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

DoOnError will only perform side effects and assuming the findById are will return a Mono.Error() if it fails something like this should work.

return userRepository.findById(id)
    .flatMap ( user -> 
        barRepository.findByUserId(user.getId())
        .map((user,bar)-> Foo.builder().msg("Already exists").build())  
        .switchIfEmpty(barRepository.save(Bar.builder().userId(user.getId()).build())
        .map(bar-> Foo.builder().msg("Created").build())

    ))
    .onErrorReturn(throwable -> Mono.just(handleError(throwable)));

The try catch will only work if you either call a blocking operation of the chain, or a runtime error occurs before you enter the reactive chain. the doOn operations do not modify the chain, they are used for side effects only. Since flatMap expects a producer, you will need to return a Mono from the call, and in this case if an error occurs, then it will just propagate the error. In all reactive chains the error will propagate unless otherwise handled.