Servlet 3特性：异步Servlet

「爱情、让人受尽委屈。」 2022-01-16 02:49 261阅读 0赞

[2019独角兽企业重金招聘Python工程师标准>>> ][2019_Python_] ![hot3.png][]

理解异步Servlet之前，让我们试着理解为什么需要它。假设我们有一个Servlet需要很多的时间来处理，类似下面的内容：

**LongRunningServlet.java**

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   <td style="border:0px;vertical-align:baseline;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;"> 
    <div> 
     <div>
       packagecom.journaldev.servlet; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       importjava.io.IOException; 
     </div> 
     <div>
       importjava.io.PrintWriter; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       importjavax.servlet.ServletException; 
     </div> 
     <div>
       importjavax.servlet.annotation.WebServlet; 
     </div> 
     <div>
       importjavax.servlet.http.HttpServlet; 
     </div> 
     <div>
       importjavax.servlet.http.HttpServletRequest; 
     </div> 
     <div>
       importjavax.servlet.http.HttpServletResponse; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       @WebServlet("/LongRunningServlet") 
     </div> 
     <div>
       publicclassLongRunningServletextendsHttpServlet { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;privatestaticfinallongserialVersionUID = 1L; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;protectedvoiddoGet(HttpServletRequest request, 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;HttpServletResponse response)throwsServletException, IOException { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;longstartTime = System.currentTimeMillis(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("LongRunningServlet Start::Name=" 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ Thread.currentThread().getName() +"::ID=" 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ Thread.currentThread().getId()); 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;String time = request.getParameter("time"); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;intsecs = Integer.valueOf(time); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// max 10 seconds 
     </div> 
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       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if(secs &gt;10000) 
     </div> 
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       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;secs =10000; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;longProcessing(secs); 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;PrintWriter out = response.getWriter(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;longendTime = System.currentTimeMillis(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;out.write("Processing done for "+ secs +" milliseconds!!"); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("LongRunningServlet Start::Name=" 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ Thread.currentThread().getName() +"::ID=" 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ Thread.currentThread().getId() +"::Time Taken=" 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ (endTime - startTime) +" ms."); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;privatevoidlongProcessing(intsecs) { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// wait for given time before finishing 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try{ 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Thread.sleep(secs); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}catch(InterruptedException e) { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;e.printStackTrace(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       } 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

如果我们的URL是：[http://localhost:8080/AsyncServletExample/LongRunningServlet?time=8000][http_localhost_8080_AsyncServletExample_LongRunningServlet_time_8000]

得到响应为“Processing done for 8000 milliseconds! !“。现在，如果你会查看服务器日志，会得到以下记录:

所以Servlet线程实际运行超过 8秒，尽管大多数时间用来处理其它Servlet请求或响应。

这可能导致线程饥饿——因为我们的Servlet线程被阻塞，直到所有的处理完成。如果服务器的请求得到了很多过程，它将达到最大Servlet线程限制和进一步的请求会拒绝连接错误。

Servlet 3.0之前，这些长期运行的线程容器特定的解决方案，我们可以产生一个单独的工作线程完成耗时的任务，然后返回响应客户。Servlet线程返回Servlet池后启动工作线程。Tomcat 的 Comet、WebLogic FutureResponseServlet 和 WebSphere Asynchronous Request Dispatcher都是实现异步处理的很好示例。

容器特定解决方案的问题在于，在不改变应用程序代码时不能移动到其他Servlet容器。这就是为什么在Servlet3.0提供标准的方式异步处理Servlet的同时增加异步Servlet支持。

**实现异步Servlet**

让我们看看步骤来实现异步Servlet，然后我们将提供异步支持Servlet上面的例子：

1.  首先Servlet,我们提供异步支持 Annotation @WebServlet 的属性asyncSupported 值为true。
2.  由于实际实现委托给另一个线程，我们应该有一个线程池实现。我们可以一个通过[Executors framework][] 创建线程池和使用servlet context listener来初始化线程池。
3.  通过ServletRequest.startAsync方法获取AsyncContext的实例。AsyncContext提供方法让ServletRequest和ServletResponse对象引用。它还提供了使用调度方法将请求转发到另一个 dispatch() 方法。
4.  编写一个可运行的实现,我们将进行重处理，然后使用AsyncContext对象发送请求到另一个资源或使用ServletResponse编写响应对象。一旦处理完成，我们通过AsyncContext.complete()方法通知容器异步处理完成。
5.  添加AsyncListener实现AsyncContext对象实现回调方法，我们可以使用它来提供错误响应客户端装进箱的错误或超时，而异步线程处理。在这里我们也可以做一些清理工作。

一旦我们将完成我们的项目对于异步Servlet示例，项目结构看起来会像下面的图片：

[![26175305_D3JH.png][]][26175305_D3JH.png 1]

