jsp统计在线人数

import java.util.HashSet;
 2 
 3 import javax.servlet.ServletContext;
 4 
 5 import javax.servlet.http.HttpSession;
 6 
 7 import javax.servlet.http.HttpSessionEvent;
 8 
 9 import javax.servlet.http.HttpSessionListener; 
10 
11 public class MySessionListener implements HttpSessionListener {       
12 
13  public void sessionCreated(HttpSessionEvent event) {             
14 
15  HttpSession session = event.getSession();             
16 
17 ServletContext application = session.getServletContext();                           
18 
19  // 在application范围由一个HashSet集保存所有的session             
20 
21  HashSet sessions = (HashSet) application.getAttribute("sessions");             
22 
23 if (sessions == null) {                    
24 
25 sessions = new HashSet();                    
26 
27 application.setAttribute("sessions", sessions);             
28 
29 }                           
30 
31 // 新创建的session均添加到HashSet集中             
32 
33  sessions.add(session);             
34 
35 // 可以在别处从application范围中取出sessions集合             
36 
37 // 然后使用sessions.size()获取当前活动的session数，即为“在线人数”      
38 
39 }       
40 
41 public void sessionDestroyed(HttpSessionEvent event) {             
42 
43 HttpSession session = event.getSession();             
44 
45  ServletContext application = session.getServletContext();             
46 
47  HashSet sessions = (HashSet) application.getAttribute("sessions");                           
48 
49  // 销毁的session均从HashSet集中移除             
50 
51 sessions.remove(session);      
52 
53 }
54 
55 }