@RequestMapping(value = "/email/servletReq", method = GET)
public void servletReq (HttpServletRequest request, HttpServletResponse response) {
    AsyncContext asyncContext = request.startAsync();
    //设置监听器:可设置其开始、完成、异常、超时等事件的回调处理
    asyncContext.addListener(new AsyncListener() {
        @Override
        public void onTimeout(AsyncEvent event) throws IOException {
            System.out.println("超时了...");
            //做一些超时后的相关操作...
        }
        @Override
        public void onStartAsync(AsyncEvent event) throws IOException {
            System.out.println("线程开始");
        }
        @Override
        public void onError(AsyncEvent event) throws IOException {
            System.out.println("发生错误："+event.getThrowable());
        }
        @Override
        public void onComplete(AsyncEvent event) throws IOException {
            System.out.println("执行完成");
            //这里可以做一些清理资源的操作...
        }
    });
    //设置超时时间
    asyncContext.setTimeout(20000);
    asyncContext.start(new Runnable() {
        @Override
        public void run() {
            try {
                Thread.sleep(10000);
                System.out.println("内部线程：" + Thread.currentThread().getName());
                asyncContext.getResponse().setCharacterEncoding("utf-8");
                asyncContext.getResponse().setContentType("text/html;charset=UTF-8");
                asyncContext.getResponse().getWriter().println("这是异步的请求返回");
            } catch (Exception e) {
                System.out.println("异常："+e);
            }
            //异步请求完成通知
            //此时整个请求才完成
            asyncContext.complete();
        }
    });
    //此时之类 request的线程连接已经释放了
    System.out.println("主线程：" + Thread.currentThread().getName());
}