Shared observable that will run a task. Every subscriber that subscribes while the task is running will share the same observable and receive the same result. When the tasks finishes the next subscriber will trigger a new task.

_output.txt

[1] subscribed at 234ms
-> Started Task at 238ms
[2] subscribed at 1238ms
[3] subscribed at 2239ms
-> Finished Task at 3242ms
[1] Received result. Value: 55
[4] subscribed at 3243ms
[2] Received result. Value: 55
-> Started Task at 3243ms
[3] Received result. Value: 55
[5] subscribed at 4247ms
-> Finished Task at 6245ms
[4] Received result. Value: 27
[5] Received result. Value: 27

Main.java

public class Main {

    public static final long START_TIME = System.currentTimeMillis();

    public static void main(String[] args) throws InterruptedException {

        SingletonTask singletonTask = singletonTask();

        singletonTask.getObservable()
                .subscribeOn(Schedulers.newThread())
                .observeOn(Schedulers.single())
                .subscribe(result -> System.out.println("[1] Received result. Value: " + result));
        System.out.println("[1] subscribed at " + (System.currentTimeMillis() - START_TIME) + "ms");

        Thread.sleep(1000);

        singletonTask.getObservable()
                .subscribeOn(Schedulers.newThread())
                .observeOn(Schedulers.single())
                .subscribe(result -> System.out.println("[2] Received result. Value: " + result));
        System.out.println("[2] subscribed at " + (System.currentTimeMillis() - START_TIME) + "ms");

        Thread.sleep(1000);

        singletonTask.getObservable()
                .subscribeOn(Schedulers.newThread())
                .observeOn(Schedulers.single())
                .subscribe(result -> System.out.println("[3] Received result. Value: " + result));
        System.out.println("[3] subscribed at " + (System.currentTimeMillis() - START_TIME) + "ms");

        Thread.sleep(1000);

        singletonTask.getObservable()
                .subscribeOn(Schedulers.newThread())
                .observeOn(Schedulers.single())
                .subscribe(result -> System.out.println("[4] Received result. Value: " + result));
        System.out.println("[4] subscribed at " + (System.currentTimeMillis() - START_TIME) + "ms");

        Thread.sleep(1000);

        singletonTask.getObservable()
                .subscribeOn(Schedulers.newThread())
                .observeOn(Schedulers.single())
                .subscribe(result -> System.out.println("[5] Received result. Value: " + result));
        System.out.println("[5] subscribed at " + (System.currentTimeMillis() - START_TIME) + "ms");

        Thread.sleep(20000);

    }
}

SingletonTask.java

public enum SingletonTask {
    INSTANCE;

    private final Random randomGenerator = new Random(System.currentTimeMillis());

    private final Observable<Integer> longRunningObservable = Observable.<Integer>create(observableEmitter -> {
        int result = randomGenerator.nextInt(100);
        System.out.println("-> Started Task at " + (System.currentTimeMillis() - Main.START_TIME) + "ms");
        Thread.sleep(3000);
        System.out.println("-> Finished Task at " + (System.currentTimeMillis() - Main.START_TIME) + "ms");
        observableEmitter.onNext(result);
        observableEmitter.onComplete();
    }).share();

    public static SingletonTask singletonTask() {
        return INSTANCE;
    }

    public Observable<Integer> getObservable() {
        return longRunningObservable;
    }
}