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")

publicclass LongRunningServlet extendsHttpServlet {

    privatestatic final long serialVersionUID = 1L;

 

    protectedvoid doGet(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.");

    }

 

    privatevoid longProcessing(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:

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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。

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

publicclass AppContextListener implementsServletContextListener {

 

    publicvoid contextInitialized(ServletContextEvent servletContextEvent) {

 

        // create the thread pool

        ThreadPoolExecutor executor = newThreadPoolExecutor(100,200, 50000L,

                TimeUnit.MILLISECONDS,newArrayBlockingQueue<Runnable>(100));

        servletContextEvent.getServletContext().setAttribute("executor",

                executor);

 

    }

 

    publicvoid 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

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packagecom.journaldev.servlet.async;

 

importjava.io.IOException;

importjava.io.PrintWriter;

 

importjavax.servlet.AsyncContext;

 

publicclass AsyncRequestProcessor implementsRunnable {

 

    privateAsyncContext asyncContext;

    privateint secs;

 

    publicAsyncRequestProcessor() {

    }

 

    publicAsyncRequestProcessor(AsyncContext asyncCtx, intsecs) {

        this.asyncContext = asyncCtx;

        this.secs = secs;

    }

 

    @Override

    publicvoid 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();

    }

 

    privatevoid longProcessing(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

publicclass AppAsyncListener implementsAsyncListener {

 

    @Override

    publicvoid onComplete(AsyncEvent asyncEvent) throwsIOException {

        System.out.println("AppAsyncListener onComplete");

        // we can do resource cleanup activity here

    }

 

    @Override

    publicvoid onError(AsyncEvent asyncEvent) throwsIOException {

        System.out.println("AppAsyncListener onError");

        //we can return error response to client

    }

 

    @Override

    publicvoid onStartAsync(AsyncEvent asyncEvent) throwsIOException {

        System.out.println("AppAsyncListener onStartAsync");

        //we can log the event here

    }

 

    @Override

    publicvoid onTimeout(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)

publicclass AsyncLongRunningServlet extendsHttpServlet {

    privatestatic final long serialVersionUID = 1L;

 

    protectedvoid doGet(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。