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.