1.实现一个监听器的接口(有N种)具体看继承哪个监听器接口

实例一:监听session个数确定网站访问人数

//监听器配置
public class OnlineCountListener implements HttpSessionListener {
    //session创建监听
    //一旦创建一个session就会触发这个事件
    public void sessionCreated(HttpSessionEvent se) {
        ServletContext context = se.getSession().getServletContext();
        Integer count = (Integer)context.getAttribute("OnlineCount");
        if (count == null) {
            count = 1;
        }else{
            count++;
        }
        context.setAttribute("OnlineCount",count);
    }

    //session销毁监听
    public void sessionDestroyed(HttpSessionEvent se) {
        ServletContext context = se.getSession().getServletContext();
        Integer count = (Integer)context.getAttribute("OnlineCount");
        if (count == null) {
            count = 0;
        }else{
            count--;
        }
        context.setAttribute("OnlineCount",count);
    }
}

xml配置

    
    
        com.zhang.listener.OnlineCountListener
    

网站初始化页面定义显示

<%@ page contentType="text/html;charset=UTF-8" language="java" %>

  
    
  
  
  
当前有<%=request.getServletContext().getAttribute("OnlineCount")%>人在线
  

显示结果两个浏览器进程访问