package com.banner.servlet;


import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;

import javax.servlet.ServletContext;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionBindingListener;

public class OnlineUserBindingListener implements HttpSessionBindingListener {
         String username;
        public OnlineUserBindingListener(){
        }
        
        public OnlineUserBindingListener(String username){
            this.username=username;
        }
        public void valueBound(HttpSessionBindingEvent event) {
            HttpSession session = event.getSession();
            ServletContext application = session.getServletContext();
            // 把用户名放入在线列表
            List onlineUserList = (List) application.getAttribute("onlineUserList");
            // 第一次使用前，需要初始化
            if (onlineUserList == null) {
                onlineUserList = new ArrayList();
                
            }
            onlineUserList.add(this.username);
            application.setAttribute("onlineUserList", onlineUserList);
        }

        public void valueUnbound(HttpSessionBindingEvent event) {
            HttpSession session = event.getSession();
            ServletContext application = session.getServletContext();

            // 从在线列表中删除用户名
            List onlineUserList = (List) application.getAttribute("onlineUserList");
            onlineUserList.remove(this.username);
            SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd hh:mm:ss");
            System.out.println(this.username + "退出系统。");
            System.out.println("退出时间"+sdf.format(new Date()));
        }
    
}

------解决方案--------------------
Java code

public class OnlineListener implements HttpSessionListener {

    private static HashMap  hUserName = new HashMap();//保存sessionID和username的映射
    
    @Override
    public void sessionCreated(HttpSessionEvent arg0) {
        
    }

    @Override
    public void sessionDestroyed(HttpSessionEvent arg0) {
        hUserName.remove(event.getSession());
    }

    /*
     * isAlreadyEnter-用于判断用户是否已经登录以及相应的处理方法 @param sUserName String-登录的用户名称
     * @return boolean-该用户是否已经登录过的标志
     */
    public static boolean isAlreadyEnter(HttpServletRequest request, String sUserName) {
        boolean flag = false;
        if (hUserName.containsValue(sUserName)) {// 如果该用户已经登录过，则使上次登录的用户掉线(依据使用户名是否在hUserName中)
            flag = true;
            // 遍历原来的hUserName，删除原用户名对应的sessionID(即删除原来的sessionID和username)
            Iterator iter = hUserName.entrySet().iterator();
            while (iter.hasNext()) {
                Map.Entry entry = (Map.Entry) iter.next();
                Object key = entry.getKey();
                Object val = entry.getValue();
                if (((String) val).equals(sUserName)) {
                    hUserName.remove(key);
                    try{
                        HttpSession session = (HttpSession)key;
                        session.invalidate();
                    }catch(Exception ex){
                        logger.error(ex.getMessage(), ex);
                    }
                    
                }
            }
            hUserName.put(request.getSession(),sUserName);// 添加现在的sessionID