flatMap() vs subscribe() in Spring webflux
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.
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.
I hope this helps.
XYZ
Updated on June 09, 2022Comments
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XYZ almost 2 years
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 !
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XYZ over 5 yearsSo, that means that flaptmap isn't blocking? Also, I have read in a number of articles that "Nothing happens until you subscribe" , i.e. no data flow is available until you subscribe. Then how does flatmap work without subscribe() ?
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Markus Appel over 5 years@XYZ I can't tell from your example where you return your
Mono
to, but it absolutely shouldn't. -
XYZ over 5 yearsI am returning this mono directly from controller. Can you please answer why flatmap() works without subscribe()?
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Markus Appel over 5 years@XYZ it doesn't. Try it in a sandbox environment (without any controllers and other side effects). I think Spring handles the
Mono
when you return it by subscribing to it. -
Ivan Rodrigues about 2 yearsI was facing the same issue but doing a search in documentation of mongoDB using reactive driver. There are an example using CountDownLatch. After use the CountDownLatch I can see all steps from log of Webflux. Before I can`t