LongRunningTaskService.java

    public <T> Mono<T> executeMono(Supplier<T> task) {
        CompletableFuture<T> future = CompletableFuture.supplyAsync(task);
        return Mono.fromFuture(future);
    }

    public <T> Flux<T> executeFlux(Supplier<List<T>> task) {
        return Flux.create((sink) -> {
            CompletableFuture<List<T>> future = CompletableFuture.supplyAsync(task);
            future.whenComplete((list, exception) -> {
                if (exception == null) {
                    list.forEach(sink::next);
                    sink.complete();
                } else {
                    sink.error(exception);
                }
            });
        });
    }