Java : Flow.Subscriber (Reactive Streams) with Examples

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)

void onComplete ()

Method invoked when it is known that no additional Subscriber method invocations will occur for a Subscription that is not already terminated by error, after which no other Subscriber methods are invoked by the Subscription.

class MySubscriber implements Flow.Subscriber<String> {

    @Override
    public void onSubscribe(Flow.Subscription subscription) {
        System.out.println("onSubscribe");
        subscription.request(3);
    }

    @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");
    }
}

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
// ↓
//-- subscribe --
//onSubscribe
//-- submit --
//onNext : abc
//onNext : 123
//onNext : XYZ
//-- close --
//onComplete

void onError (Throwable throwable)

Method invoked upon an unrecoverable error encountered by a Publisher or Subscription, after which no other Subscriber methods are invoked by the Subscription.

class MySubscriber implements Flow.Subscriber<String> {

    @Override
    public void onSubscribe(Flow.Subscription subscription) {
        System.out.println("onSubscribe");
        subscription.request(3);
    }

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

        if ("123".equals(item)) {
            throw new IllegalArgumentException("Error!");
        }
    }

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

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

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
// ↓
//-- subscribe --
//onSubscribe
//-- submit --
//onNext : abc
//onNext : 123
//onError : java.lang.IllegalArgumentException: Error!
//-- close --

void onNext (T item)

Method invoked with a Subscription's next item.

Please see onComplete().

void onSubscribe (Flow.Subscription subscription)

Method invoked prior to invoking any other Subscriber methods for the given Subscription.

Please see onComplete().