Web监听器

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java web

监听器的主要作用就是监听，当目标发生变化的时候就会触发一定的方法，这也就是所谓的事件触发机制。在这种机制中要有三个要素，也就是事件，事件源，处理事件的方法。这三要素之间是相互制约的。一旦事件处理方法被触发必定有事件发生，也就可以得到触发的事件，通过事件也就可以得到事件源，也就谁触发了事件，这样据可以将这三个要素联系在一起了。一个事件源可以有多个触发事件，而一个触发事件的发生将有多个方法来调用，这也就是所谓的一对多关系，通过一对多关系我们很容易从多端得到一端，相反从一端无法得到多端，这个道理大家都是很清楚的，所以也就可以很好的理解事件处理方法可以得到事件对象，事件对象又可以得到事件源。

1. web监听器的开发步骤

首先建立一个类实现listenner接口

public class ContextTestListener implements ServletContextListener {

	public void contextInitialized(ServletContextEvent sce) {
                //具体实现类
        }

	public void contextDestroyed(ServletContextEvent sce) {
	       //具体实现类 
       }
}

其次，在web.xml中配置监听器

  <listener>
    <listener-class>com.jason.listener.ContextTestListener</listener-class>
  </listener>

2. web监听器的分类，如图所示，目前分如下几种监听器 ServletContext，Session，Request的

3. 各种监听器的详细介绍及用法示例

(1) 与ServletContext相关的监听器接口为ServletContextListener，ServletContextAttributeListener，该监听器针对的是整个web应用程序的生命周期，其范围最大的。一般用来在应用程序初始化时读取数据库连接对象，初始化应用程序程序的配置等。其接口定义如下：

public interface ServletContextListener {
	public void contextInitialized(ServletContextEvent sce)；
	public void contextDestroyed(ServletContextEvent sce)；
}

典型应用：需求统计一个网页请求的访问量，该web应用停止时，将访问量数值写到配置文件中，改应用程序重新启动，数值从上次访问量开始。

<1> 实现ServletContextListener接口，在初始化话方法中读取配置文件中访问量的信息，在销毁方法中存储访问量的信息。因为在应用程序启动时，会调用contextInitialized方法，在应用程序停止时，会调用contextDestroyed方法。该例子应用了properties的操作，不熟悉properties请google下用法,特别注意读取文件和存储文件的写法，代码实现如下:

public class ContextTestListener implements ServletContextListener {

	private static final String COUNT_NAME = "count";
	
	private static final String PROPERTIES_NAME = "WEB-INF/count.properties";

	/*
	 * (non-Javadoc)
	 * 
	 * @see
	 * javax.servlet.ServletContextListener#contextInitialized(javax.servlet
	 * .ServletContextEvent)
	 */
	public void contextInitialized(ServletContextEvent sce) {

		ServletContext context = sce.getServletContext();
		InputStream inputstream = context.getResourceAsStream(PROPERTIES_NAME);
		
		Properties prop = new Properties();
		try {
			prop.load(inputstream);
		} catch (IOException e) {
			throw new RuntimeException(e);
		}
		
		int count = Integer.parseInt(prop.getProperty(COUNT_NAME));
		context.setAttribute(COUNT_NAME, count);
	}

	/*
	 * (non-Javadoc)
	 * 
	 * @see javax.servlet.ServletContextListener#contextDestroyed(javax.servlet.
	 * ServletContextEvent)
	 */
	public void contextDestroyed(ServletContextEvent sce) {

		ServletContext context = sce.getServletContext();
		Object countAttr = context.getAttribute(COUNT_NAME);
		if(countAttr != null){
			int count = (Integer) context.getAttribute(COUNT_NAME);
			
			InputStream inputstream = context.getResourceAsStream(PROPERTIES_NAME);
			Properties prop = new Properties();
			try {
				prop.load(inputstream);
				prop.setProperty(COUNT_NAME, String.valueOf(count));
			} catch (IOException e) {
				throw new RuntimeException(e);
			}
	
