一、监听器简介

　　监听器是web容器对某一些对象的监听，当某些对象发生创建、修改、删除的动作时，

　　web容器会自动调用对应的监听器中的方法。

　　例如A是监听器，B是被监听对象，A的监听任务是B回家时来开门。

　　那么当B一回家，监听器A就会执行开门的操作。

二、监听器用法

　　2.1.1 ServletContextListener

　　用于监听ServletContext对象的创建及销毁，实际上就是监听Web应用的生命周期。

　　首先我们要创建一个类，实现ServletContextListener接口，实现里面的两个方法：

　　public void contextInitialized(ServletContextEvent sce)//web程序初始化时执行

　　public void contextDestroyed(ServletContextEvent sce)//web程序销毁时执行

ContextListener：

import java.io.PrintStream;

import javax.servlet.ServletContextEvent;

public class ServletContextListener implements javax.servlet.ServletContextListener{

    PrintStream out = System.out;

    @Override

    public void contextInitialized(ServletContextEvent sce) {

        // TODO Auto-generated method stub

        out.println("ContextInit");

    }

    @Override

    public void contextDestroyed(ServletContextEvent sce) {

        // TODO Auto-generated method stub

        out.println("ContextDestroyed");

    }

}

web.xml

 <listener>

      <listener-class>com.listener.ServletContextListener</listener-class>

  </listener>

当打开Tomcat时，监听器监听到ServletContext开始初始化，会执行contextInitialized（），

打印出ContextInit。当关闭Tomcat时，监听器监听到ServeltContext已关闭，会执行contextDestroyed（），

打印出ContextDestroyed.

      

　　2.1.2ServletContextAttrubute

　　监听ServletContext属性列表发送更改。

　　比如属性的添加(setAttrubute("a","a")、

　　修改（setAttribute("a","A")）、删除（removeAttribute("a")）。

　　ServletContextAttribute中有三个方法：

　　attributeAdded(ServletContextAttributeEvent scae)；//监听添加属性

　　attributeRemoved(ServletContextAttributeEvent scae)//监听删除属性

　　attributeReplaced(ServletContextAttributeEvent scae)//监听修改属性

import java.io.PrintStream;

import javax.servlet.ServletContextAttributeEvent;

import javax.servlet.ServletContextAttributeListener;

import javax.servlet.ServletContextEvent;

public class ServletContextListener implements ServletContextAttributeListener{

    PrintStream out = System.out;

    @Override

    public void attributeAdded(ServletContextAttributeEvent scae) {

        // TODO Auto-generated method stub

        out.println("AddAttribute");

    }

    @Override

    public void attributeRemoved(ServletContextAttributeEvent scae) {

        // TODO Auto-generated method stub

        out.println("RemovedAttribute");

    }

    @Override

    public void attributeReplaced(ServletContextAttributeEvent scae) {

        // TODO Auto-generated method stub

        out.println("ReplacedAddAttribute");

    }

}

LoginServlet:

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.ServletContext;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import com.util.HttpPageUtil;

public class LoginServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        PrintWriter out = response.getWriter();

        String username = request.getParameter("username");

        ServletContext sc = this.getServletContext();

        //添加

        sc.setAttribute("Attribute", "addAttribute");

        //修改

        sc.setAttribute("Attribute", "reAttribute");

        //删除

        sc.removeAttribute("Attribute");

        HttpPageUtil.printHtmlPage(out, true);

        out.println("SerlvetContextAttribute");

        HttpPageUtil.printHtmlPage(out, false);

        System.out.println("Servlet");

    }

    /**

     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

     */

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        // TODO Auto-generated method stub

        doGet(request, response);

    }

}

控制台输出：

当LoginServlet中向ServletContext添加属性时，对应监听器监听到添加动作，

执行方法attributeAdded()方法输出AddAttribute，修改属性时执行attributeReplaced()方法

输出ReplacedAddAttribute，删除属性时执行attributeRemoved()方法，输出RemoveAttribute。

　　2.2.1HttpSessionListener

　　HttpSessionnLister主要用于监听Session的创建及销毁。

　   public void sessionCreated(HttpSessionEvent se)//session创建时执行。

　　public void sessionDestroyed(HttpSessionEvent se)//session销毁时执行。

import java.io.PrintStream;

import javax.servlet.http.HttpSessionEvent;

public class HttpSessionListener implements javax.servlet.http.HttpSessionListener{

    PrintStream out = System.out;

    @Override

    public void sessionCreated(HttpSessionEvent se) {

        // TODO Auto-generated method stub

        out.println("SeesionCreater");

