Listener (easy to understand)

I. Introduction

Listeners are used less and less in javaweb, and more are used in Android development. As a java programmer, it can be used as an understanding.

Second, use steps

Implement a listener interface (there are countless kinds of interfaces)

1. Write a listener and implement the listener interface

//统计网站在线人数：统计session
public class OnlineCountListener implements HttpSessionListener {
    
    
    //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 se) {
    
    
        ServletContext ctx = se.getSession().getServletContext();
        Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");
        if (onlineCount == null){
    
    
            onlineCount = new Integer(1);
        }else {
    
    
            int count = onlineCount.intValue();
            onlineCount = new Integer(count ++);
        }
        ctx.setAttribute("OnlineCount",onlineCount);
    }

    //销毁session监听
    //一旦销毁session就会触发一次这个事件
    public void sessionDestroyed(HttpSessionEvent se) {
    
    
        ServletContext ctx = se.getSession().getServletContext();
        Integer onlineCount = (Integer) ctx.getAttribute("OnlineCount");
        if (onlineCount == null){
    
    
            onlineCount = new Integer(0);
        }else {
    
    
            int count = onlineCount.intValue();
            onlineCount = new Integer(count --);
        }
        ctx.setAttribute("OnlineCount",onlineCount);
    }
}

2. Configure the listener in web.xml

    <!--注册监听器-->
    <listener>
        <listener-class>com.xu.listener.OnlineCountListener</listener-class>
    </listener>