JavaWeb(Servlet)中实现监听器

1.实现listener接口（当然有很多类型）:

package com.tang.listener;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;

public class javaListener implements HttpSessionListener {
          
   
    @Override
    public void sessionCreated(HttpSessionEvent se) {
          
   
        ServletContext sct = se.getSession().getServletContext();

        Integer onlineCount = (Integer) sct.getAttribute("OnlineCount");

        if(onlineCount == null){
          
   
            onlineCount = new Integer(1);
        }
        else {
          
   
            int count = onlineCount.intValue();
            onlineCount = new Integer(count+1);
        }

        sct.setAttribute("OnlineCount",onlineCount);
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
          
   
        ServletContext sct = se.getSession().getServletContext();

        Integer onlineCount = (Integer) sct.getAttribute("OnlineCount");

        if(onlineCount == null){
          
   
            onlineCount = new Integer(0);
        }
        else {
          
   
            int count = onlineCount.intValue();
            onlineCount = new Integer(count-1);
        }

        sct.setAttribute("OnlineCount",onlineCount);
    }
}

2、在web.xml中注册监听器才能使用：

<listener>
        <listener-class>com.tang.listener.javaListener</listener-class>
   	</listener>