监听器就是实现一个特定接口的java程序，此程序专门用来监听另一个类方法的调用。java的awt大量的运用到了此种模式，例如可以给button对象添加一个点击事件，当鼠标点击时，就会调用事件处理程序。又如：在javascript事件中也运用到了此种模式，当用户点击鼠标时，会触发一个鼠标点击事件去调用程序员定义的事件处理程序。下面就以GUI编程来说明监听器。监听器都使用到了观察者模式，观察者模式所定义的对象间一对多的依赖关系，当一个对象的状态发生改变时，所有依赖他的对象都会得到通知自动更新。
package cn.zq.demo;
import java.awt.Container;
import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.JButton;
import javax.swing.JFrame;
public class DemoFrame extends JFrame{
/**
*
*/
private static final long serialVersionUID = -7552482706279772458L;
public DemoFrame() {
JButton button = new JButton("点击试试");
System.out.println("button ->" + button.hashCode());
//设置默认的关闭操作：关闭时退出程序
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
//给button添加一个监听器
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
System.out.println("监听到了 ->"+e.getSource().hashCode());
}
});
//拿到容器
Container container = getContentPane();
//设置为流式布局
container.setLayout(new FlowLayout());
container.add(button);
setSize(200, 100);
setVisible(true);
}
public static void main(String[] args) {
new DemoFrame();
}
}
用button.addActionListener方法给按钮添加了一个监听器，当用户点击鼠标时，监听器中的actionPerformed(ActionEvent e)方法就会被执行，传过来的参数ActionEvent对象包含了当前正在被监听的对象。监听器存在以下三个类：
被监听Person
package cn.zq.demo;
public class Person {
private PersonListener l;
public void addPersonListener(PersonListener l){
this.l = l;
}
public void run() {
//先执行监听器对象方法
if(l != null){
//传递事件源对象
l.run(new PersonEvent(this));
}
System.out.println("Person->"+this + "->run...");
}
}
package cn.zq.demo;
public interface PersonListener {
void run(PersonEvent e);
}
package cn.zq.demo;
public class PersonEvent {
private Object src;
//构造事件对象时把被监听对象传进来
public PersonEvent(Object src) {
this.src = src;
}
public Object getSource(){
return src;
}
}
测试类：
package cn.zq.demo;
public class Main {
public static void main(String[] args) {
Person p = new Person();
p.addPersonListener(new PersonListener() {
public void run(PersonEvent e) {
System.out.println("event->"+e.getSource() + "->run....");
}
});
//触发用户的run方法
p.run();
}
}
在javaweb中主要监听这三个对象：ServletContext. HttpSession, ServletRequest，
主要的监听器如下表：
第一步：创建一个类，这个类实现ServletRequestListener
package cn.zq.listener;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
public class RequestListener implements ServletRequestListener{
public void requestInitialized(ServletRequestEvent e) {
System.out.println("request创建了!");
System.out.println("这个request是"+e.getServletRequest());
}
public void requestDestroyed(ServletRequestEvent e) {
System.out.println("request销毁了!");
System.out.println("这个request是"+e.getServletRequest());
}
}
<listener> <listener-class>cn.zq.listener.RequestListener</listener-class> </listener>
request创建了! 这个request是org.apache.catalina.connector.RequestFacade@19845fb request销毁了! 这个request是org.apache.catalina.connector.RequestFacade@19845fb
分析：只要有人访问，创建一个session就是一个会话，一个会话就是一个在线人数。所以需要监听session的创建和销毁。
第一步：编写一个类实现HttpSessionListener接口
package cn.zq.listener;
import javax.servlet.ServletContext;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
public class MyHttpSessionListener implements HttpSessionListener{
public void sessionCreated(HttpSessionEvent e) {
System.out.println("有人访问本站点了...");
HttpSession session = e.getSession();
System.out.println("session->"+session+"("+session.getId()+")");
ServletContext sc = session.getServletContext();
Integer onlineCount = (Integer) sc.getAttribute("onlineCount");
if(onlineCount == null){
onlineCount = 0;
}
sc.setAttribute("onlineCount", ++onlineCount);
}
public void sessionDestroyed(HttpSessionEvent e) {
System.out.println("有人退出了 ->"+e.getSession().getId());
ServletContext sc = e.getSession().getServletContext();
Integer onlineCount = (Integer) sc.getAttribute("onlineCount");
if(onlineCount != null && onlineCount > 0){
sc.setAttribute("onlineCount", --onlineCount);
}
}
}
<listener> <listener-class>cn.zq.filter.MyHttpSessionListener</listener-class> </listener>
<p>在线人数：${applicationScope.onlineCount }</p><session-config> <!-- 分钟 --> <session-timeout>1</session-timeout> </session-config>
第一步：编写一个类实现HttpSessionAttributeListener
package cn.zq.listener;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
/**
* 监听session属性变化
