Spring Reactive WebFlux - how to customize the BadRequest error message

Question

In my request handler, if the passed-in accountId cannot be converted to a valid ObjectId I want to catch the error and send back a meaningful message; however, doing so causes the return type to be incompatible, and I cannot figure out how to achieve this pretty trivial use case.

My code:

 @GetMapping("/{accountId}") public Mono<ResponseEntity<Account>> get(@PathVariable String accountId) { log.debug(GETTING_DATA_FOR_ACCOUNT, accountId); try { ObjectId id = new ObjectId(accountId); return repository.findById(id) .map(ResponseEntity::ok) .switchIfEmpty(Mono.just(ResponseEntity.notFound().build())); } catch (IllegalArgumentException ex) { log.error(MALFORMED_OBJECT_ID, accountId); // TODO(marco): find a way to return the custom error message. This seems to be currently // impossible with the Reactive API, as using body(message) changes the return type to // be incompatible (and Mono<ResponseEntity<?>> does not seem to cut it). return Mono.just(ResponseEntity.badRequest().build()); } }

The body(T body) method changes the type of the returned Mono so that it is (assuming one just sends a String) a Mono<ResponseEntity<String>>; however, changing the method's return type to Mono<ResponseEntity<?>> does not work either:

 ... return Mono.just(ResponseEntity.badRequest().body( MALFORMED_OBJECT_ID.replace("{}", accountId)));

as it gives an "incompatible type" error on the other return statement:

error: incompatible types: Mono<ResponseEntity<Account>> cannot be converted to Mono<ResponseEntity<?>> .switchIfEmpty(Mono.just(ResponseEntity.notFound().build()));

Obviously, changing the return type of the method to Mono<?> would work, but the response then is the serialized JSON of the ResponseEntity which is NOT what I want.

I have also tried using the onErrorXxxx() methods, but they do not work here either, as the conversion error happens even before the Flux is computed, and I just get a "vanilla" 400 error with an empty message.

The only way I can think of working around this would be to add a message field to my Account object and return that one, but it's genuinely a horrible hack.

you need to read up on the basics of reactive programming. You should not use try/catch, if you wish to return an error to the client should return a Mono.error containing a ResponseStatusException. I suggest reading the getting started guide in the reactor documentation. projectreactor.io/docs/core/release/reference/#getting-started — Toerktumlare
– Toerktumlare, Commented Sep 1, 2020 at 8:33

Marco Massenzio · Accepted Answer · 2020-09-02 05:07:45Z

@thomas-andolf's answer helped me figure out the actual solution.

For anyone stumbling upon this in future, here is how I actually solved the puzzle (and, yes, you still need the try/catch to intercept the error thrown by the ObjectId constructor):

 @GetMapping("/{accountId}") public Mono<ResponseEntity<Account>> get(@PathVariable String accountId) { return Mono.just(accountId) .map(acctId -> { try { return new ObjectId(accountId); } catch (IllegalArgumentException ex) { throw new ResponseStatusException(HttpStatus.BAD_REQUEST, MALFORMED_OBJECT_ID)); } }) .flatMap(repository::findById) .map(ResponseEntity::ok) .switchIfEmpty(Mono.just(ResponseEntity.notFound().build())); }

To actually see the message in the returned body, you will need to add server.error.include-message=always in application.properties (see here).

Using onError() won't work here (I did try that, in all its variants) as it requires a Mono<ResponseEntity<Account>> and there is no way to generate one from the error status (when adding the message body).

Not sure why someone down-voted this, I literally tried the code in the "accepted" answer and it doesn't compile (nor it does what I intended to do in my original question). This one compiles and does return the error message to the caller.
I had the same issue and your question helped to solve mine (stackoverflow.com/a/67413913/2096986). your solution with try/catch works although it is not using the functional style which is the best choice since you are using reactive programming.
glad it helped, @Felipe - if it did, please do upvote it, so it will help others: someone (who didn't even bother to comment and explain themselves) down-voted it, so it shows lower than the (upvoted, but incorrect) one. As I mention also above, the try/catch is necessary here because the library method I'm calling does not use functional/reactive style: that method is given and cannot be modified; not using a try/catch here causes the exception to propagate and breaks the reactive functional chain.

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