onSubscribe (tid=32)
//-- submit --
//  onNext (tid=32) : abc
//  onNext (tid=32) : 123
//  onNext (tid=32) : XYZ
//-- close --
//  onComplete (tid=32)

Methods

void cancel ()

Causes the Subscriber to (eventually) stop receiving messages.

class MySubscriber implements Flow.Subscriber<String> {

    private Flow.Subscription subscription;

    @Override
    public void onSubscribe(Flow.Subscription subscription) {
        System.out.println("onSubscribe");
        this.subscription = subscription;
    }

    @Override
    public void onNext(String item) {
        System.out.println("onNext : " + item);
    }

    @Override
    public void onError(Throwable throwable) {
        System.out.println("onError : " + throwable);
    }

    @Override
    public void onComplete() {
        System.out.println("onComplete");
    }

    void request(long n) {
        if (subscription != null) {
            subscription.request(n);
        }
    }

    void cancel() {
        if (subscription != null) {
            subscription.cancel();
        }
    }
}

try (final var publisher = new SubmissionPublisher<String>()) {

    System.out.println("-- subscribe --");

    final var subscriber = new MySubscriber();
    publisher.subscribe(subscriber);

    TimeUnit.SECONDS.sleep(1);

    System.out.println("-- submit (a1, a2) --");

    publisher.submit("a1");
    publisher.submit("a2");

    TimeUnit.SECONDS.sleep(1);

    System.out.println("-- request 3 --");
    subscriber.request(3);

    TimeUnit.SECONDS.sleep(1);

    System.out.println("-- cancel --");
    subscriber.cancel();

    TimeUnit.SECONDS.sleep(1);

    System.out.println("-- submit (a3, a4) --");

    publisher.submit("a3");
    publisher.submit("a4");

    TimeUnit.SECONDS.sleep(1);

    System.out.println("-- close --");
}

// Result
// ↓
//-- subscribe --
//onSubscribe
//-- submit (a1, a2) --
//-- request 3 --
//onNext : a1
//onNext : a2
//-- cancel --
//-- submit (a3, a4) --
//-- close --

void request (long n)

Adds the given number n of items to the current unfulfilled demand for this subscription.

class MySubscriber implements Flow.Subscriber<String> {

    private Flow.Subscription subscription;

    @Override
    public void onSubscribe(Flow.Subscription subscription) {
        System.out.println("onSubscribe");
        this.subscription = subscription;
    }

    @Override
    public void onNext(String item) {
        System.out.println("onNext : " + item);
    }

    @Override
    public void onError(Throwable throwable) {
        System.out.println("onError : " + throwable);
    }

    @Override
    public void onComplete() {
        System.out.println("onComplete");
    }

    void request(long n) {
        if (subscription != null) {
            subscription.request(n);
        }
    }
}

try (final var publisher = new SubmissionPublisher<String>()) {

    System.out.println("-- subscribe --");

    final var subscriber = new MySubscriber();
    publisher.subscribe(subscriber);

    TimeUnit.SECONDS.sleep(1);

    System.out.println("-- submit (a1, a2, a3, a4) --");

    publisher.submit("a1");
    publisher.submit("a2");
    publisher.submit("a3");
    publisher.submit("a4");

    TimeUnit.SECONDS.sleep(1);

    System.out.println("-- request 1 --");
    subscriber.request(1);

    TimeUnit.SECONDS.sleep(1);

    System.out.println("-- request 2 --");
    subscriber.request(2);

    TimeUnit.SECONDS.sleep(1);

    System.out.println("-- request 3 --");
    subscriber.request(3);

    TimeUnit.SECONDS.sleep(1);

    System.out.println("-- submit (a5, a6) --");

    publisher.submit("a5");
    publisher.submit("a6");

    TimeUnit.SECONDS.sleep(1);

    System.out.println("-- close --");
}

// Result
// ↓
//-- subscribe --
//onSubscribe
//-- submit (a1, a2, a3, a4) --
//-- request 1 --
//onNext : a1
//-- request 2 --
//onNext : a2
//onNext : a3
//-- request 3 --
//onNext : a4
//-- submit (a5, a6) --
//onNext : a5
//onNext : a6
//-- close --
//onComplete

API Examples

Java : Flow.Subscription (Reactive Streams) with Examples

Summary

Methods

void cancel ()

void request (long n)

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