    }

    @Override

    public void sessionDestroyed(HttpSessionEvent se) {

        // TODO Auto-generated method stub

        out.println("SeesionDestroy");

    }

}

控制台输出：

但Session创建销毁时，监听器会监听到对应的动作并执行对方法。

　　2.2.2HttpSessionAttributeListener

　　主要监听Session对象属性的添加、修改、删除操作。

　　public void attributeAdded(HttpSessionBindingEvent se);//添加session中属性时执行　　

　　public void attributeRemoved(HttpSessionBindingEvent se);//删除session中属性时执行

　　public void attributeReplaced(HttpSessionBindingEvent se)；//修改session中属性时执行

HttpSessionAttributeListener
public class HttpSessionListener implements javax.servlet.http.HttpSessionAttributeListener{

    PrintStream out = System.out;

    @Override

    public void attributeAdded(HttpSessionBindingEvent se) {

        // TODO Auto-generated method stub

        out.println("SessionAttributeAdd");

    }

    @Override

    public void attributeRemoved(HttpSessionBindingEvent se) {

        // TODO Auto-generated method stub

        out.println("SessionAttributeRemove");

    }

    @Override

    public void attributeReplaced(HttpSessionBindingEvent se) {

        // TODO Auto-generated method stub

        out.println("SessionAttributeReplace");

    }

}

LoginServlet 

import java.io.IOException;

import java.io.PrintWriter;

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.util.HttpPageUtil;;

//  http://localhost:8080/TestServlet/Filter/Login

public class LoginServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        PrintWriter out = response.getWriter();

        String username = request.getParameter("username");

        HttpSession session = request.getSession();

        //添加

        session.setAttribute("attribute", "add");

        //修改

        session.setAttribute("attribute", "replace");

        //删除

        session.removeAttribute("attribute");

        HttpPageUtil.printHtmlPage(out, true);

        out.println("SessionAttributeListener");

        HttpPageUtil.printHtmlPage(out, false);

        System.out.println("Servlet");

    }

    /**

     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

     */

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        // TODO Auto-generated method stub

        doGet(request, response);

    }

}

控制台输出：

监听器会添加session中属性的增加、修改、删除，并执行对应方法

输出对应方法中的语句。

　　2.3.1ServletRequestListener

　　ServletRequestListener主要监听request的初始化和销毁。

　　public void requestDestroyed(ServletRequestEvent sre)；//request初始化时执行。

　　public void requestInitialized(ServletRequestEvent sre)；//request销毁时执行。

import java.io.PrintStream;

import javax.servlet.ServletRequestEvent;

public class ServletRequest implements javax.servlet.ServletRequestListener{

    PrintStream out = System.out;

    @Override

    public void requestDestroyed(ServletRequestEvent sre) {

        // TODO Auto-generated method stub

        out.println("RequestDestroy");

    }

    @Override

    public void requestInitialized(ServletRequestEvent sre) {

        // TODO Auto-generated method stub

        out.println("RequestInit");

    }

}

LoginServlet:

import java.io.IOException;

import java.io.PrintWriter;

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.util.HttpPageUtil;;

//  http://localhost:8080/TestServlet/Filter/Login

public class LoginServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        System.out.println("intoServlet");

    }

    /**

     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

     */

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        // TODO Auto-generated method stub

        doGet(request, response);

    }

}

但在地址栏通过URL访问LoginServlet时：

控制台输出结果：

当发送一个访问请求时，请求初始化被监听器监听到，执行对应方法输出“RequestInit”。

然后到达LoginServlet进行输出"intoServlet",最后请求结束被监听器监听到，执行requestDestroyed方法

输出“RequestDestroy”。

　　2.3.2 ServletRequestAttributeListener

　　ServletResqeustAttributerListener主要监听reques中属性的创建、修改、删除。

　　public void attributeAdded(HttpSessionBindingEvent se);//添加session中属性时执行　　

　　public void attributeRemoved(HttpSessionBindingEvent se);//删除session中属性时执行

　　public void attributeReplaced(HttpSessionBindingEvent se)；//修改session中属性时执行

RequestAttributeListener:

import java.io.PrintStream;

import javax.servlet.ServletRequestAttributeEvent;

public class ServletRequest implements javax.servlet.ServletRequestAttributeListener{

    PrintStream out = System.out;

    @Override

    public void attributeAdded(ServletRequestAttributeEvent srae) {

        // TODO Auto-generated method stub

        out.println("AddAttribute");

