How to handle errors in Spring reactor Mono or Flux?

问题

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.

回答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.

回答2:

The last line of the code of @James Ralston is wrong. The correct code should be:

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(Mono.just(handleError(throwable)));

回答3:

@Gianluca Pinto's last line of code is also incorrect. The code won't be compiled. onErrorReturn is not suitable for complicated error handling. What you should use is onErrorResume.

see: https://grokonez.com/reactive-programming/reactor/reactor-handle-error#21_By_falling_back_to_another_Flux

onErrorResume will fall back to another Flux and let you catch and manage the exception thrown by previous Flux. if look into the implementation of onErrorReturn, you will find onErrorReturn is actually using onErrorResume.

So here the code should be:

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

来源：https://stackoverflow.com/questions/51024279/how-to-handle-errors-in-spring-reactor-mono-or-flux

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Spring

project-reactor

reactive-streams