Spring Webflux with Spring MVC - nulls emitted when using doOnEach

Question

I'm experimenting a bit with Spring Webflux and Spring MVC, and encountered an interesting case.

Starting with a simple controller:

@GetMapping
public Mono<String> list(final Model model) {
    Flux<User> users = this.userRepository.findAll();
    model.addAttribute("users", users);
    return Mono.just("users/list");
}

The userReposutory is a custom ConcurrentHashMap-based implementation. Here you can find the findAll method:

@Override
public Flux<User> findAll() {
    return Flux.fromIterable(this.users.values());
}

Whenever I try to return to access the "users/list" view, everything seems to be working properly.

But, if I try to rewrite the controller using an idiomatic reactive approach, problems start appearing:

@GetMapping
public Mono<String> list(final Model model) {
    return this.userRepository.findAll()
      .collectList()
      .doOnEach(users -> model.addAttribute("users", users.get()))
      .map(u -> "users/list");
}

If I hit the endpoint, I'm getting this in logs:

java.lang.IllegalArgumentException: ConcurrentModel does not support null attribute value
    at org.springframework.util.Assert.notNull(Assert.java:193)
    at org.springframework.ui.ConcurrentModel.addAttribute(ConcurrentModel.java:75)
    at org.springframework.ui.ConcurrentModel.addAttribute(ConcurrentModel.java:39)
    at com.baeldung.lss.web.controller.UserController.lambda$list$0(UserController.java:37)
    at reactor.core.publisher.FluxDoOnEach$DoOnEachSubscriber.onError(FluxDoOnEach.java:132)

Apparently, some stray null is making its way there. Let's filter out all of them eagerly then:

@RequestMapping
public Mono<String> list(final Model model) {
    return this.userRepository.findAll()
      .filter(Objects::nonNull)
      .collectList()
      .filter(Objects::nonNull)
      .doOnEach(users -> model.addAttribute("users", users.get()))
      .map(u -> "users/list");
}

Same problem, but... if I squeeze everything in a map() call, everything works again:

@GetMapping
public Mono<String> list(final Model model) {
    return this.userRepository.findAll()
      .collectList()
      .map(users -> {
          model.addAttribute("users", users);
          return "users/list";
      });
}

Although, placing side-effects in map is not optimal.

Any ideas what's wrong with the doOnEach() here?

Answer 1

Very nice question. Let's see what the JavaDocs tell about doOnEach:

public final Mono<T> doOnEach(Consumer<? super Signal<T>> signalConsumer)

Add behavior triggered when the Mono emits an item, fails with an error or completes successfully. All these events are represented as a Signal that is passed to the side-effect callback

Curious. The users in doOnEach(users -> ...) is not an List<User> but a Signal<List<User>>. This Signal<T> object won't be null, which explains why the filter methods in the second version don't work.

The JavaDocs for Signal<T> says that the get() method is explicitly marked as @Nullable and will return a non-null value only on next item arrives. If the completion or error signal is generated, then it will return null.

Solutions:

1. Use doOnNext instead: You are interested in the next value, not any signal that comes from the source stream.
2. Do a null-check in doOnEach lambda: This will work too, but since you're not interested in other events, is superfluous.