**在监听中初始化线程池**

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   <td style="border:0px;vertical-align:baseline;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;"> 
    <div> 
     <div>
       packagecom.journaldev.servlet.async; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       importjava.util.concurrent.ArrayBlockingQueue; 
     </div> 
     <div>
       importjava.util.concurrent.ThreadPoolExecutor; 
     </div> 
     <div>
       importjava.util.concurrent.TimeUnit; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       importjavax.servlet.ServletContextEvent; 
     </div> 
     <div>
       importjavax.servlet.ServletContextListener; 
     </div> 
     <div>
       importjavax.servlet.annotation.WebListener; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       @WebListener 
     </div> 
     <div>
       publicclassAppContextListenerimplementsServletContextListener { 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;publicvoidcontextInitialized(ServletContextEvent servletContextEvent) { 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// create the thread pool 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ThreadPoolExecutor executor =newThreadPoolExecutor(100,200, 50000L, 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;TimeUnit.MILLISECONDS,newArrayBlockingQueue&lt;Runnable&gt;(100)); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;servletContextEvent.getServletContext().setAttribute("executor", 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;executor); 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;publicvoidcontextDestroyed(ServletContextEvent servletContextEvent) { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ThreadPoolExecutor executor = (ThreadPoolExecutor) servletContextEvent 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;.getServletContext().getAttribute("executor"); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;executor.shutdown(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       } 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

实现很直接，如果你不熟悉ThreadPoolExecutor 框架请读[线程池的ThreadPoolExecutor][Executors framework] 。关于listeners 的更多细节，请阅读[教程Servlet Listener][Servlet Listener]。

**工作线程实现**

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    <div> 
     <div>
       packagecom.journaldev.servlet.async; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       importjava.io.IOException; 
     </div> 
     <div>
       importjava.io.PrintWriter; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       importjavax.servlet.AsyncContext; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       publicclassAsyncRequestProcessorimplementsRunnable { 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;privateAsyncContext asyncContext; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;privateintsecs; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;publicAsyncRequestProcessor() { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;publicAsyncRequestProcessor(AsyncContext asyncCtx,intsecs) { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;this.asyncContext = asyncCtx; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;this.secs = secs; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;@Override 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;publicvoidrun() { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("Async Supported? " 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ asyncContext.getRequest().isAsyncSupported()); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;longProcessing(secs); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try{ 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;PrintWriter out = asyncContext.getResponse().getWriter(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;out.write("Processing done for "+ secs +" milliseconds!!"); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}catch(IOException e) { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;e.printStackTrace(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//complete the processing 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;asyncContext.complete(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;privatevoidlongProcessing(intsecs) { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// wait for given time before finishing 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try{ 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Thread.sleep(secs); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}catch(InterruptedException e) { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;e.printStackTrace(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       } 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

**注意：**在请求和响应时使用AsyncContext对象，然后在完成时调用 asyncContext.complete() 方法。

**AsyncListener 实现**

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   <td style="border:0px;vertical-align:baseline;font-family:Consolas, 'Bitstream Vera Sans Mono', 'Courier New', Courier, monospace;"> 
    <div> 
     <div>
       packagecom.journaldev.servlet.async; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       importjava.io.IOException; 
     </div> 
     <div>
       importjava.io.PrintWriter; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       importjavax.servlet.AsyncEvent; 
     </div> 
     <div>
       importjavax.servlet.AsyncListener; 
     </div> 
     <div>
       importjavax.servlet.ServletResponse; 
     </div> 
     <div>
       importjavax.servlet.annotation.WebListener; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       @WebListener 
     </div> 
     <div>
       publicclassAppAsyncListenerimplementsAsyncListener { 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;@Override 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;publicvoidonComplete(AsyncEvent asyncEvent)throwsIOException { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("AppAsyncListener onComplete"); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// we can do resource cleanup activity here 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;@Override 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;publicvoidonError(AsyncEvent asyncEvent)throwsIOException { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("AppAsyncListener onError"); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//we can return error response to client 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;@Override 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;publicvoidonStartAsync(AsyncEvent asyncEvent)throwsIOException { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("AppAsyncListener onStartAsync"); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//we can log the event here 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;@Override 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;publicvoidonTimeout(AsyncEvent asyncEvent)throwsIOException { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("AppAsyncListener onTimeout"); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;//we can send appropriate response to client 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ServletResponse response = asyncEvent.getAsyncContext().getResponse(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;PrintWriter out = response.getWriter(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;out.write("TimeOut Error in Processing"); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       } 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

