import javax.servlet.ServletContext;import javax.servlet.http.HttpSession;import javax.servlet.http.HttpSessionEvent;import javax.servlet.http.HttpSessionListener;public class OnlineListener implements HttpSessionListener {    public void sessionCreated(HttpSessionEvent se) {        HttpSession session = se.getSession();        if(session.isNew()){            ServletContext context = session.getServletContext();            //第几个访问者            Integer caller = (Integer) context.getAttribute("caller");            //在线人数            Integer counter = (Integer) context.getAttribute("counter");            if(caller==null){                caller = 1;            }else{                caller++;            }            if(counter==null){                counter = 1;            }else{                counter++;            }            context.setAttribute("counter",counter);            session.setAttribute("caller",counter);        }    }    public void sessionDestroyed(HttpSessionEvent se) {    }}

<%@ page language="java" pageEncoding="UTF-8"%>                你是第${caller}访问者
    当前在线${counter}人