			String filepath = context.getRealPath("/");
			FileOutputStream fos = null;
			try {
				fos = new FileOutputStream(filepath + PROPERTIES_NAME);
			} catch (FileNotFoundException e) {
				throw new RuntimeException(e);
			}
	
			try {
				prop.store(fos, "add the count");
			} catch (IOException e) {
				throw new RuntimeException(e);
			} finally {
				if (fos != null) {
					try {
						fos.close();
					} catch (IOException e) {
						System.err.println("Cant close stream :" + e.getMessage());
					}
				}
			}
		}
	}
}

<2> 定义TestCountServlet，作为请求访问网页的请求URL，其实现代码如下，记录方法量，同时做跳转到访问量页面，代码如下：

package com.jason.servlet;

import java.io.IOException;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

/**
 * Servlet implementation class TestAccountServlet
 */
public class TestAccountServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;
 
	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doPost(request,response);
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		ServletContext context = request.getSession().getServletContext();
		Integer count = (Integer) context.getAttribute("count");
		if(count == null){
			count = 1;
		}else{
			count ++;
		}
		context.setAttribute("count",count);
		// do other things
		response.sendRedirect(request.getContextPath() + "/count.jsp");
	}
}

<3>webxml中的配置，webxml中配置了监听器，servlet及welcoen页面，内容如下：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>WebListener</display-name>
  <listener>
    <listener-class>com.jason.listener.ContextTestListener</listener-class>
  </listener>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
  <servlet>
    <description></description>
    <display-name>TestAccountServlet</display-name>
    <servlet-name>TestAccountServlet</servlet-name>
    <servlet-class>com.jason.servlet.TestAccountServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>TestAccountServlet</servlet-name>
    <url-pattern>/TestAccountServlet</url-pattern>
  </servlet-mapping>
</web-app>

<4> 在WEB-INF 目录下，建立count.properties，内容如下：

count=0

<5>建立index和count页面，index页面如下：

<%@ page language="java" contentType="text/html; charset=UTF-8" pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset= UTF-8">
<title>index page</title>
</head>
<body>
	<a href="${pageContext.request.contextPath}/TestAccountServlet">visit it to test </a>
</body>
</html>

在index页面中注意${pageContext.request.contextPath}，在jsp页面调用servlet的URL时，如果没设定basepath，前面一定要加上contextpath的路径，否则，servlet的地址是不对的。或者也可以直接用<c：out value=“/TestAccountServlet”/〉，因为jstl 中该标签直接在其前面添加contextpath路径。count页面的内容如下：

<%@ page language="java" contentType="text/html; charset=UTF-8"
    pageEncoding="UTF-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset= UTF-8">
<title>count here </title>
</head>
<body>
    visit count：${applicationScope.count}
    <%
    System.out.println(application.getAttribute("count"));
	%> 
</body>
</html>

利用${applicationScope.count}获取数值，该程序运行，每点击访问一次count页面，其访问量加1.

（2）与Session相关的监听器有有个四个HttpSessionListener，HttpSessionAttributeListener ，HttpSessionBindListener和HttpSessionActivationListener

<1>HttpSessionListener 是生命周期监听器，如果想对Session对象创建或结束时，做相应的动作可实现HttpSessionListener，该接口的定义如下：

public interface HttpSessionListener {
    public void sessionCreated(HttpSessionEvent se) ；
    public void sessionDestroyed(HttpSessionEvent se) ；
}

关于该监听器的用法，例如我们防止用户重新登录时，常常在数据库中定义字段状态，来表明该用户是否登录，在注销时，我们将其状态修改，但好多情况下，我们直接关闭浏览器，造成数据库状态没法更新，同时，用户登录session也有存活期限，此时我们希望在session销毁时，改变数据库状态，我们可以在HttpSessionListener监听器的sessionDestroyed（se）方法中在session中获取该用户，操作数据库，将该用户的登录状态设置为未登录。