* @author zq
*
*/
public class SessionAttributeListener implements HttpSessionAttributeListener{
/**
* 属性被添加后执行此方法
*/
public void attributeAdded(HttpSessionBindingEvent event) {
System.out.println("添加session属性");
HttpSession session = event.getSession();
System.out.println("session ->" + session);
//属性名
String name = event.getName();
//属性值
Object value = event.getValue();
System.out.println(name + "=" + value);
}
/**
* 属性被移除时执行这个方法
*/
public void attributeRemoved(HttpSessionBindingEvent event) {
System.out.println("移除session属性");
HttpSession session = event.getSession();
System.out.println("session ->" + session);
//属性名
String name = event.getName();
//属性值
Object value = event.getValue();
System.out.println(name + "=" + value);
}
public void attributeReplaced(HttpSessionBindingEvent event) {
System.out.println("替换session属性");
HttpSession session = event.getSession();
System.out.println("session ->" + session);
//属性名
String name = event.getName();
//属性值
Object value = event.getValue();
System.out.println("替换前："+name + "=" + value);
System.out.println("替换后："+name + "=" + session.getAttribute(name));
}
}
第二步：在web.xml中配置这个监听器
<listener> <listener-class>cn.zq.listener.SessionAttributeListener</listener-class> </listener>
<%
session.setAttribute("name", "RiccioZhang");
session.setAttribute("name", "zq");
session.removeAttribute("name");
%>添加session属性 session ->org.apache.catalina.session.StandardSessionFacade@1a8739b name=RiccioZhang 替换session属性 session ->org.apache.catalina.session.StandardSessionFacade@1a8739b 替换前：name=RiccioZhang 替换后：name=zq 移除session属性 session ->org.apache.catalina.session.StandardSessionFacade@1a8739b name=zq
（1）显示登录人数
（2）显示登录人的ip，session的创建时间、最后访问时间
（3）可以踢出某个用户
分析：只要有人登录（输入了用户名）就会放到session中，Session.setAttribute("user", "zq");
实现第一个核心类：
package cn.zq.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;
public class MySessionAttrListener implements HttpSessionAttributeListener{
public void attributeAdded(HttpSessionBindingEvent event) {
String name = event.getName();
if("user".equals(name)){
ServletContext sc = event.getSession()
.getServletContext();
//将session缓存到map中
@SuppressWarnings("unchecked")
Map<String, HttpSession> m = (Map<String, HttpSession>) sc.getAttribute("login");
if(m == null){
m = new HashMap<String, HttpSession>();
sc.setAttribute("login", m);
}
m.put(event.getSession().getId(), event.getSession());
}
}
public void attributeRemoved(HttpSessionBindingEvent event) {
String name = event.getName();
if("user".equals(name)){
HttpSession session = event.getSession();
ServletContext sc = session.getServletContext();
@SuppressWarnings("unchecked")
Map<String, HttpSession> m = (Map<String, HttpSession>) sc.getAttribute("login");
if(m != null){
m.remove(session.getId());
}
}
}
public void attributeReplaced(HttpSessionBindingEvent event) {}
}
配置：
<listener> <listener-class>cn.zq.listener.MySessionAttrListener</listener-class> </listener>
（1）登录页面
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>主页</title>
</head>
<body>
<c:choose>
<c:when test="${empty sessionScope.user}">
<form action="<c:url value='/Login'/>" method="post">
<input type="text" name="name"><br/>
<input type="submit" >
</form>
</c:when>
<c:otherwise>
欢迎您,${sessionScope.user }<br/>
<a href="<c:url value='/Login'/>">退出</a><br/>
</c:otherwise>
</c:choose>
<a href="<c:url value='/ShowServlet'/>">显示所有登录人数</a>
</body>
</html>
（2）开发处理登录、退出的servlet
package cn.zq.servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
public class LoginServlet extends HttpServlet {
private static final long serialVersionUID = 3059445154848670189L;
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.getSession()
.removeAttribute("user");
response.sendRedirect(request.getContextPath() + "/index.jsp");
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
request.setCharacterEncoding("UTF-8");
String name = request.getParameter("name");
request.getSession().setAttribute("user", name);
request.getSession().setAttribute("ip", request.getRemoteAddr());
response.sendRedirect(request.getContextPath() + "/index.jsp");
}
}
<servlet>
<servlet-name>ShowServlet</servlet-name>
