Servlet 3特性：异步Servlet

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

package com.journaldev.servlet;   import java.io.IOException; import java.io.PrintWriter;   import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;   @WebServlet("/LongRunningServlet") public class LongRunningServlet extends HttpServlet {     private static final long serialVersionUID = 1L;       protected void doGet(HttpServletRequest request,             HttpServletResponse response) throws ServletException, IOException {         long startTime = System.currentTimeMillis();         System.out.println("LongRunningServlet Start::Name="                 + Thread.currentThread().getName() + "::ID="                 + Thread.currentThread().getId());           String time = request.getParameter("time");         int secs = Integer.valueOf(time);         // max 10 seconds         if (secs > 10000)             secs = 10000;           longProcessing(secs);           PrintWriter out = response.getWriter();         long endTime = 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.");     }       private void longProcessing(int secs) {         // 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:

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. 1. 
      
2. 首先Servlet,我们提供异步支持 Annotation @WebServlet 的属性asyncSupported 值为true。
3. 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。
4. 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。
5. 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。
6. 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。
   1. 
      

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

package com.journaldev.servlet.async;   import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit;   import javax.servlet.ServletContextEvent; import javax.servlet.ServletContextListener; import javax.servlet.annotation.WebListener;   @WebListener public class AppContextListener implements ServletContextListener {       public void contextInitialized(ServletContextEvent servletContextEvent) {           // create the thread pool         ThreadPoolExecutor executor = new ThreadPoolExecutor(100, 200, 50000L,                 TimeUnit.MILLISECONDS, new ArrayBlockingQueue<Runnable>(100));         servletContextEvent.getServletContext().setAttribute("executor",                 executor);       }       public void contextDestroyed(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

package com.journaldev.servlet.async;   import java.io.IOException; import java.io.PrintWriter;   import javax.servlet.AsyncContext;   public class AsyncRequestProcessor implements Runnable {       private AsyncContext asyncContext;     private int secs;       public AsyncRequestProcessor() {     }       public AsyncRequestProcessor(AsyncContext asyncCtx, int secs) {         this.asyncContext = asyncCtx;         this.secs = secs;     }       @Override     public void run() {         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();     }       private void longProcessing(int secs) {         // 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

package com.journaldev.servlet.async;   import java.io.IOException; import java.io.PrintWriter;   import javax.servlet.AsyncEvent; import javax.servlet.AsyncListener; import javax.servlet.ServletResponse; import javax.servlet.annotation.WebListener;   @WebListener public class AppAsyncListener implements AsyncListener {       @Override     public void onComplete(AsyncEvent asyncEvent) throws IOException {         System.out.println("AppAsyncListener onComplete");         // we can do resource cleanup activity here     }       @Override     public void onError(AsyncEvent asyncEvent) throws IOException {         System.out.println("AppAsyncListener onError");         //we can return error response to client     }       @Override     public void onStartAsync(AsyncEvent asyncEvent) throws IOException {         System.out.println("AppAsyncListener onStartAsync");         //we can log the event here     }       @Override     public void onTimeout(AsyncEvent asyncEvent) throws IOException {         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。

package com.journaldev.servlet.async;   import java.io.IOException; import java.util.concurrent.ThreadPoolExecutor;   import javax.servlet.AsyncContext; import javax.servlet.ServletException; import javax.servlet.annotation.WebServlet; import javax.servlet.http.HttpServlet; import javax.servlet.http.HttpServletRequest; import javax.servlet.http.HttpServletResponse;   @WebServlet(urlPatterns = "/AsyncLongRunningServlet", asyncSupported = true) public class AsyncLongRunningServlet extends HttpServlet {     private static final long serialVersionUID = 1L;       protected void doGet(HttpServletRequest request,             HttpServletResponse response) throws ServletException, IOException {         long startTime = 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");         int secs = Integer.valueOf(time);         // max 10 seconds         if (secs > 10000)             secs = 10000;           AsyncContext asyncCtx = request.startAsync();         asyncCtx.addListener(new AppAsyncListener());         asyncCtx.setTimeout(9000);           ThreadPoolExecutor executor = (ThreadPoolExecutor) request                 .getServletContext().getAttribute("executor");           executor.execute(new AsyncRequestProcessor(asyncCtx, secs));         long endTime = 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：

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：

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 Exception in thread "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。

这是所有异步Servlet内容，希望你喜欢它。​​下载 AsyncServletExample 工程​​

本文由 ​​ImportNew​​​ - ​​彭秦进​​​ 翻译自 ​​journaldev​​