How to handle errors in Spring reactor Mono or Flux?

Question

I have below code retuning Mono<Foo>:

try {
    return userRepository.findById(id)  // step 1
        .flatMap(user -> barRepository.findByUserId( user.getId())  // step 2
        .map(bar-> Foo.builder().msg("Already exists").build())  // step 3
            .switchIfEmpty(barRepository.save(Bar.builder().userId(user.getId()).build())  // step 4
                .map(bar-> Foo.builder().msg("Created").build())   // step 5 
            ))
            .doOnError(throwable -> Mono.just(handleError(throwable)));
    } catch(Exception e) {

        log.error("from catch block");
        return Mono.just(handleError(e));

    }

If error occurs in step 1 (e.g. user does not exist by the specified id), will it be caught by doOnError or by try catch block or none of these two?

Same question if error happens in step 2, step3, step 4.

What is the correct code so that error is always caught by doOnError and eliminate try catch?

I am using public interface UserRepository extends ReactiveMongoRepository<User, String> same for barRepository.

handleError(throwable) simply does log.error(e.getMessage() and retuns Foo.

Answer 1

I think the first error is in the title: "Mono or Flux" is not related with the error handling.

• Mono can only emit one item at the most (streams one element)
• Flux can emit more complex stuff (i.e. List)

To handle errors you can follow this example:

return webClient.get()
                .uri(url)
                .retrieve()
                .bodyToMono(ModelYouAreRetrieving.class)
                .doOnError(throwable -> logger.error("Failed for some reason", throwable))
                .onErrorReturn(new ModelYouAreRetrieving(...))
                .block();