JavaWeb实现同一帐号同一时间只能一个地点登陆（类似QQ登录的功能）

package com.weirhp;
import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import javax.servlet.http.HttpSession;
import javax.servlet.http.HttpSessionAttributeListener;
import javax.servlet.http.HttpSessionBindingEvent;
import javax.servlet.http.HttpSessionEvent;
import javax.servlet.http.HttpSessionListener;
public class RecordSessionListener implements HttpSessionAttributeListener,
HttpSessionListener {
private static List sessions;
public static String loginFlag = "loginUser";
static {
if (sessiOns== null) {
sessiOns= Collections.synchronizedList(new ArrayList());
}
}
public void attributeAdded(HttpSessionBindingEvent e) {
HttpSession session = e.getSession();
System.out.println("-------------*start added*-----------------------");
String attrName = e.getName();
// 登录
if (attrName.equals(loginFlag)) {
User nowUser = (User) e.getValue();
User sUser = (User)session.getAttribute(loginFlag);
// 遍历所有session
for (int i = sessions.size()-1; i >= 0; i--) {
SessionAndUser tem = sessions.get(i);
if (tem.getUserID().equals(nowUser.getName())) {
tem.getSession().invalidate();//自动调用remove
break;
}
}
SessionAndUser sau = new SessionAndUser();
sau.setUserID(nowUser.getName());
sau.setSession(session);
sau.setSid(session.getId());
sessions.add(sau);
}
}
public void attributeRemoved(HttpSessionBindingEvent e) {
HttpSession session = e.getSession();
System.out.println("-------------*start Removed*-----------------------");
String attrName = e.getName();
// 登录
if (attrName.equals(loginFlag)) {
User nowUser = (User) e.getValue();
// 遍历所有session
for (int i = sessions.size()-1; i >= 0; i--) {
SessionAndUser tem = sessions.get(i);
if (tem.getUserID().equals(nowUser.getName())) {
sessions.remove(i);
break;
}
}
}
}
public void attributeReplaced(HttpSessionBindingEvent e) {
HttpSession session = e.getSession();
System.out.println("-------------*start replace*-----------------------");
String attrName = e.getName();
int delS=-1;
// 登录
if (attrName.equals(loginFlag)) {
// User nowUser = (User) e.getValue();//old value
User nowUser = (User)session.getAttribute(loginFlag);//当前session中的user
// 遍历所有session
for (int i = sessions.size()-1; i >= 0; i--) {
SessionAndUser tem = sessions.get(i);
if (tem.getUserID().equals(nowUser.getName())&&!tem.getSid().equals(session.getId())) {
System.out.println("Remove:invalidate 1!");
delS=i;
}else if(tem.getSid().equals(session.getId())){
tem.setUserID(nowUser.getName());
}
}
if (delS!=-1) {
sessions.get(delS).getSession().invalidate();//失效时自动调用了remove方法。也就会把它从sessions中移除了
}
}
}
public void sessionCreated(HttpSessionEvent e) {
}
public void sessionDestroyed(HttpSessionEvent e) {
}
}

recordSession
com.weirhp.RecordSessionListener