listener 作用

简介

Listener是监听器，它可以监听客户端的请求、服务端的操作等。通过监听器，可以自动激发一些操作
监听系统启动，做一些初始化动作比如监听在线的用户的数量

Listener 创建模板

创建的模板如下：

选择这个 整个生命周期的监测

package servlet;

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

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


    /**
     * Default constructor. 
     */
    public ListenerTest() {
        // TODO Auto-generated constructor stub
    }

	/**
     * @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

		}
    

}

添加对应的业务

package servlet;

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

/**
 * Application Lifecycle Listener implementation class ListenerTest
 *
 */
public class ListenerTest implements HttpSessionListener {
	private static int activeSessions = 0;

    /**
     * Default constructor. 
     */
    public ListenerTest() {
        // TODO Auto-generated constructor stub
    }

	/**
     * @see HttpSessionListener#sessionCreated(HttpSessionEvent)
     */
    public void sessionCreated(HttpSessionEvent se)  { 
         // TODO Auto-generated method stub
    	activeSessions++;
    	System.out.println("sessionCreated");
    }

	/**
     * @see HttpSessionListener#sessionDestroyed(HttpSessionEvent)
     */
    public void sessionDestroyed(HttpSessionEvent se)  { 
         // TODO Auto-generated method stub
    	if (activeSessions>0) {
    		activeSessions--;
        	System.out.println("sessionDestroyed");
		}
    }
	
    public static int getActiveSessions() {
        return activeSessions;
    }
}

webxml中添加listener

  <listener>
    <listener-class>servlet.ListenerTest</listener-class>
  </listener>

jsp如下：

<%@ page language="java" import="java.util.*" pageEncoding="utf-8"%>
<%@ page import="servlet.ListenerTest"%>


<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
  <head> 
    <title>My JSP 'ApplicationTest.jsp' starting page</title>
    
    <meta http-equiv="pragma" content="no-cache">
    <meta http-equiv="cache-control" content="no-cache">
    <meta http-equiv="expires" content="0">    
    <meta http-equiv="keywords" content="keyword1,keyword2,keyword3">
    <meta http-equiv="description" content="This is my page">
    <!--
    <link rel="stylesheet" type="text/css" href="styles.css">
    -->

  </head>
  
  <body>
        在线人数为：<%=ListenerTest.getActiveSessions() %>
  </body>
</html>

结果如下：

posted @ 2021-01-26 13:01  测开工程师成长之路  阅读(350)  评论(0)  编辑  收藏  举报