在用tomcat来搞servlet异步化请求，当然异步化下异常处理也很重要，servlet3默认带一个监听器，可以监听onComplete，onError，onStartAsync，onTimeout事件，当然onTimeout可以选择在系统server.xml当中Connector 设置asyncTimeout="100"单位是毫秒，如果不设置，默认就是10秒，当然
Java代码
AsyncContext asyncContext = request.startAsync();  
asyncContext.setTimeout(100);  
在代码中也可以去设置，tomcat会以此为准，当然你必须得addListener。
 
 
调试中发现这100毫秒不会生效，当服务端返回超过100ms，可以请求就是不会报超时，这是为什么呢？看tomcat源码了
找啊找啊找，一切都明了了，原来超时线程在那sleep（1000）呢，因此要得到很短的超时并且能够生效，要么自己搞，要么改tomcat的源码吧
 
bio && apr实现

Java代码

/** 
* Async timeout thread 
*/  
protected class AsyncTimeout implements Runnable {  
   /** 
	* The background thread that checks async requests and fires the 
	* timeout if there has been no activity. 
	*/  
   @Override  
   public void run() {  

	   // Loop until we receive a shutdown command  
	   while (running) {  
		   try {  
			   Thread.sleep(1000);  
		   } catch (InterruptedException e) {  
			   // Ignore  
		   }  
		   long now = System.currentTimeMillis();  
		   Iterator<SocketWrapper<Socket>> sockets =  
			   waitingRequests.iterator();  
		   while (sockets.hasNext()) {  
			   SocketWrapper<Socket> socket = sockets.next();  
			   long access = socket.getLastAccess();  
			   if (socket.getTimeout() > 0 &&  
					   (now-access)>socket.getTimeout()) {  
				   processSocketAsync(socket,SocketStatus.TIMEOUT);  
			   }  
		   }  

		   // Loop if endpoint is paused  
		   while (paused && running) {  
			   try {  
				   Thread.sleep(1000);  
			   } catch (InterruptedException e) {  
				   // Ignore  
			   }  
		   }  

	   }  
   }  
}  
 

nio实现 
Java代码 
Poller线程当中，存在  
//process timeouts  
timeout(keyCount,hasEvents);  
也是在有新的事件到来，或者selectorTimeout=1000ms超时后，都会去触发  

// Async requests with a timeout of 0 or less never timeout  
  long delta = now - ka.getLastAccess();  
  long timeout = (ka.getTimeout()==-1)?((long) socketProperties.getSoTimeout()):(ka.getTimeout());  
  boolean isTimedout = delta > timeout;  
  if (isTimedout) {  
	  // Prevent subsequent timeouts if the timeout event takes a while to process  
	  ka.access(Long.MAX_VALUE);  
	  processSocket(ka.getChannel(), SocketStatus.TIMEOUT, true);  
  }