<servlet-class>cn.zq.servlet.ShowServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>LoginServlet</servlet-name>
<url-pattern>/Login</url-pattern>
</servlet-mapping>（1）获取登录人集合并将其转换成一个MapList，将封装好的信息传递到显示页面
package cn.zq.servlet;
import java.io.IOException;
import java.text.DateFormat;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
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;
public class ShowServlet extends HttpServlet {
/**
*
*/
private static final long serialVersionUID = 8867391453104991999L;
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request, response);
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
ServletContext sc = getServletContext();
@SuppressWarnings("unchecked")
Map<String, HttpSession> m = (Map<String, HttpSession>)
sc.getAttribute("login");
List<Map<String, Object>> list = new ArrayList<Map<String,Object>>();
if(m != null){
DateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");
for(Iterator<Map.Entry<String, HttpSession>> it = m.entrySet().iterator();
it.hasNext();){
Entry<String, HttpSession> entry = it.next();
Map<String, Object> mapBean = new HashMap<String, Object>();
mapBean.put("sessionid", entry.getKey());
mapBean.put("user", entry.getValue().getAttribute("user"));
mapBean.put("creationTime", df.format(new Date(entry.getValue().getCreationTime())));
mapBean.put("lastAccessTime", df.format(new Date(entry.getValue().getLastAccessedTime())));
mapBean.put("ip", entry.getValue().getAttribute("ip"));
list.add(mapBean);
}
}
request.setAttribute("list", list);
request.getRequestDispatcher("/page/show.jsp")
.forward(request, response);
}
}
<%@ page language="java" import="java.util.*" pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/functions" prefix="fn"%>
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>显示登录人数</title>
</head>
<body>
<p>欢迎您：<c:out value="${user }"></c:out>，共${fn:length(list) }人，以下是所有登录人数：</p>
<table border="1">
<tr>
<th>姓名</th>
<th>登录时间</th>
<th>最后访问时间</th>
<th>登录ip</th>
<th>操作</th>
</tr>
<c:forEach items="${list}" var="m">
<tr>
<td>${m.user}</td>
<td>${m.creationTime}</td>
<td>${m.lastAccessTime}</td>
<td>${m.ip}</td>
<td>
<a href="<c:url value='/KickupServlet?id=${m.sessionid }'/>">踢出</a>
</td>
</tr>
</c:forEach>
</table>
</body>
</html>
package cn.zq.servlet;
import java.io.IOException;
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;
public class KickupServlet extends HttpServlet {
private static final long serialVersionUID = 1007302413238302848L;
public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
doPost(request, response);
}
public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
String id = request.getParameter("id");
ServletContext sc = getServletContext();
Map<String, HttpSession> login = (Map<String, HttpSession>) sc.getAttribute("login");
HttpSession session = login.get(id);
if(session != null){
session.removeAttribute("user");
}
login.remove(id);
response.sendRedirect(request.getContextPath() + "/ShowServlet");
}
}
<servlet>
<servlet-name>KickupServlet</servlet-name>
<servlet-class>cn.zq.servlet.KickupServlet</servlet-class>
</servlet>
<servlet-mapping>
<servlet-name>KickupServlet</servlet-name>
<url-pattern>/KickupServlet</url-pattern>
</servlet-mapping>
完成以上步骤，并测试自己写的程序。
这个监听器很有用，比如可以在web容器启动的是否加载一些配置文件，执行一些初始化操作，web容器停止时，执行一些清楚操作。spring就是使用监听器在web容器启动时执行初始化的。
ServletContextListener有2个方法：
做一个案例：开发一个显示网站的访问次数的小功能
分析：应该将访问次数存放到servletContext中，为了避免访问次数的丢失，应当将服务器停止时将访问次数放到文件中保存，服务器启动时再从文件中读取，这个操作应当在ServletContextListener监听器中做。用户每发送一次请求就算是一次访问，记录访问次数的增加，可以再过滤器中做，也可以在ServletRequestListener中做（创建一个request对象就是一次访问），因为在学习监听器，所以我选择后者。
（1）开发一个ServletContextListener和一个ServletRequestListener
package cn.zq.listener;
import java.io.BufferedReader;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContextListener;
public class MyServletContextListener implements ServletContextListener{
public void contextInitialized(ServletContextEvent sce) {
System.out.println("ServletContext:"+sce.getServletContext()+"->init...");
String path = MyServletContextListener.class.getClassLoader().getResource("vist.txt").getFile();
try {
BufferedReader br = new BufferedReader(new FileReader(path));
String line = br.readLine();
