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。