标题起得比较诡异，其实并不是坑，而是jetty似乎压根就没做对异步request的ServletRequestListener的特殊处理，如果文中有错误欢迎提出，可能自己有所疏漏了。
之前遇到了一个bug，在Listener中重写requestDestroyed清理资源后，这些资源在异步任务中就不可用了。
这与预期不符，直觉上request应该在任务完成之后才触发requestDestroyed，而不应该是开始异步操作返回后就触发。
正确的触发时机应该是异步任务完成之后。
后来查阅了下，发现servlet 3的规范中只是增加了异步servlet和异步filter的支持，listener如何处理没有定义，也就是各个容器的实现可能会差异（我们自己使用的是jetty）。
但讲道理的话，合理的实现应该判断当前request是否处于异步处理中，如果是的话将销毁的触发的时机放置在AsyncContext完成之后。
自己试了下使用两个容器：Tomcat 8.5.15和Jetty 9.4.7.v20170914。
测试代码比较简单，为了去除其他框架的影响使用纯的servlet api。
servlet：
@WebServlet(asyncSupported = true, urlPatterns = { "/test" })
public class TestServlet extends HttpServlet {private static final long serialVersionUID = 7395865716615939512L;private ExecutorService pool = Executors.newCachedThreadPool();@Overridepublic void destroy() {pool.shutdown();}@Overrideprotected void doGet(HttpServletRequest request, HttpServletResponse response)throws ServletException, IOException {System.out.println(LocalDateTime.now() + " get start " + request);AsyncContext context = request.startAsync();context.addListener(new AsyncListener() {@Overridepublic void onTimeout(AsyncEvent event) throws IOException {System.out.println(LocalDateTime.now() + " async onTimeout " + event.getSuppliedRequest());}@Overridepublic void onStartAsync(AsyncEvent event) throws IOException {System.out.println(LocalDateTime.now() + " async onStartAsync " + event.getSuppliedRequest());}@Overridepublic void onError(AsyncEvent event) throws IOException {System.out.println(LocalDateTime.now() + " async onError " + event.getSuppliedRequest());}@Overridepublic void onComplete(AsyncEvent event) throws IOException {System.out.println(LocalDateTime.now() + " async onComplete " + event.getSuppliedRequest());}}, request, response);pool.execute(() -> {try {Thread.sleep(TimeUnit.SECONDS.toMillis(5));System.out.println(LocalDateTime.now() + " job done");context.complete();} catch (InterruptedException e) {e.printStackTrace();}});System.out.println(LocalDateTime.now() + " get return");}}listener:
@WebListener
public class TestListener implements ServletRequestListener {@Overridepublic void requestDestroyed(ServletRequestEvent event) {System.out.println(LocalDateTime.now() + " TestListener requestDestroyed " + event.getServletRequest());}@Overridepublic void requestInitialized(ServletRequestEvent event) {System.out.println(LocalDateTime.now() + " TestListener requestInitialized " + event.getServletRequest());}}使用tomcat返回的结果:
2017-11-19T16:42:04.256 TestListener requestInitialized org.apache.catalina.connector.RequestFacade@7c2df671
2017-11-19T16:42:04.257 get start org.apache.catalina.connector.RequestFacade@7c2df671
2017-11-19T16:42:04.261 get return
2017-11-19T16:42:09.261 job done
2017-11-19T16:42:09.261 async onComplete org.apache.catalina.connector.RequestFacade@7c2df671
2017-11-19T16:42:09.261 TestListener requestDestroyed org.apache.catalina.connector.RequestFacade@7c2df671使用jetty返回的结果：
2017-11-19T16:46:35.231 TestListener requestInitialized (GET //localhost:8080/test)@1124787543 org.eclipse.jetty.server.Request@430ae557
2017-11-19T16:46:35.232 get start (GET //localhost:8080/test)@1124787543 org.eclipse.jetty.server.Request@430ae557
2017-11-19T16:46:35.234 get return
2017-11-19T16:46:35.235 TestListener requestDestroyed [GET //localhost:8080/test]@1124787543 org.eclipse.jetty.server.Request@430ae557
2017-11-19T16:46:40.235 job done
2017-11-19T16:46:58.019 async onComplete [GET //localhost:8080/test]@1124787543 org.eclipse.jetty.server.Request@430ae557jetty果然没有讲道理... ...
先看下讲道理的tomcat，果不其然他做了特殊判断。
在StandardHostValve的invoke方法中：
if (!request.isAsync() && !asyncAtStart) {context.fireRequestDestroyEvent(request.getRequest());
}并且销毁的时机和预想的一样是在AsyncContext完成之后：
public void fireOnComplete() {List<AsyncListenerWrapper> listenersCopy = new ArrayList<>();listenersCopy.addAll(listeners);ClassLoader oldCL = context.bind(Globals.IS_SECURITY_ENABLED, null);try {for (AsyncListenerWrapper listener : listenersCopy) {try {listener.fireOnComplete(event);} catch (Throwable t) {ExceptionUtils.handleThrowable(t);log.warn("onComplete() failed for listener of type [" +listener.getClass().getName() + "]", t);}}} finally {context.fireRequestDestroyEvent(request.getRequest());clearServletRequestResponse();context.unbind(Globals.IS_SECURITY_ENABLED, oldCL);}}而jetty没有处理，在ContextHandler的doHandle中没有异步请求的判断：
finally{// Handle more REALLY SILLY request events!if (new_context){if (!_servletRequestListeners.isEmpty()){final ServletRequestEvent sre = new ServletRequestEvent(_scontext,request);for (int i=_servletRequestListeners.size();i-->0;)_servletRequestListeners.get(i).requestDestroyed(sre);}if (!_servletRequestAttributeListeners.isEmpty()){for (int i=_servletRequestAttributeListeners.size();i-->0;)baseRequest.removeEventListener(_servletRequestAttributeListeners.get(i));}}}new_content用来在第一次调用返回true之后是false，和异步处理无关。
网上查了些也不清楚为什么jetty没有像tomcat那样实现这个，还是jetty希望用户通过AsyncListener来做处理呢。