通知的实现在 Timeout()方法，通过它发送超时响应给客户端。

**Async Servlet 实现**

这是我们的异步Servlet实现，注意使用AsyncContext和ThreadPoolExecutor进行处理。

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    <div> 
     <div>
       packagecom.journaldev.servlet.async; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       importjava.io.IOException; 
     </div> 
     <div>
       importjava.util.concurrent.ThreadPoolExecutor; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       importjavax.servlet.AsyncContext; 
     </div> 
     <div>
       importjavax.servlet.ServletException; 
     </div> 
     <div>
       importjavax.servlet.annotation.WebServlet; 
     </div> 
     <div>
       importjavax.servlet.http.HttpServlet; 
     </div> 
     <div>
       importjavax.servlet.http.HttpServletRequest; 
     </div> 
     <div>
       importjavax.servlet.http.HttpServletResponse; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       @WebServlet(urlPatterns ="/AsyncLongRunningServlet", asyncSupported =true) 
     </div> 
     <div>
       publicclassAsyncLongRunningServletextendsHttpServlet { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;privatestaticfinallongserialVersionUID = 1L; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;protectedvoiddoGet(HttpServletRequest request, 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;HttpServletResponse response)throwsServletException, IOException { 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;longstartTime = System.currentTimeMillis(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("AsyncLongRunningServlet Start::Name=" 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ Thread.currentThread().getName() +"::ID=" 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ Thread.currentThread().getId()); 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;request.setAttribute("org.apache.catalina.ASYNC_SUPPORTED",true); 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;String time = request.getParameter("time"); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;intsecs = Integer.valueOf(time); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// max 10 seconds 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if(secs &gt;10000) 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;secs =10000; 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;AsyncContext asyncCtx = request.startAsync(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;asyncCtx.addListener(newAppAsyncListener()); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;asyncCtx.setTimeout(9000); 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;ThreadPoolExecutor executor = (ThreadPoolExecutor) request 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;.getServletContext().getAttribute("executor"); 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;executor.execute(newAsyncRequestProcessor(asyncCtx, secs)); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;longendTime = System.currentTimeMillis(); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;System.out.println("AsyncLongRunningServlet End::Name=" 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ Thread.currentThread().getName() +"::ID=" 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ Thread.currentThread().getId() +"::Time Taken=" 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;+ (endTime - startTime) +" ms."); 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;} 
     </div> 
     <div>
       &nbsp; 
     </div> 
     <div>
       } 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

**Run Async Servlet**

现在，当我们将上面运行servlet URL：

[http://localhost:8080/AsyncServletExample/AsyncLongRunningServlet?time=8000][http_localhost_8080_AsyncServletExample_AsyncLongRunningServlet_time_8000]

得到响应和日志：

如果运行时设置time=9999，在客户端超时以后会得到响应超时错误处理和日志：

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    <div> 
     <div>
       AsyncLongRunningServlet Start::Name=http-bio-8080-exec-44::ID=117 
     </div> 
     <div>
       AsyncLongRunningServlet End::Name=http-bio-8080-exec-44::ID=117::Time Taken=1 ms. 
     </div> 
     <div>
       Async Supported?true 
     </div> 
     <div>
       AppAsyncListener onTimeout 
     </div> 
     <div>
       AppAsyncListener onError 
     </div> 
     <div>
       AppAsyncListener onComplete 
     </div> 
     <div>
       Exceptioninthread"pool-5-thread-6"java.lang.IllegalStateException: The request associated with the AsyncContext has already completed processing. 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;at org.apache.catalina.core.AsyncContextImpl.check(AsyncContextImpl.java:439) 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;at org.apache.catalina.core.AsyncContextImpl.getResponse(AsyncContextImpl.java:197) 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;at com.journaldev.servlet.async.AsyncRequestProcessor.run(AsyncRequestProcessor.java:27) 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:895) 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:918) 
     </div> 
     <div>
       &nbsp;&nbsp;&nbsp;&nbsp;at java.lang.Thread.run(Thread.java:680) 
     </div> 
    </div> </td> 
  </tr> 
 </tbody> 
</table>

**注意：**Servlet线程执行完，很快就和所有主要的处理工作是发生在其他线程。

转载于:https://my.oschina.net/u/865921/blog/196216

[2019_Python_]: https://my.oschina.net/u/2663968/blog/3061697
[hot3.png]: /images/20220114/b036500a34f94ce6a573f21be41fc295.png
[http_localhost_8080_AsyncServletExample_LongRunningServlet_time_8000]: http://localhost:8080/AsyncServletExample/LongRunningServlet?time=8000
[Executors framework]: http://mjava.org/1417.shtml
[26175305_D3JH.png]: /images/20220114/940ffd00737a49cb9d36c585794d474f.png
[26175305_D3JH.png 1]: http://www.importnew.com/?attachment_id=8903
[Servlet Listener]: http://www.journaldev.com/1945/servlet-listener-example-servletcontextlistener-httpsessionlistener-and-servletrequestlistener
[http_localhost_8080_AsyncServletExample_AsyncLongRunningServlet_time_8000]: http://localhost:8080/AsyncServletExample/AsyncLongRunningServlet?time=8000