对相同 url 的请求在 http nio 线程池中被阻止

Question

我简单的在springboot(1.5.4.RELEASE)应用中创建了一个RestController，测试了多个请求进来时如何工作。让我困惑的是：

相同url：第二个请求阻塞，直到第一个请求执行
不同url：非阻塞，2个请求几乎同时执行

我的问题是谁阻止了第二个请求，为什么？

测试代码：

@GetMapping(value = "/sleep")
public String sleep(HttpServletRequest request, @RequestParam boolean status) 
{
    String requestId = request.toString();
    logger.info("request  [{}] in  and status = {}.", requestId, status);
    String result;
    if (status)
    {
        Thread.currentThread().sleep(10 * 1000);
        result = "slept";
    }
    else
    {
        result = "stay up";
    }
    logger.info("response [{}] out and result = [{}].", requestId, result);
    return result;
}

测试结果：

不同url：不阻塞，几乎同时开始执行

http://localhost:20002/sleep?status=false AND http://localhost:20002/sleep?status=true

2018-08-14 15:04:14.139  [http-nio-20002-exec-5]: connection [RequestFacade@46515328] in  and status = true.
2018-08-14 15:04:16.452  [http-nio-20002-exec-6]: connection [RequestFacade@1140f857] in  and status = false.
2018-08-14 15:04:16.452  [http-nio-20002-exec-6]: connection [RequestFacade@1140f857] out and result = [stay up].
2018-08-14 15:04:24.139  [http-nio-20002-exec-5]: connection [RequestFacade@46515328] out and result = [slept].

相同url：块，第一个请求完成后，第二个请求才会执行。

http://localhost:20002/sleep?status=true AND http://localhost:20002/sleep?status=true

2018-08-14 15:10:29.943  [http-nio-20002-exec-9]: connection [RequestFacade@46515328] in  and status = true.
2018-08-14 15:10:39.944  [http-nio-20002-exec-9]: connection [RequestFacade@46515328] out and result = [slept].
2018-08-14 15:10:39.960  [http-nio-20002-exec-1]: connection [RequestFacade@1140f857] in  and status = true.
2018-08-14 15:10:49.960  [http-nio-20002-exec-1]: connection [RequestFacade@1140f857] out and result = [slept].

我调试了tomcat-embed-core的代码，发现当有不同url的request进来时，Poller线程可以立即捕获并处理；当具有相同 url 的请求进入时，轮询器在返回第一个连接之前无法获取它。

Answer 1

感谢@M.Deinum，阻塞与NIO无关。是 chrome 阻止了第二个请求，出于原因，我在下面找到一些评论：

this behavior is due to Chrome locking the cache and waiting to see the result of one 
request before requesting the same resource again.

并且通过chrome Network -> Timing 证明，第2个请求处于[stalled]状态，直到第1个响应返回。

connection_stalled

对相同 url 的请求在 http nio 线程池中被阻止

Requests to same url blocked in http nio thread pool

tomcat

nio

spring-boot

spring-restcontroller