How to throw an Error explicitly while processing Mono

Question

I need to know how to throw an exception explicitly while executing the Mono. I'm not able to subscribe, block Mono because it has end (down-stream) subscriber. Just need to validate the data and throw exception explicitly to perform retry.

    public Mono<Response> handleResponse() {
        return userService.getUser()
                .doOnNext(response -> validate(response.getData()))
                .onErrorResume(ex -> {
                    // not working
                    throw Exceptions.propagete(ex);
                });
    }
    
    private void validate() {
        .....
        throw new RuntimeException();
    }

@Retyable(value = {RuntimeException.class})

If no exception, it will return Mono to next level. I can use Mono retry itself but need to know is there any way to solve this.

Answer 1

To produce an error you can use Mono.error(throwable) .

But the another problem in your code that you do the validation in doOnNext , which is not supposed to change the flow. Better to use flatMap .

You can do something like:

userService.getUser()
            .flatMap(response -> {
               try {
                  validate(response.getData());
                  return Mono.just(response);
               } catch (Throwable t) {
                  return Mono.error(t);
               }
            })

Answer 2

The simplest way would be the following:

getUser()
    .map(response -> {
        validate(response);
        return response;
    })

If it's possible for you to make changes to the validate method, I would suggest doing it in the following manner:

getUser()
    .handle((response, sink) -> {
        if (isValid(response)) {
            sink.next(response);
        } else {
            sink.error(new RuntimeException("..."));
        }
    })

// where
public boolean isValid(User user) {
    //...
}

Answer 3

What if you try something like this:

    public Mono<Response> handleResponse() {
            return Mono.fromFuture(userService.getUser()
                    .doOnNext(response -> validate(response.getData()))
                    .handle((response, throwable) -> {
                          if (throwable == null) {
                              return response;
                          } else {
                              throw new RuntimeException();
                          }
                     });
    }