Spring 5 的 Reactive WebClient 不那么异步？

Question

我遇到了奇怪的 Spring WebClient 行为。我有两个网址，慢的和快的。机器人什么都不做，但慢一点，只需等待十秒钟即可响应。当我使用 WebClient 同时调用它们时，我希望快速 URL 会比慢速 URL 更早完成，但实际上它们都是同时完成的。最糟糕的是 - 有时它会按预期工作。有没有人想过，为什么它会以这种方式运行，以及如何使它正常工作？这是我的例子

fun main() {
    val webClient = WebClient.create()
    println("${LocalDateTime.now()} [${Thread.currentThread().name}] Start")

    webClient.get().uri("http://callback-mock/slow-url").exchange()
       .subscribe { response -> 
            println("${LocalDateTime.now()} [${Thread.currentThread().name}] Executed callback slow URL with result ${response.statusCode()}") 
       }
    webClient.get().uri("http://callback-mock/fast-url").exchange()
       .subscribe { response -> 
            println("${LocalDateTime.now()} [${Thread.currentThread().name}] Executed callback fast URL with result ${response.statusCode()}")
       }
    println("${LocalDateTime.now()} [${Thread.currentThread().name}] Waiting for exit")
    Thread.sleep(15_000)
    println("${LocalDateTime.now()} [${Thread.currentThread().name}] Exit")
}

结果（大多数情况下）

2019-10-02T13:04:34.536 [main] Start
2019-10-02T13:04:35.173 [main] Waiting for exit
2019-10-02T13:04:44.791 [reactor-http-nio-4] Executed callback slow URL with result 200 OK
2019-10-02T13:04:44.791 [reactor-http-nio-2] Executed callback fast URL with result 200 OK
2019-10-02T13:04:50.193 [main] Exit

Process finished with exit code 0

在极少数情况下，它会按预期工作

2019-10-02T13:23:35.619 [main] Start
2019-10-02T13:23:36.300 [main] Waiting for exit
2019-10-02T13:23:36.802 [reactor-http-nio-2] Executed callback fast URL with result 200 OK
2019-10-02T13:23:45.810 [reactor-http-nio-4] Executed callback slow URL with result 200 OK
2019-10-02T13:23:51.308 [main] Exit

Process finished with exit code 0

Answer 1

以下非常简单的测试表明fast 总是首先返回。 （Reactor Netty 用作 HTTP 服务器）

    @Test
    public void test() throws InterruptedException {
        DisposableServer server =
                HttpServer.create()
                        .port(0)
                        .route(r -> r.get("/fast", (req,res) -> res.sendString(Mono.just("test")))
                                    .get("/slow", (req,res) -> res.sendString(Mono.just("test").delayElement(Duration.ofSeconds(10)))))
                        .bindNow();

        WebClient webClient = WebClient.create();
        System.out.println(LocalDateTime.now() + " " + Thread.currentThread().getName() + " Start");

        webClient.get().uri("http://localhost:" + server.port() + "/slow").exchange()
                .subscribe(response ->
                        System.out.println(LocalDateTime.now() + " " + Thread.currentThread().getName() +
                                " Executed callback slow URL with result " + response.statusCode()));

        webClient.get().uri("http://localhost:" + server.port() + "/fast").exchange()
                .subscribe(response ->
                        System.out.println(LocalDateTime.now() + " " + Thread.currentThread().getName() +
                                " Executed callback fast URL with result " + response.statusCode()));

        System.out.println(LocalDateTime.now() + " " + Thread.currentThread().getName() + " Waiting for exit");
        Thread.sleep(15_000);
        System.out.println(LocalDateTime.now() + " " + Thread.currentThread().getName() + " Exit");

        server.disposeNow();
    }