How to check Flux<Object> is empty or not?

I have a api for kubernetes to call and detect service is available or not.In the api,first call a interface to get host of other service,the interface return a Flux,if the result is empty api return SERVICE_UNAVAILABLE other return ok.My current code as below:

@GetMapping(value = "/gateway/readiness")
public Mono<Long> readiness(ServerHttpResponse response) {
    Flux<Map<String, List<String>>> hosts = hostProvider.getHosts();
    List<String> hostProviders = new ArrayList<>();
    
    // the below code has a warning: Calling subscribe in a non-blocking scope
    hosts.subscribe(new Consumer<Map<String, List<String>>>() {
        @Override
        public void accept(Map<String, List<String>> stringListMap) {
            hostProviders.addAll(stringListMap.keySet());
        }
    });
    if (hostProviders.isEmpty()) {
        response.setStatusCode(HttpStatus.SERVICE_UNAVAILABLE);
    }
    return routeLocator.getRoutes().count();
}

Has graceful to do this?

Please, try the following:

@GetMapping(value = "/gateway/readiness")
public Mono<ServerResponse> readiness() {
    return hostProvider.getHosts()
            .map(Map::keySet)
            .flatMap(set -> Flux.fromStream(set.stream()))
            .collectList()
            .flatMap(hostProviders -> 
               // response whatever you want to the client
               ServerResponse.ok().bodyValue(routeLocator.getRoutes().count())
            )
            // if no value was propagated from the above then send 503 response code
            .switchIfEmpty(ServerResponse.status(HttpStatus.SERVICE_UNAVAILABLE).build());
}