/**
 * Application Lifecycle Listener implementation class OnlinePepoleListener
 *
 */
public class OnlinePepoleListener implements HttpSessionListener {

	/**
     * @see HttpSessionListener#sessionCreated(HttpSessionEvent)
     */
    public void sessionCreated(HttpSessionEvent se) {
        // TODO Auto-generated method stub
    }

	/**
     * @see HttpSessionListener#sessionDestroyed(HttpSessionEvent)
     */
    public void sessionDestroyed(HttpSessionEvent se) {
        // TODO Auto-generated method stub
    	HttpSession session = se.getSession();
    	String user = (String)session.getAttribute("login");
    	//修改数据库用户登录状态为未登录
    }
}

典型应用：统计当前用户在线数

（1）index页面

<%@ page language="java" contentType="text/html; charset=utf-8"
    pageEncoding="utf-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
	<body>
		<form action="login.jsp" method="post">
		   userName：<input type="text" name="username" />
		    <br />
		    <input type="submit" value="login" />
		</form>
	</body>
</html>

2.login页面

<%@ page language="java" contentType="text/html; charset=utf-8" pageEncoding="utf-8"%>
<%@ page import="java.util.*"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
	<body>
	<%
	    request.setCharacterEncoding("UTF-8");
	    // 取得登录的用户名
	    String username = request.getParameter("username");
	    // 把用户名保存进session
	    session.setAttribute("username", username);
	    // 把用户名放入在线列表
	    List onlineUserList = (List) application.getAttribute("onlineUserList");
	    // 第一次使用前，需要初始化
	    if (onlineUserList == null) {
	        onlineUserList = new ArrayList();
	        application.setAttribute("onlineUserList", onlineUserList);
	    }
	    onlineUserList.add(username);
	    // 成功
	    response.sendRedirect("result.jsp");
	%>
	</body>
</html>

3.result页面

<%@ page language="java" contentType="text/html; charset=utf-8" pageEncoding="utf-8"%>
<%@ page isELIgnored="false"%>
<%@page import="java.util.List"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
	<h3>您好：${username} [<a href="logout.jsp">注销</a>]</h3>
	当前在线用户：
	<table>
	<%
	   List onlineUserList = (List) application.getAttribute("onlineUserList");
	   for (int i = 0; i < onlineUserList.size(); i++) {
	   String onlineUsername = (String) onlineUserList.get(i);
	%>
	   <tr>
	       <td><%=onlineUsername%></td>
	   </tr>
	<%
	}
	%>
	</table>
</body>
</html>

4、logout页面

<%@ page language="java" contentType="text/html; charset=utf-8" pageEncoding="utf-8"%>
<%@ page import="java.util.*"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
<%
    // 取得登录的用户名
    String username = (String) session.getAttribute("username");
    // 销毁session
    session.invalidate();
    // 从在线列表中删除用户名
    List onlineUserList = (List) application.getAttribute("onlineUserList");
    onlineUserList.remove(username);
    // 成功
    response.sendRedirect("index.jsp");
%>
</body>
</html>

5、监听器代码

package com.jason.listener;

import java.util.List;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

/**
 * Application Lifecycle Listener implementation class OnlineUserListener
 *
 */
public class OnlineUserListener implements HttpSessionListener {
	
	public void sessionCreated(HttpSessionEvent event) {

        System.out.println("新建session:"+event.getSession().getId());
    }

    public void sessionDestroyed(HttpSessionEvent event) {

        HttpSession session = event.getSession();
        ServletContext application = session.getServletContext();
        // 取得登录的用户名
        String username = (String) session.getAttribute("username");
        // 从在线列表中删除用户名
        List onlineUserList = (List) application.getAttribute("onlineUserList");
        onlineUserList.remove(username);
        System.out.println(username+"已经退出！");
    }

}

6 web的配置

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xmlns:web="http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd" id="WebApp_ID" version="2.5">
  <display-name>OnlinePepoleListener</display-name>
  <welcome-file-list>
    <welcome-file>index.jsp</welcome-file>
  </welcome-file-list>
  <listener>
    <listener-class>com.jason.listener.OnlineUserListener</listener-class>
  </listener>
</web-app>

