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

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. 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

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