Spring WebFlux: Emit exception upon null value in Spring Data MongoDB reactive repositories?

Question

I'm trying to learn how to use MongoDB reactive repositories using spring-boot 2.0.0.M2, but I fear that I'm not doing things as intended.

This is one of my methods, which tries to find a User by their email. But if there is none, the method should throw an exception.

@Override
public Mono<User> findByEmail(String email) {
    User user = repository.findByEmail(email).block();
    if(user == null) {
        throw new NotFoundException("No user account was found with email: " + email);
    }
    return Mono.just(user);
}

The repository extends ReactiveCrudRepository<User, String>, but I fear that by using .block() I'm preventing this method from being reactive. I'm new to reactive programming, and I'm having a hard time to find good documentation on it. Can someone please point me in the right direction?

mp911de · Accepted Answer · 2017-07-29 18:54:39Z

23

Reactive programming requires a flow that is end-to-end reactive to gain the actual benefits that come from a reactive programming model. Calling .block() within the flow enforces synchronization and is considered an anti-pattern.

For your code, just propagate the Mono you obtain from ReactiveCrudRepository and apply the switchIfEmpty operator to emit an error signal if the Mono terminates without emitting a value. null values are not allowed in the Reactive Streams spec (the spec that Project Reactor is based on). If a result yields no value, then the Publisher does not emit a value.

@Override
public Mono<User> findByEmail(String email) {

    Mono<User> fallback = Mono.error(new NotFoundException("No user account was found with email: " + email));
    return repository.findByEmail(email).switchIfEmpty(fallback);
}

1 Comment

Andreas Over a year ago

Thanks, I had totally missed the switchIfEmpty(...) method.

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