Servlet中的异步处理_servlet 支持异步-优快云博客

本文链接：https://blog.youkuaiyun.com/JavaLixy/article/details/70162488

Servlet 3引入了异步处理特性，允许Servlet在新的线程中执行耗时任务，避免阻塞主线程。文章介绍了如何开启异步处理、设置超时时间、使用AsyncContext以及监听器。示例代码展示了如何编写异步Servlet，包括发送进程更新和处理异步事件。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

Servlet 3引入一项新的特性，可以让Servlet异步处理请求，本文来介绍这新技术

一.概述

WebServlet 和 WebFilter 注解类型可以包含 asyncSupport属性。为了编写能够支持异步的Servlet和Filter ，该

属性必须为true

支持异步处理的Servlet或者Filter 可以通过在ServletRequest 中调用startAsync 方法来启动新的线程。

startAsync 有两个重载方法：

AsyncContext startAsync ()

AsyncContext startAsync(ServletRequest request ,ServletResponse response)

注意：重复调用startAsync方法将会启动同一个线程，将会抛出一个java.lang.lllegalStateException 异常

调用 AsyncContext的start方法不会造成阻塞，因此它派发的线程还没启动，也会继续执行下一行代码

二.编写异步的Servlet

如果你有一个任务需要相对比较长的时间才能完成，最好成绩一个Servlet或者Filter 。在异步的Servlet或者Filter类中

需要完成以下工作：

1.在ServletRequest 中调用startAsync 方法来启动新的线程

2.在AsyncContext中调用setTimeout（）方法，设置一个任务必须在指定的时间内完成的毫秒数

3.调用asyncContext.start方法，传递一个执行长时间任务的Runnable

4.任务完成时，通过Runnable调用asyncContext.complete方法或者asyncContext.dispatch方法

举个例子

import java.io.IOException;

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(name="AsyncDispatchServlet",
urlPatterns="/asyncDispatch",
asyncSupported=true)
public class AsyncDispatchServlet extends HttpServlet {

	private static final long serialVersionUID=222L;
	
	@Override
	protected void doGet(final HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		// TODO Auto-generated method stub
		final AsyncContext asyncContext=req.startAsync();
		req.setAttribute("mainThread", Thread.currentThread().getName());
		asyncContext.setTimeout(5000);
		asyncContext.start(new Runnable() {
			
			@Override
			public void run() {
				// TODO Auto-generated method stub
				try{
					Thread.sleep(4000);
				}catch(InterruptedException e){
					
				}
				req.setAttribute("workerThread", Thread.currentThread().getName());
				asyncContext.dispatch("/threadNames.jsp");
			}
		});
	}
}

上述代码在任务里休眠3秒。将主线程和工作线程发送到test.jsp页面，显示他们的线程名称

<body>
   Main thread:${mainThread}
   <br>
   Worker Thread:${workerThread}
  </body>

三.发送进程更新的异步

这个Servlet每秒钟发送一次进程更新，以便用户能够追踪进程

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

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(asyncSupported=true,urlPatterns="/asyncComplete")
public class AsyncCompleteServlet extends HttpServlet {
	private static final long serialVersionUID=78234L;
	
	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		// TODO Auto-generated method stub
		resp.setContentType("text/html");
		final PrintWriter write=resp.getWriter();
		write.println("<html><head><title>"
				+"Async Servlet</title></head>");
		write.println("<body><div id='progress'></div>");
		final AsyncContext asyncContext=req.startAsync();
		asyncContext.setTimeout(60000);
		asyncContext.start(new Runnable() {
			
			@Override
			public void run() {
				// TODO Auto-generated method stub
				System.out.println("new Thread:"+Thread.currentThread());
				for(int i=0;i<10;i++){
					write.println("<script>");
					write.print("document.getElementById('progress').innerHTML='"
							+(i*10)+"% complete'");
					write.println("</script>");
					write.flush();
					try{
						Thread.sleep(1000);
					}catch(InterruptedException e){}
				}
				write.println("<script>");
				write.println("document.getElementById('progress').innerHTML='DONE'");
				write.println("</script>");
				write.println("</body></html>");
				asyncContext.complete();
			}
		});
	}

}

四.同步监听器

除了Servlet和Filter执行异步操作之外，Servlet 3.0还新增了一个AsyncListener接口，以便通知用户在异步处理期间

发生的情况。

AsyncListener接口定义的方法：

void onStartAsync(AsyncEvent event) 在刚启动一个异步操作时调用

void onComplete(AsyncEvent event) 当一个异步操作已经完成时调用

void onError(AsyncEvent event) 当一个异步操作失败是调用

void onTimeout(AsyncEvent event) 当一个异步操作已经超时的时候调用

这四个方法都会受到一个AsyncEvent事件，可以通过调用它的getAsyncContext,getSuppliedRequest .getSuppliedResponse

方法从中获取相关的AsyncContext,ServletRequest,ServletResponse实例

举个例子:MyAsyncListener类实现了AsyncListener接口，以便当异步操作中有事件发生时能够受到通知，

与其他监听器不同，它没有用@WebListener标注AsyncListener的实现

import java.io.IOException;

import javax.servlet.AsyncEvent;
import javax.servlet.AsyncListener;
//do not annotate with @WebListener
public class MyAsyncListener implements AsyncListener {

	@Override
	public void onComplete(AsyncEvent arg0) throws IOException {
		// TODO Auto-generated method stub

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

	@Override
	public void onError(AsyncEvent arg0) throws IOException {
		// TODO Auto-generated method stub

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

	@Override
	public void onStartAsync(AsyncEvent arg0) throws IOException {
		// TODO Auto-generated method stub

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

	@Override
	public void onTimeout(AsyncEvent arg0) throws IOException {
		// TODO Auto-generated method stub

		System.out.println("onTimeout");
	}

}

import java.io.IOException;

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(name="AsyncListenerServlet",urlPatterns="/asyncListener",
asyncSupported=true)
public class AsyncListenerServlet extends HttpServlet {
	@Override
	protected void doGet(final HttpServletRequest req, HttpServletResponse resp)
			throws ServletException, IOException {
		// TODO Auto-generated method stub
		final AsyncContext asyncContext=req.startAsync();
		asyncContext.setTimeout(5000);
		
		asyncContext.addListener(new MyAsyncListener());
		asyncContext.start(new Runnable() {
			
			@Override
			public void run() {
				// TODO Auto-generated method stub
				try{
					Thread.sleep(3000);
					
				}catch(InterruptedException  e){
					
				}
				String greeting="hi from listener";
			    System.out.println("wait....");
			    req.setAttribute("greeting", greeting);
			    asyncContext.dispatch("/index.jsp");
			}
		});
	}

	
}