Log what method in reactive chain thew an exception

Question

I have some methods in my reactive chain that can throw exception. If an exception happens - I don't want to continue executing the chain, and I want to log - what exact action threw the exception.

I wrote a code, that looks something like this:

   Mono<Void> mono = Mono.just(10)
            .map(a -> action1(a))
            .doOnError((t) -> System.out.println("action 1 threw an exception"))
            .onErrorStop()
            .map(a -> action2(a)) 
            .doOnError((t) -> System.out.println("action 2 threw an exception"))
            .then();
    
    mono.subscribe();

but when action1 throws an exception - I get the following output:

action 1 threw an exception
action 2 threw an exception

what's not what I expected

Errors are propagated through the reactive chain. If you need these logs only to have a proper stack trace, then check out the debugging section in the reference: https://projectreactor.io/docs/core/release/reference/#debugging — Martin Tarjányi, Dec 17 '20 at 21:40

score 1 · Accepted Answer · answered Dec 21 '20 at 06:26

You need to think the reactive chain as a pipeline or a data flow.In the pipeline , the error is also a data as same as others.
The flatMap() method change the signal to another (Mono or Flux) when data is available.

Mono<Void> mono = Mono.just(10)
                .map(a -> action1(a))
                .doOnError((t) -> System.out.println("action 1 threw an exception"))
                .flatMap(a -> Mono.just(a)
                        .map(value -> action2(value))
                        .doOnError((t) -> System.out.println("action 2 threw an exception"))
                        .then()
                )
                .then();
        mono.subscribe();

Log what method in reactive chain thew an exception

1 Answers1