long vistCount = 0;
if(line != null && line.length() > 0){
vistCount = Long.parseLong(line);
}
sce.getServletContext().setAttribute("vistCount", vistCount);
System.out.println("初始化的访问次数："+vistCount);
br.close();
} catch (IOException e) {
e.printStackTrace();
}
}
public void contextDestroyed(ServletContextEvent sce) {
System.out.println("ServletContext:"+sce.getServletContext()+"->destroy...");
String path = MyServletContextListener.class.getClassLoader().getResource("vist.txt").getFile();
try {
FileWriter fw = new FileWriter(path);
Long vistCount = (Long) sce.getServletContext()
.getAttribute("vistCount");
if(vistCount != null){
fw.write(vistCount.toString());
}
fw.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
package cn.zq.listener;
import javax.servlet.ServletContext;
import javax.servlet.ServletRequestEvent;
import javax.servlet.ServletRequestListener;
public class RequestListener implements ServletRequestListener {
public void requestInitialized(ServletRequestEvent e) {
System.out.println(e.getServletRequest()+"->创建了");
ServletContext sc = e.getServletContext();
Long vistCount = (Long) sc.getAttribute("vistCount");
if(vistCount == null){
vistCount = 0L;
}
sc.setAttribute("vistCount", ++vistCount);
}
public void requestDestroyed(ServletRequestEvent e) {
System.out.println(e.getServletRequest() + "->销毁了");
}
}
<listener> <listener-class>cn.zq.listener.RequestListener</listener-class> </listener> <listener> <listener-class> cn.zq.listener.MyServletContextListener</listener-class> </listener>
要监听一个对象是否被绑定到了session对象中，这个对象需要实现HttpSessionbindingListener这个接口，注意：只需要实现这个接口，不需要配置到web.xml文件中。
（1）先开发一个Person类实现这个接口
package cn.zq.domain;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;
public class Person implements HttpSessionBindingListener{
public void valueBound(HttpSessionBindingEvent event) {
String name = event.getName();
Object value = event.getValue();
System.out.println(value+"被绑定到session中了");
System.out.println(name + "=" + value);
}
public void valueUnbound(HttpSessionBindingEvent event) {
System.out.println(event.getValue()+"从session中解绑了");
}
}
<%
Person p = new Person();
session.setAttribute("p", p);
session.removeAttribute("p");
%>这个监听器用处并不大，使用了这个监听器，那么javabean就与这个接口耦合起来了，增加了耦合度，那么意味着这个javabean就必须依赖servel api而存在，没有了就会出现错误
实现此接口的javabean可以感知活化（从硬盘到内存）或钝化（从内存到硬盘）的过程，如果同时需要保存在session中的javabean，则此javabean需要实现Serializable接口，实现此接口的javabean，不需要配置到web.xml文件中。在session被活化或钝化时，保存在session中的相应javabean执行的方法。
（1）编写一个Person类实现此接口，并且实现序列化接口
package cn.zq.domain;
import java.io.Serializable;
import javax.servlet.http.HttpSessionActivationListener;
import javax.servlet.http.HttpSessionEvent;
public class Person implements HttpSessionActivationListener,
Serializable{
private static final long serialVersionUID = 7606213806247294106L;
private String name;
public Person(String name) {
this.name = name;
}
public Person() {}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public String toString() {
return "Person [name=" + name + "]";
}
public void sessionDidActivate(HttpSessionEvent e) {
System.out.println("------------------------");
System.out.println(this+"活了...");
System.out.println("这个session是"+e.getSession());
}
public void sessionWillPassivate(HttpSessionEvent e) {
System.out.println("------------------------");
System.out.println(this+"钝化了...");
System.out.println("这个session是"+e.getSession());
}
}
（2）在tomcat的目录下的conf\Catalina\localhost配置这个项目
<Context docBase="D:\\code\\j2ee\\listener\\WebRoot"> <Manager className="org.apache.catalina.session.PersistentManager" saveOnRestart="true"> <Store className="org.apache.catalina.session.FileStore" directory="c:/a"> </Store> </Manager> </Context>
<%
if(session.getAttribute("p") == null){
int rand = new Random().nextInt(50);
Person p = new Person();
p.setName("p" + rand);
session.setAttribute("p", p);
}
//在浏览器设置这个cookie能确保，下次访问时，通知服务器当前的cookie
Cookie cookie = new Cookie("JSESSIONID", session.getId());
cookie.setMaxAge(60*10);
cookie.setPath(request.getContextPath());
response.addCookie(cookie);
%>
${p }
<hr/>
<%=session.getId() %>
<hr/>（4）测试并查看结果，将每次的结果进行对比。
原文：http://blog.csdn.net/ricciozhang/article/details/43833401