Servlet 3特性：异步Servlet

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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:

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：

首先Servlet,我们提供异步支持 Annotation @WebServlet 的属性asyncSupported 值为true。
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。
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。
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。
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

原文链接： journaldev 翻译： ImportNew.com - 彭秦进
译文链接： http://www.importnew.com/8864.html
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关于作者：彭秦进

转载于:https://www.cnblogs.com/clds/p/5006572.html