Java Reactor Flux/Mono, when does doOnNext get triggered before or after element is emitted?

Question

I have this confusion, when does doOnNext is triggered before or after of element emission by Publisher (Flux/Mono).

Answer 1

It's after the publication of the element - and it has to be after by definition, otherwise the Consumer passed to doOnNext wouldn't have access to the element emitted.

However, doOnNext() is called before the subscriber. For example:

Flux.just("first", "second")
        .doOnNext(x -> System.out.println(x + " onNext"))
        .subscribe(System.out::println);

...would output:

first onNext
first
second onNext
second

Answer 2

I am afraid you haven't understood reactive properly, and that's perfectly fine :). In Rx, the subscription goes bottom-up. Consider the following code:

Flux<Integer> f = Flux.fromIterable(Arrays.asList(1,2,3));

Since there is no subscriber, nothing happens. No item would be emitted. Now consider this:

f.subscribe(System.out::println)

What happens is that the subscriber subscribes to the flux, and items are emitted.

Now consider this:

f.doOnNext(System.out::println).subscribe(System.out.println);

Remember, the subscription happens in a bottom-up manner. So over here, the subscriber subscribes to the doOnNext(), and the doOnNext() subscribes to the original flux, which then starts emitting events. doOnNext() then intercepts each event and performs some side-effect.