12 监听器

实现一个监听器的接口

1. 编写一个监听器

   package com.karl.listener;
   
   import javax.servlet.ServletContext;
   import javax.servlet.http.HttpSessionEvent;
   import javax.servlet.http.HttpSessionListener;
   
   //统计网站在线人数: 统计session
   public class OnlineCountListener implements HttpSessionListener {
       @Override
       //创建session监听:看你的一举一动
       //一旦创建Session就会触发一次这个事件!
       public void sessionCreated(HttpSessionEvent httpSessionEvent) {
           ServletContext servletContext = httpSessionEvent.getSession().getServletContext();
           Integer onlineCount = (Integer)servletContext.getAttribute("OnlineCount");
           if (null == onlineCount){
               onlineCount = 1;
           }else {
               int count = onlineCount;
               onlineCount = count + 1;
           }
           servletContext.setAttribute("OnlineCount", onlineCount);
       }
   
       @Override
       //销毁session监听
       //一旦销毁Session就会触发一次这个事件!
       public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
       }
   }
   

2. web.xml中配置监听器

   <listener>
       <listener-class>com.karl.listener.OnlineCountListener</listener-class>
   </listener>
   

3. 看情况使用