监听器

实现一个监听器的接口：

1、编写一个监听器

public class OnlineCountListener implements HttpSessionListener {
    //创建session的监听
    @Override
    public void sessionCreated(HttpSessionEvent se) {
        ServletContext context = se.getSession().getServletContext();
        Integer OnlineCount = (Integer) context.getAttribute("OnlineCount");
        if( OnlineCount == null){
            OnlineCount = 1;
        }else {
            int count = OnlineCount.intValue();
            OnlineCount = new Integer(count+1);
        }

        context.setAttribute("OnlineCount",OnlineCount);

    }
    //销毁session监听
    @Override
    public void sessionDestroyed(HttpSessionEvent se) {
        ServletContext context = se.getSession().getServletContext();
        Integer OnlineCount = (Integer) context.getAttribute("OnlineCount");
        if( OnlineCount == null){
            OnlineCount = 0;
        }else {
            int count = OnlineCount.intValue();
            OnlineCount = new Integer(count-1);
        }

        context.setAttribute("OnlineCount",OnlineCount);
    }
}

//复习销毁session
//getsession().invalidate();
//web.xml配置自动线销毁

2、配置监听器（注册）

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