事件监听之统计当前在线人数案例

 1 import javax.servlet.ServletContext;
 2 import javax.servlet.ServletContextEvent;
 3 import javax.servlet.ServletContextListener;
 4 import javax.servlet.annotation.WebListener;
 5 import javax.servlet.http.HttpSessionAttributeListener;
 6 import javax.servlet.http.HttpSessionEvent;
 7 import javax.servlet.http.HttpSessionListener;
 8 import javax.servlet.http.HttpSessionBindingEvent;
 9 
10 @WebListener()
11 public class CountListener implements HttpSessionListener {
12     private int count=0;
13     public void  sessionCreated(HttpSessionEvent hse) {
14         count++;
15         ServletContext context = hse.getSession ().getServletContext ();
16         context.setAttribute ("count",new Integer (count));
17     }
18 
19     @Override
20     public void sessionDestroyed(HttpSessionEvent hse) {
21         count--;
22         ServletContext context = hse.getSession ().getServletContext ();
23         context.setAttribute ("count",new Integer (count));
24     }
25 }

index.jsp

 1 <%@ page contentType="text/html;charset=UTF-8" language="java" %>
 2 <html>
 3   <head>
 4     <title>$Title$</title>
 5   </head>
 6   <body>
 7     <h3>
 8       当前在线人数为：<%=application.getAttribute("count")%>
 9     </h3>
10     <a href="<%=response.encodeURL ("logout.jsp")%>">退出系统</a>
11   </body>
12 </html>

logout.jsp

 1 <%@ page contentType="text/html;charset=UTF-8" language="java" %>
 2 <html>
 3 <head>
 4     <title>Title</title>
 5 </head>
 6 <body>
 7   <%session.invalidate();%>
 8     <h3>您已退出系统</h3>
 9 </body>
10 </html>

采用不同的浏览器访问，防止公用一个session对象