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.
@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).