Servlet 3特性：异步Servlet

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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packagecom.journaldev.servlet;   importjava.io.IOException; importjava.io.PrintWriter;   importjavax.servlet.ServletException; importjavax.servlet.annotation.WebServlet; importjavax.servlet.http.HttpServlet; importjavax.servlet.http.HttpServletRequest; importjavax.servlet.http.HttpServletResponse;   @WebServlet("/LongRunningServlet") publicclassLongRunningServletextendsHttpServlet {     privatestaticfinallongserialVersionUID = 1L;       protectedvoiddoGet(HttpServletRequest request,             HttpServletResponse response)throwsServletException, IOException {         longstartTime = System.currentTimeMillis();         System.out.println("LongRunningServlet Start::Name="                 + Thread.currentThread().getName() +"::ID="                 + Thread.currentThread().getId());           String time = request.getParameter("time");         intsecs = Integer.valueOf(time);         // max 10 seconds         if(secs >10000)             secs =10000;           longProcessing(secs);           PrintWriter out = response.getWriter();         longendTime = System.currentTimeMillis();         out.write("Processing done for "+ secs +" milliseconds!!");         System.out.println("LongRunningServlet Start::Name="                 + Thread.currentThread().getName() +"::ID="                 + Thread.currentThread().getId() +"::Time Taken="                 + (endTime - startTime) +" ms.");     }       privatevoidlongProcessing(intsecs) {         // wait for given time before finishing         try{             Thread.sleep(secs);         }catch(InterruptedException e) {             e.printStackTrace();         }     }   }

如果我们的URL是：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:

1 2	LongRunningServlet Start::Name=http-bio-8080-exec-34::ID=103 LongRunningServlet Start::Name=http-bio-8080-exec-34::ID=103::Time Taken=8002 ms.

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：

在监听中初始化线程池

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packagecom.journaldev.servlet.async;   importjava.util.concurrent.ArrayBlockingQueue; importjava.util.concurrent.ThreadPoolExecutor; importjava.util.concurrent.TimeUnit;   importjavax.servlet.ServletContextEvent; importjavax.servlet.ServletContextListener; importjavax.servlet.annotation.WebListener;   @WebListener publicclassAppContextListenerimplementsServletContextListener {       publicvoidcontextInitialized(ServletContextEvent servletContextEvent) {           // create the thread pool         ThreadPoolExecutor executor =newThreadPoolExecutor(100,200, 50000L,                 TimeUnit.MILLISECONDS,newArrayBlockingQueue<Runnable>(100));         servletContextEvent.getServletContext().setAttribute("executor",                 executor);       }       publicvoidcontextDestroyed(ServletContextEvent servletContextEvent) {         ThreadPoolExecutor executor = (ThreadPoolExecutor) servletContextEvent                 .getServletContext().getAttribute("executor");         executor.shutdown();     }   }

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

工作线程实现

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

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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packagecom.journaldev.servlet.async;   importjava.io.IOException; importjava.io.PrintWriter;   importjavax.servlet.AsyncEvent; importjavax.servlet.AsyncListener; importjavax.servlet.ServletResponse; importjavax.servlet.annotation.WebListener;   @WebListener publicclassAppAsyncListenerimplementsAsyncListener {       @Override     publicvoidonComplete(AsyncEvent asyncEvent)throwsIOException {         System.out.println("AppAsyncListener onComplete");         // we can do resource cleanup activity here     }       @Override     publicvoidonError(AsyncEvent asyncEvent)throwsIOException {         System.out.println("AppAsyncListener onError");         //we can return error response to client     }       @Override     publicvoidonStartAsync(AsyncEvent asyncEvent)throwsIOException {         System.out.println("AppAsyncListener onStartAsync");         //we can log the event here     }       @Override     publicvoidonTimeout(AsyncEvent asyncEvent)throwsIOException {         System.out.println("AppAsyncListener onTimeout");         //we can send appropriate response to client         ServletResponse response = asyncEvent.getAsyncContext().getResponse();         PrintWriter out = response.getWriter();         out.write("TimeOut Error in Processing");     }   }

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

Run Async Servlet

现在，当我们将上面运行servlet URL：

http://localhost:8080/AsyncServletExample/AsyncLongRunningServlet?time=8000

得到响应和日志：

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AsyncLongRunningServlet Start::Name=http-bio-8080-exec-50::ID=124 AsyncLongRunningServlet End::Name=http-bio-8080-exec-50::ID=124::Time Taken=1 ms. Async Supported?true AppAsyncListener onComplete

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

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。