session_counter.java

2．打开“命令行提示符”； 3．输入mysql –u root - p命令

package com.ch9;  

import javax.servlet.*;  
import javax.servlet.http.*;  

public class Session_Counter implements HttpSessionListener 
{  

	private static int activeSessions = 0;  

	//有新用户访问，计数器加1
	public void sessionCreated(HttpSessionEvent se) 
	{  
		activeSessions++;  
	}  
	
	//有用户离开，计数器减1
	public void sessionDestroyed(HttpSessionEvent se) 
	{  
		if(activeSessions > 0) 
		{
			activeSessions--;  
		} 		
	}  
	
	//获取计数器的值
	public static int getActiveSessions() 
	{  
		return activeSessions;  
	}  
}