    }

    @Override

    public void attributeRemoved(ServletRequestAttributeEvent srae) {

        // TODO Auto-generated method stub

        out.println("RemoveAttribute");

    }

    @Override

    public void attributeReplaced(ServletRequestAttributeEvent srae) {

        // TODO Auto-generated method stub

        out.println("ReplaceAttribute");

    }

}

LoginServlet:

import java.io.IOException;

import java.io.PrintWriter;

import java.util.Enumeration;

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.util.HttpPageUtil;;

public class LoginServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        //添加

        request.setAttribute("attribute", "add");

        //修改

        request.setAttribute("attribute", "replace");

        //删除

        request.removeAttribute("attribute");

    }

    /**

     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

     */

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        // TODO Auto-generated method stub

        doGet(request, response);

    }

}

通过URL调用LoginServlet，

控制台输出：

当在reques中设置属性时，对应监听器会监听到，并调用attributeAdded()方法，输出"AddAttribute".

当修改reqest中属性时，会调用attributeReplaced()方法，输出“ReplaceAttribute”.

当删除reques中属性时会嗲用attributeRemoved()方法，输出“RemoveAttribute”。

三、监听器实例（在线人数统计）

　　首先在线人数应该是全站可见的，所以人数应该存放在作用域最大的ServletContexte中。

　　每登入一个用户我们就创建一个Session，Session监听器中人数+1。当一个session销毁时

　　监听器中人数-1.

OnLineListener:

import java.util.Set;

import java.util.TreeSet;

import javax.servlet.ServletContext;

import javax.servlet.ServletContextEvent;

import javax.servlet.http.HttpSessionEvent;

public class OnLineListener  implements javax.servlet.ServletContextListener, javax.servlet.http.HttpSessionListener{

    private ServletContext sc = null;

    private int count = 0;//记录登陆人数

    @Override

    public void sessionCreated(HttpSessionEvent se) {

        // TODO Auto-generated method stub

        //当创建用户session时，count++

        count++;

        //将人数设置在ServletContext中

        sc.setAttribute("count", new Integer(this.count));

    }

    @Override

    public void sessionDestroyed(HttpSessionEvent se) {

        // TODO Auto-generated method stub

        //当ession销毁时，count--

        count--;

        //将人数设置在ServletContext中

        sc.setAttribute("count", new Integer(this.count));

    }

    @Override

    public void contextInitialized(ServletContextEvent sce) {

        // TODO Auto-generated method stub

        //Tomcat启动时获取ServletContext，并设置一个登陆人数（初值为0）；

        this.sc = sce.getServletContext();

        this.sc.setAttribute("count", new Integer(this.count));

    }

    @Override

    public void contextDestroyed(ServletContextEvent sce) {

        // TODO Auto-generated method stub

        sc = null;

    }

}

LoginServlet:

import java.io.IOException;

import java.io.PrintWriter;

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.util.*;

//  http://localhost:8080/TestServlet/Filter/Login

public class LoginServlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        PrintWriter out = response.getWriter();

        String username = request.getParameter("username");

        HttpSession session = request.getSession();

        //获取ServletContext

        ServletContext sc = this.getServletContext();

        HttpPageUtil.printHtmlPage(out, true);

        //从ServletContext中获取人数。

        out.println("欢迎" + username +  "当前在线人数" + ((Integer)sc.getAttribute("count")).intValue());

        HttpPageUtil.printHtmlPage(out, false);

    }

    /**

     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

     */

    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        // TODO Auto-generated method stub

        doGet(request, response);

    }

}

login.jsp

<%@ page language="java" contentType="text/html; charset=UTF-8"

    pageEncoding="UTF-8"%>

<!DOCTYPE html>

<html>

<head>

<meta charset="UTF-8">

<title>Login</title>

</head>

<body>

    <!--在.../Page/下重定向 不加 /的相对路径 -->

    <!--  http://localhost:8080/TestServlet/Page/Filter/Login -->

    <form name = f1 action ="/TestServlet/Filter/Login" method = "post">

        username:<input type = "text" name = "username" value = "${un}"></input><br>

        password:<input type = "password" name = "password" value = "${pw}"></input><br>

    <!--      记住密码 ：<input type = "radio"  name = "SavePassword" value = "1"><br> -->

        <input type = "submit" value = "登录">

    </form>

</body>

</html>

每登录一个用户都会为其创建一个Session，这里测试时最好用不同的浏览器。

如果是一个浏览器，此例中多次登录会被认为是一个session，不会触发Session创建监听。

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