Issue with use of project reactor's flatMap and switchIfEmpty operators

问题

I have an issue with a reactive chain relying on flatMap() and switchIfEmpty(). For some reason, one of the Mono does not emit anything...

This is the public handler method calling the others:

//Throws: NoSuchElementException: Source was empty
public Mono<ServerResponse> createUser(ServerRequest serverRequest) {
    Hooks.onOperatorDebug();
    Mono<User> userMono = serverRequest.bodyToMono(User.class);
    return validateUser(userMono)
        .switchIfEmpty(saveUser(userMono))
        .single();
}

This is the first method called by createUser. Note that it is not called from a switchIfEmpty() (see above) and it does emit an error if there is any.

private Mono<ServerResponse> validateUser(Mono<User> userMono) {
    return userMono
        .map(this::computeErrors)
        .filter(AbstractBindingResult::hasErrors)
        .flatMap(err ->
            status(BAD_REQUEST)
                .contentType(APPLICATION_JSON)
                .body(BodyInserters.fromObject(err.getAllErrors()))
        );
}

This is just a helper method:

private AbstractBindingResult computeErrors(User user) {
    AbstractBindingResult errors = new BeanPropertyBindingResult(user, User.class.getName());
    userValidator.validate(user, errors);
    return errors;
}

This is the saveUser method. It does not emit any result!!. It is called from a switchIfEmpty (see above).

private Mono<ServerResponse> saveUser(Mono<User> userMono) {
    return userMono
        .flatMap(userRepository::save)
        .flatMap(newUser -> status(CREATED)
            .contentType(APPLICATION_JSON)
            .body(BodyInserters.fromObject(newUser))
        );
}

However, if I just call the saveUser method directly, it will emit a result.

//Works fine
public Mono<ServerResponse> createUser(ServerRequest serverRequest) {
    Hooks.onOperatorDebug();
    Mono<User> userMono = serverRequest.bodyToMono(User.class);
    return saveUser(userMono) // Compare this to the above version
        .single();
}

Can anyone please help figure out why the saveUser method does not emit anything when called from a switchIfEmpty()?

Here is the error I get:

java.util.NoSuchElementException: Source was empty
        at reactor.core.publisher.MonoSingle$SingleSubscriber.onComplete(MonoSingle.java:165) ~[reactor-core-3.3.0.RC1.jar:3.3.0.RC1]
        Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Assembly trace from producer [reactor.core.publisher.MonoSingleMono] :
        reactor.core.publisher.Mono.single(Mono.java:3898)
        org.example.contracttestingdemo.handler.UserHandler.createUser(UserHandler.java:55)

回答1:

It's not to do with flatMap() or switchIfEmpty() directly - it's because you're trying to consume the same Mono twice:

Mono<User> userMono = serverRequest.bodyToMono(User.class);
return validateUser(userMono)
    .switchIfEmpty(saveUser(userMono))
    .single();

In the above example, you're passing userMono first to validateUser(), and then to saveUser() (but by that point the User has already been emitted.)

If you want the publisher to be subscribed to multiple times, and output the same result, you'll need to cache it by calling serverRequest.bodyToMono(User.class).cache();.

来源：https://stackoverflow.com/questions/57836568/issue-with-use-of-project-reactors-flatmap-and-switchifempty-operators

标签

reactive-programming

project-reactor