Java之Listener
Listener监听器
监听器有很多种，大部分还是在GUI用的比较多，这里简单记录一点关于HttpSessionListener的
统计session count Listener。主要通过ServletContext上下文来存储session count，在listener中create和destroyed方法实现session count的逻辑，主要是将ServletContext作为第三方来进行session count值的存储与改动。通过jsp中application对象get上下文属性来获取count值实现实时显示session的数量。
public class OnlineCountListener implements HttpSessionListener { //创建session监听
//一旦创建session就会触发该事件
public void sessionCreated(HttpSessionEvent httpSessionEvent) {
System.out.println(httpSessionEvent.getSession().getId());
//每次有session创建，代表用户数量+1
HttpSession session = httpSessionEvent.getSession(); //获取上下文，将人数OnlineCount存入上下文中方便读写
ServletContext sct = session.getServletContext(); Integer OnlineCount = (Integer) sct.getAttribute("OnlineCount"); if (OnlineCount==null){
OnlineCount = new Integer(1);
}else {
int count = OnlineCount.intValue();
OnlineCount = new Integer(count+1);
}
//数值通过属性存入上下文中
sct.setAttribute("OnlineCount", OnlineCount);
} //销毁session监听
//一旦销毁session就会触发该事件
public void sessionDestroyed(HttpSessionEvent httpSessionEvent) {
//每次有session创建，代表用户数量+1
HttpSession session = httpSessionEvent.getSession(); //获取上下文，将人数OnlineCount存入上下文中方便读写
ServletContext sct = session.getServletContext(); Integer OnlineCount = (Integer) sct.getAttribute("OnlineCount"); if (OnlineCount==null){
OnlineCount = new Integer(0);
}else {
int count = OnlineCount.intValue();
OnlineCount = new Integer(count-1);
}
//数值通过属性存入上下文中
sct.setAttribute("OnlineCount", OnlineCount);
}
}
web.xml配置Listener
<listener>
<listener-class>com.zh1z3ven.listener.OnlineCountListener</listener-class>
</listener>
Index.jsp
<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
<head>
<title>OnlineCount</title>
</head>
<body>
<h2 align="center">当前在线人数为：<span><%= application.getAttribute("OnlineCount")%></span></h2>
</body>
</html>
