WebFlux functional: How to detect an empty Flux and return 404?

Question

I'm having the following simplified handler function (Spring WebFlux and the functional API using Kotlin). However, I need a hint how to detect an empty Flux and then use noContent() for 404, when the Flux is empty.

fun findByLastname(request: ServerRequest): Mono<ServerResponse> {
    val lastnameOpt = request.queryParam("lastname")
    val customerFlux = if (lastnameOpt.isPresent) {
        service.findByLastname(lastnameOpt.get())
    } else {
        service.findAll()
    }
    // How can I detect an empty Flux and then invoke noContent() ?
    return ok().body(customerFlux, Customer::class.java)
}

Answer 1

From a Mono:

return customerMono
           .flatMap(c -> ok().body(BodyInserters.fromObject(c)))
           .switchIfEmpty(notFound().build());

From a Flux:

return customerFlux
           .collectList()
           .flatMap(l -> {
               if(l.isEmpty()) {
                 return notFound().build();

               }
               else {
                 return ok().body(BodyInserters.fromObject(l)));
               }
           });

Note that collectList buffers data in memory, so this might not be the best choice for big lists. There might be a better way to solve this.

Answer 2

In addition to the solution of Brian, if you are not want to do an empty check of the list all the time, you could create a extension function:

fun <R> Flux<R>.collectListOrEmpty(): Mono<List<R>> = this.collectList().flatMap {
        val result = if (it.isEmpty()) {
            Mono.empty()
        } else {
            Mono.just(it)
        }
        result
    }

And call it like you do it for the Mono:

return customerFlux().collectListOrEmpty()
                     .switchIfEmpty(notFound().build())
                     .flatMap(c -> ok().body(BodyInserters.fromObject(c)))

Answer 3

I'm not sure why no one is talking about using the hasElements() function of Flux.java which would return a Mono.

Answer 4

Use Flux.hasElements() : Mono<Boolean> function:

return customerFlux.hasElements()
                   .flatMap {
                     if (it) ok().body(customerFlux)
                     else noContent().build()
                   }