How can I return a Mono<Map<String, Flux<Integer>>> response in Spring Webflux?

Question

So right now, I am returning a response looking like

    @GetMapping("/integers")
    @ResponseStatus(code = HttpStatus.OK)
    public Mono<Map<String, Flux<Integer>>> getIntegers() {
        Mono<Map<String, Flux<Integer>>> integers = 
               Mono.just(Map.of("Integers", integerService.getIntegers()));
        return integers;
    }

which gives me a response of

{"Integers":{"scanAvailable":true,"prefetch":-1}}

where I expected it to stream the Flux<Integer> part as well, but it didn't. How would I do this in Spring webflux?

Brian Clozel · Accepted Answer · 2018-07-11 19:27:34Z

6

Spring WebFlux can only deal with one reactive type and won't deal with nested reactive types (like a Mono<Flux<Integer>>). Your controller method can return a Mono<Something>, a Flux<Something>, a ResponseEntity<Mono<Something>>, a Mono<ResponseEntity<Something>>, etc - but never nested reactive types.

The strange data you're seeing in the response is Jackson actually trying to serialize a reactive type (so you're looking at the promise for the data, not the data itself).

In this case, you could rewrite your method like this:

@GetMapping("/integers")
@ResponseStatus(code = HttpStatus.OK)
public Mono<Map<String, Flux<Integer>>> getIntegers() {
    Flux<Integer> integers = integerService.getIntegers();
    Mono<Map<String, List<Integer>>> result = integers
            // this will buffer and collect all integers in a Mono<List<Integer>>
            .collectList()
            // we can then map that and wrap it into a Map
            .map(list -> Collections.singletonMap("Integers", list));
    return result;
}

You can read more about the supported return values in the Spring WebFlux reference documentation.

answered Jul 11, 2018 at 19:27

Brian Clozel

59.7k15 gold badges180 silver badges184 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

katiex7 Over a year ago

I see, well I ended up chainging the @Getmapping to produce MediaType.APPLICATION_STREAM_JSON_VALUE because I was not sure, if returning a pure Flux<Integer> would stream the data to the client and not do any sort of buffering. Largely, due to the fact that i am returning a billion heavy objects(integer was just a placeholder for the actual business object) In your case, could the integers.collectToList() throw a Out of Heap Memory exception if the Flux<Integer> integers has a lot of things it's going to stream

katiex7 Over a year ago

sorry, edited the comment above. I pressed enter accidentally, before writing everything

Brian Clozel Over a year ago

Haha, I'm often hitting that problem with SO comments. You're right. In this case, streaming values is the right choice - Flux.collectList does buffer everything in memory, so this is not the best choice for large collections/objects.

katiex7 Over a year ago

Hehe, well I've noticed you around a lot of spring webflux related questions on SO, so your hands must be keystroke-happy by now That makes sense, thanks!

Collectives™ on Stack Overflow

How can I return a Mono<Map<String, Flux<Integer>>> response in Spring Webflux?

1 Answer 1

4 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

4 Comments

Your Answer

Sign up or log in

Post as a guest

Related