1.8（学习笔记）监听器（Listener）

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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原文地址：https://www.cnblogs.com/huang-changfan/p/10320746.html