onErrorResume not working as expected

Question

I'm trying to use WebFlux and I'm seeing a behavior I don't quite understand, I suspect this is a bug in WebFlux or possibly Reactor, but I need confirmation.

I've attempted to create a minimally reproducible case that consist of a very simple HandlerFunction that attempts to return an 200 response, but throws an exception during body creation and then attempts to use onErrorResume to instead return a 404 response.

The handler looks like so:

public Mono<ServerResponse> reproduce(ServerRequest request){
        return ServerResponse.ok()
        .contentType(APPLICATION_JSON)
        .body(Mono.fromCallable(this::trigger),String.class)
        .onErrorResume(MinimalExampleException.class,e->ServerResponse.notFound().build());
}

I would expect when calling the associated endpoint that I would get a 404 response. Instead what I'm seeing is a 500 response with log messages indicating Spring believes there was an unhandled exception during request processing.

When I breakpoint inside of onErrorResume I see two handlers being registered, the one I register in the method above, as well as one that's being registered by Spring (inside of RouterFunctions.toHttpHandler) for instances of ResponseStatusException. Then during processing of the request I see only the second handler (the one registered by Spring) being called, not matching on the exception being thrown and then falling through to the root level handler.

Near as I can tell, Spring is overwriting onErrorResume at the router level preventing the one I registered in the Handler from being called. Is this the expected behavior? Is there a better way to accomplish what I'm attempting?

Answer 1

The framework indeed has a "catch-all" that is invoked before whatever follows .body(...) can be invoked. This is by design and I think it will be hard to avoid.

I see 3 solutions:

1. Turn the operators chain around

Like so:

return Mono.fromCallable(this::trigger)
           .flatMap(s -> ServerResponse.ok()
                        .contentType(MediaType.TEXT_PLAIN)
                        .syncBody(s))
           .onErrorResume(MinimalExampleException.class,
                        e -> ServerResponse.notFound().build());

2. Use a ResponseStatusException

You could put the onErrorResume at the same level as the fromCallable() (inside the body call) to transform the specific error into a ResponseStatusException via an error mono:

Mono.fromCallable(this::trigger)
    .onErrorResume(MinimalExampleException.class,
        e->Mono.error(new ResponseStatusException(404, "reason message", e)))

3. Use an exception handler

You should be able to annotate a method with @ExceptionHandler to deal with your particular exception.