一旦监听器发现调用了sessionDestoryed方法就会把其用户从在线人数中delete，在下面两种情况下会发生sessionDestoryed事件

a.执行session.invalidate()方法时

logout.jsp中调用了 session.invalidate()方法

b.session会话超时

session的默认超时事件是30分钟，30分钟后自动销毁session

2、HttpSessionBindingListener

HttpSessionBindingListener虽然叫做监听器，但使用方法与HttpSessionListener完全不同。我们实际看一下它是如何使用的。新建类OnlineUserBindingListener，实现HttpSessionBindingListener接口，构造方法传入username参数，HttpSessionBindingListener内有两个方法valueBound(HttpSessionBindingEvent event)和valueUnbound(HttpSessionBindingEvent event)，前者为数据绑定，后者为取消绑定所谓对session进行数据绑定，就是调用session.setAttribute()把HttpSessionBindingListener保存进session中。

在login.jsp中做这一步：

<%@page import="com.test.OnlineUserBindingListener"%>
<%@ page contentType="text/html;charset=utf-8"%>
<%@ page import="java.util.*"%>
<%
    request.setCharacterEncoding("UTF-8");
    // 取得登录的用户名
    String username = request.getParameter("username");
       // 把用户名放入在线列表
    session.setAttribute("onlineUserBindingListener", new OnlineUserBindingListener(username));
    // 成功
    response.sendRedirect("result.jsp");
%>

这就是HttpSessionBindingListener和HttpSessionListener之间的最大区别：HttpSessionListener只需要设置到web.xml中就可以监听整个应用中的所有session。HttpSessionBindingListener必须实例化后放入某一个session中，才可以进行监听。从监听范围上比较，HttpSessionListener设置一次就可以监听所有session，HttpSessionBindingListener通常都是一对一的。正是这种区别成就了HttpSessionBindingListener的优势，我们可以让每个listener对应一个username，这样就不需要每次再去session中读取username，进一步可以将所有操作在线列表的代码都移入listener，更容易维护。

package com.test;

import java.util.ArrayList;
import java.util.List;
import javax.servlet.ServletContext;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;

public class OnlineUserBindingListener implements HttpSessionBindingListener {
    String username;
   
    public OnlineUserBindingListener(String username){
        this.username=username;
    }

    public void valueBound(HttpSessionBindingEvent event) {
        HttpSession session = event.getSession();
        ServletContext application = session.getServletContext();
        // 把用户名放入在线列表
        List onlineUserList = (List) application.getAttribute("onlineUserList");
        // 第一次使用前，需要初始化
        if (onlineUserList == null) {
            onlineUserList = new ArrayList();
            application.setAttribute("onlineUserList", onlineUserList);
        }
        onlineUserList.add(this.username);
    }

    public void valueUnbound(HttpSessionBindingEvent event) {
        HttpSession session = event.getSession();
        ServletContext application = session.getServletContext();

        // 从在线列表中删除用户名
        List onlineUserList = (List) application.getAttribute("onlineUserList");
        onlineUserList.remove(this.username);
        System.out.println(this.username + "退出。");
    }
}

这里可以直接使用listener的username操作在线列表，不必再去担心session中是否存在username。

valueUnbound的触发条件是以下三种情况：

a.执行session.invalidate()时。

b.session超时，自动销毁时。

c.执行session.setAttribute("onlineUserListener", "其他对象");或session.removeAttribute("onlineUserListener");将listener从session中删除时。

因此，只要不将listener从session中删除，就可以监听到session的销毁。

典型事例：强行把用户踢出例子

1、监听器代码如下：

package com.jason.listener;

import java.util.HashMap;
import java.util.Map;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;

import com.jason.domain.User;

/**
 * Application Lifecycle Listener implementation class SessionAttributeListener
 *
 */
public class SessionAttributeListener implements HttpSessionAttributeListener {

