How to return an object from Mono SpringWebFlux

Question

I am using Spring Webflux, and I need to return the ID of user upon successful save.

Repository is returning the Mono

Mono<User> savedUserMono = repository.save(user);

But from controller, I need to return the user ID which is in object returned from save() call.

I have tried using doOn*, and subscribe(), but when I use return from subscribe, there is an error 'Unexpected return value'

Possible duplicate of How to get username from mono<user> on spring boot webflux? — K.Nicholas
– K.Nicholas, Commented Aug 6, 2019 at 19:09

Michael Berry · Accepted Answer · 2019-08-07 07:43:11Z

5

Any time you feel the need to transform or map what's in your Mono to something else, then you use the map() method, passing a lambda that will do the transformation like so:

Mono<Integer> userId = savedUserMono.map(user -> user.getId());

(assuming, of course, that your user has a getId() method, and the id is an integer.)

In this simple case, you could also use optionally use the double colon syntax for a more concise syntax:

Mono<Integer> userId = savedUserMono.map(User::getId);

The resulting Mono, when subscribed, will then emit the user's ID.

answered Aug 7, 2019 at 7:43

Michael Berry

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Comments

Toerktumlare · Accepted Answer · 2019-08-07 09:48:58Z

2

what you do is

Mono<String> id = user.map(user -> {
        return user.getId();
    });

and then you return the Mono<String> to the client, if your id is a string

edited Aug 7, 2019 at 9:48

answered Aug 6, 2019 at 22:04

Toerktumlare

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4 Comments

user3595026 Over a year ago

User<String> id should be a Mono<String> id

Toerktumlare Over a year ago

yes thank for pointing it out, was a bit too fast there. i have edited it

Simon Baslé Over a year ago

for readability I'd recommend using method reference: user.map(User::getId)

Toerktumlare Over a year ago

for readability id say this is more readable, your suggestion is less readable but what i prefer. But since the thread starter doesn't seam to have much experience the more verbose version above is more understandable.

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