flatMap() vs subscribe() in Spring webflux

Question

I am a newbie to Spring WebFlux and trying to convert my spring MVC application to webflux. I return a Mono mono from my service :

    List<Store> stores = new ArrayList();

When I do:

    mono.subscribe(stores::addAll);
    dataexchange.put("stores", stores);
    return Mono.just(dataexchange);

Then stores is populated as empty list in response. However, I can verify that subscribe() is working after returning response.

When I do :

    return mono.flatmap( (response) -> {
        dataexchange.put("stores", response));
        return Mono.just(dataexchange);
    });

Then stores is populated in the response.

Can someone please explain me what is the difference between the two? Is flatMap blocking? Thanks in advance !

Answer 1

mono.subscribe(stores::addAll);

is asynchronous. That means, you tell the mono that it can now start evaluating.

What you do is continue processing stores right away - with a huge chance that the Mono hasn't evaluated yet.

So, how can you fix this?

You can block until the Mono has completed:

mono.doOnNext(stores::addAll).block()

Of course, this defeats the purpose of reactive programming. You are blocking the main thread until an action completes, which can be achieved without Reactor in a much simpler fashion.

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The right way is to change the rest of your code to be reactive too, from head to toe. This is similar to your second example, where your call to dataexchange is part of the Mono, thus being evaluated asynchronously, too.

The important lesson to be learned is that operations like map or flatMap are not operating on the result of the Mono, but create a new Mono that adds another transformation to the execution of the original Mono. As long as the Mono doesn't evaluate, the flatMap or map operations are not actually doing anything.

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I hope this helps.