how to return Flux<T> as ResponseEntity in Spring Web Flux

Question

I am unable to return Flux<T> in ResponseEntity. In my repository I return Flux<Student>. Now I want to handle this response in controller with switchIfEmpty to handle 404 error code using switchIfEmpty. Below is what I tried:

repository

public interface StudentRepository extends R2dbcRepository<Student, Integer>{

    Flux<Student> findByFirstName(String firstName);
    
}

service

public Flux<Student> getStudentByName(String name){
        return repo.findByFirstName(name);
    }

Controller (this is where I am not able produce response)

@GetMapping(value="student/name/{name}")
    public Flux<ResponseEntity<Student>> getStudentByName(@PathVariable(name = "name",required = true)String name) {
        Flux<ResponseEntity<Student>> map = service.getStudentByName(name)
                .switchIfEmpty(Mono.error(new NoSuchElementException("student not found"))) 
                .map(stu->new ResponseEntity<>(Mono.just(stu),HttpStatus.OK));
    return map;
    }

As I was getting Only a single ResponseEntity supported I tried with below code:

@GetMapping(value="student/name/{name}")
    public Flux<ResponseEntity<Student>> getStudentByName(@PathVariable(name = "name",required = true)String name) {
         Flux<ResponseEntity<Student>> flatMap = service.getStudentByName(name)
        .switchIfEmpty(Mono.error(new NoSuchElementException("student not found"))) 
        .flatMap(stu->Flux.just(new ResponseEntity<Student>(stu,HttpStatus.OK)));
        return flatMap;
    }

I am still getting same exception i.e. Only a single ResponseEntity supported

I have handled NoSuchElementException as global exception.

How do I return Flux<Student> wrapped inside ResponseEntity.. or vice versa as ResponseEntity wrapped inside Flux. What approach should I go with to return Flux object.

A Flux<ResponseEntity<Student> doesn't really make sense a http response can only have a single status not multiple. You should have a ResponseEntity<Flux<Student>>. Next to that what you are returning isn't a Flux<ResponseEntity<Student>> either, looking at your mapping it is more of a Flux<ResponseEntity<Mono<Student>>>. Which is even more illogical. — M. Deinum
– M. Deinum, Commented Jun 9 at 15:19

JPG · Accepted Answer · 2025-06-09 07:34:04Z

0

below code resolved my problem.

    @GetMapping(value="student/name/{name}",produces = "text/event-stream")
    public ResponseEntity<Flux<Student>> getStudentByName(@PathVariable(name = "name",required = true)String name) {
        Flux<Student> flatMap2 = service.getStudentByName(name)
                .switchIfEmpty(Mono.error(new NoSuchElementException("student not found"))) 
                .flatMap(stu->Flux.just(stu));

        return new ResponseEntity<Flux<Student>>(flatMap2,HttpStatus.OK);
    }

answered Jun 9 at 7:34

JPG

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Comments

Abhijit Sarkar · Accepted Answer · 2025-06-09 07:47:08Z

0

You should be able to return a Flux<Student> directly just as you did in the service, without wrapping it in a ResponseEntity, more so because you hardcoded the status code. That said, returning HTTP 200 when the resource is not found is a fly in the face of REST.

answered Jun 9 at 7:47

Abhijit Sarkar

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4 Comments

JPG Jun 9 at 8:49

Mr. Sarkar, actually when the resource is not found, I am checking it inside switchIfEmpty and redirecting it to global exception handling for 404 status. Also if any other exception occurs. But you have raised a good question on how to handle errors in reactive, means there is something onError. How to use that with REST.

Toerktumlare Jun 9 at 19:13

if you are returning a Flux<Student> you are implicitly stating that there could be multiple Students returned. Depending on the mediatype set. if mediatype header is set to application/json webflux will buffer the returned students and when all are buffered it will return a list of students. if set to text/event-stream server will return students on by one as a server sent event (SSE stream). And if application/stream+json it will stream new line delimited json objects.

Toerktumlare Jun 9 at 19:13

Returning 404 tells the calling client that the endpoint called does not exist. Meaning, If you insist on return a flux with students, you should returning a empty list if someone asks for students that dont exist. Not 404

JPG Jun 10 at 7:47

ok @Toerktumlare I noted your both points. Also I am seeing some more approaches of handling exceptions discussed here stackoverflow.com/a/53596358/3422726

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