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

  
    servlet.ListenerTest
  

jsp如下：

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




   
    
    
    
    
        
    
    
    

  
  
  
        在线人数为：<%=ListenerTest.getActiveSessions() %>
  

结果如下：