servlet3.0异步servlet

使用异步servlet主要原因就是因为，在service方法中业务逻辑如果碰到io操作时间比较长的操作，这样这个service方法就会长时间占用tomcat容器线程池中的线程，这样是不利于其他请求的处理的，当线程池中的线程处理任务时，任务由于长时间io操作，肯定会阻塞线程处理其他任务，引入异步servlet的目的就是将容器线程池和业务线程池分离开。在处理大io的业务操作的时候，把这个操作移动到业务线程池中进行，释放容器线程，使得容器线程处理其他任务，在业务逻辑执行完毕之后，然后在通知tomcat容器线程池来继续后面的操作，这个操作应该是把处理结果commit到客户端或者是dispatch到其他servlet上。

    上图是原始servlet和容器线程池的模型，下图是异步servlet，容器线程池和业务线程池的模型，从图中可以看出，原始模型在处理业务逻辑的过程中会一直占有容器线程池，而异步servlet模型，可以看出在业务线程池处理的过程中，有一段时间容器线程池中的那个线程是空闲的，这种设计大大提高了容器的处理请求的能力。

    异步servlet的开启在service中开启，对于一般请求，在service方法之后，都会commit response结果到客户端，但是在异步servlet中这个commit是没有意义的，因为输出还没产生，在业务线程池中还未处理完毕，这时需要把当前处理环境保存起来，以便业务线程池处理完毕后，再次找到这个处理环境继续处理。

    下面来看一个异步servlet的实例：

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;

public class AsyncServlet 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 AsyncRequestProcessorThread(asyncCtx, secs));
		long endTime = System.currentTimeMillis();
		System.out.println("AsyncLongRunningServlet End::Name="
				+ Thread.currentThread().getName() + "::ID="
				+ Thread.currentThread().getId() + "::Time Taken="
				+ (endTime - startTime) + " ms.");
		request.getRequestDispatcher("/index.jsp").forward(request, response);
	}

}

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;


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");
	}

}

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;


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();
	}

}

package com.journaldev.servlet.async;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.AsyncContext;

public class AsyncRequestProcessorThread implements Runnable {

	private AsyncContext asyncContext;
	private int secs;

	public AsyncRequestProcessorThread() {
	}

	public AsyncRequestProcessorThread(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();
		}
	}
}

执行下面两个URL

http://localhost:8080/asyncServlet/LongRunningServlet?time=1000

http://localhost:8080/asyncServlet/async?time=1000

发现对于用户来说，响应时间是没有变化的，变化的是后台占用servlet主线程的事件，时间缩短了很多，这样servlet就能处理更多的请求，而不必在大量IO操作的时候等待

参考：http://blog.chinaunix.net/uid-27767798-id-3806685.html

http://blog.csdn.net/kuyuyingzi/article/details/18843487

servlet3.0异步servlet,布布扣,bubuko.com

servlet3.0异步servlet

原文：http://blog.csdn.net/shanhuhau/article/details/20535029