Reactor fireAndForget Mono 传递上下文

Question

I am trying to pass subscriber Context to the fireAndForget method which is called inside the doOnNext . The fireAndForget is run also async non-blocking. How this context might be passed so the value for "key" is present? When I run the following test it passes. However, in the logs I can see that for both doOnNext I get:

reactor.core.Exceptions$ErrorCallbackNotImplemented: java.util.NoSuchElementException: Context is empty

@Test
void shouldPassContextToFireAndForget() {
  final Mono<String> helloWorldMono = Mono.just("hello")
      .doOnNext(this::fireAndForget)
      .doOnNext(name -> Mono.deferContextual(contextView -> fireAndForget(contextView, name)).subscribe())
      .flatMap(name -> Mono.deferContextual(contextView -> Mono.just(name + " " + contextView.get("key"))))
      .contextWrite(Context.of("key", "world"));

  StepVerifier.create(helloWorldMono)
      .expectNext("hello world")
      .verifyComplete();
}

private Mono<String> fireAndForget(ContextView context, String name) {
  return Mono.just(name)
      .flatMap(value -> Mono.deferContextual(contextView -> Mono.just(value + contextView.get("key"))))
      .contextWrite(context);
}

private void fireAndForget(String name) {
  Mono.just(name)
      .flatMap(value -> Mono.deferContextual(contextView -> Mono.just(value + contextView.get("key"))))
      .subscribe();
}

Answer 1

Context is a subscribe-time concept. There are two possible approaches.

You can expose the ContextView at the middle of the chain using transformDeferredContextual :

 final Mono<String> helloWorldMono = Mono.just("hello")
            .transformDeferredContextual((original, cntx) -> original.doOnNext(name-> fireAndForget(cntx, name).subscribe()))
            .flatMap(name -> Mono.deferContextual(contextView -> Mono.just(name + " " + contextView.get("key"))))
            .contextWrite(Context.of("key", "world"));

Alternatively, you could take advantage of Mono.deferContextual in order to expose the ContextView at the start of the chain like this:

 final Mono<String> helloWorldMono = Mono.deferContextual(context ->
        Mono.just("hello")
            .doOnNext(name -> fireAndForget(context, name).subscribe())
            .flatMap(name -> Mono.just(name + " " + context.get("key")))
    ).contextWrite(Context.of("key", "world"));