Is the subscribe() method blocking?

Question

I'm starting with Project Reactor and I cant understand why is this code infinitely prints "Hello" and never returns the Disposable object.

Flux<Object> flux = Flux.generate(sink -> sink.next("Hello"));
Disposable disposable = flux.subscribe(System.out::println);
disposable.dispose();
System.out.println("This doesn't print");

I thought that when it comes to call subscribe() method it must immediately return the Disposable object with which I can unsubscribe if I want. I know that code inside this subscribe method running in the same thread and if I substitute the delayElementsmethod before the subscribe call then the code below will work because it runs in a separate daemon thread, so can any explain why does it stop at the subscribemethod and not return Disposable and are there any ways to manage a subscription by calling the subscribe method? Is it possible to make it so that, by analogy with the delayElements method, this is executed in a separate thread and the result of calling the subscribe method returns Disposable immediately?

I couldn't find an answer specifically to this question. In all the examples that I saw there was either a finite data stream or the delayElements method was used.

Answer 1

Yes, subscribe() is blocking in this particular scenario.

Reactor is concurrency agnostic which means by default it doesn't enforce any threading/asynchronicity on you and executes the pipeline on the calling thread (in this particular case the main thread).

You can change this explicitly by using subscribeOn or publishOn, or implicitly by using some operators like delayElements.

Flux<Object> flux = Flux.generate(sink -> sink.next("Hello")).publishOn(Schedulers.parallel());
Disposable disposable = flux.subscribe(System.out::println);
disposable.dispose();
System.out.println("This doesn't print");