	/**
     * @see HttpSessionAttributeListener#attributeRemoved(HttpSessionBindingEvent)
     */
    public void attributeRemoved(HttpSessionBindingEvent arg0) {
        // TODO Auto-generated method stub
    }

	/**
     * @see HttpSessionAttributeListener#attributeAdded(HttpSessionBindingEvent)
     */
    public void attributeAdded(HttpSessionBindingEvent se) {
        // TODO Auto-generated method stub
    	System.out.println("start");
    	Object obj = se.getValue();
        if(obj instanceof User){
        	HttpSession session = se.getSession();
            ServletContext application = session.getServletContext();
            Map map = (Map) application.getAttribute("userMap");
            if(map == null){
               map = new HashMap();
               application.setAttribute("userMap", map);
            }
            User user = (User) obj;
            map.put(user.getUserName(), session);
        }
    }

	/**
     * @see HttpSessionAttributeListener#attributeReplaced(HttpSessionBindingEvent)
     */
    public void attributeReplaced(HttpSessionBindingEvent arg0) {
        // TODO Auto-generated method stub
    }
	
}

2.loginservlet

package com.jason.servlet;

import java.io.IOException;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Map;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import com.jason.domain.User;

/**
 * Servlet implementation class LoginServlet
 */
public class LoginServlet extends HttpServlet {

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doPost(request,response);
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		String username = request.getParameter("username");
	    String password = request.getParameter("password");
	    User user = new User();
	    user.setUserName(username);
	    user.setPassword(password);
	    HttpSession session= request.getSession();
	    session.setAttribute("user", user);
	    response.sendRedirect(request.getContextPath()+"/user.jsp");
	}

}

3、kickoutServelt

package com.jason.servlet;

import java.io.IOException;
import java.util.Map;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

/**
 * Servlet implementation class KickUserServlet
 */
public class KickUserServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;

	/**
	 * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		doPost(request,response);
	}

	/**
	 * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
	 */
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		// TODO Auto-generated method stub
		String username = request.getParameter("username");
	    Map map = (Map) request.getServletContext().getAttribute("userMap");
	    HttpSession session = (HttpSession) map.get(username);
	       if(session != null){
	           session.invalidate();
	           map.remove(username);
	       }
	       request.getRequestDispatcher("/listUser.jsp").forward(request, response);
	}
}

6.index页面

<%@ page language="java" contentType="text/html; charset=utf-8" pageEncoding="utf-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Insert title here</title>
</head>
<body>
<form method="post" action="${pageContext.request.contextPath }/LoginServlet">
   用户名：<input type="text" name="username" /><br/>
   密码: <input type="password" name="password" /><br/>
   <input type="submit" value="登陆" /><br/>
</form>
</body>
</html>

2 user 页面

<%@ page language="java" contentType="text/html; charset=utf-8"
    pageEncoding="utf-8"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=ISO-8859-1">
<title>Insert title here</title>
</head>
<body>
 欢迎您：${user.userName}
<a href="listUser.jsp" >user list</a>
</body>
</html>

3 userlist页面

<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>
<%@ page language="java" contentType="text/html; charset=utf-8" pageEncoding="utf-8"%>
<%@page import="java.util.*"%>
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>Insert title here</title>
</head>
<body>
欢迎您：${user.userName}
<br/>
当前登陆用户有：<br/>
<%
	Map onlineUserMap = (Map) application.getAttribute("userMap");
	System.out.println(onlineUserMap.size());
%>
<c:forEach var="me" items="${userMap}">
    ${me.key} <a href="${pageContext.request.contextPath}/KickUserServlet?username=${me.key}" >踢死你</a>
</c:forEach>
</body>
</html>

（3）与HttpServletRequest的监听器有ServletRequestListener，ServletRequestAttributeListener和异步处理的相关监听器，该监听器是3.0新加的。

关于ServletRequestListener，更多应用于统计访问量等

这里就介绍这些。