public class OnlineListener implements HttpSessionListener,
    HttpSessionAttributeListener {
  private static List<SessionAndUser> sessions;
  static int delS = -1;
  static boolean flag = false;
  static {
    if (sessions == null) {
      sessions = Collections
          .synchronizedList(new ArrayList<SessionAndUser>());
    }
  }
  public void sessionCreated(HttpSessionEvent hse) {
    System.out.println(hse.getSession() + "-" + new Date());
    System.out.println(hse.getSession() + "-" + new Date());
  }
  public void sessionDestroyed(HttpSessionEvent hse) {
    System.out.println("-------------sessionDestroyed()-----------");
    System.out.println(hse.getSession() + " "
        + new Date(hse.getSession().getLastAccessedTime()));
    System.out.println(hse.getSession() + " " + new Date());
  }
  public void attributeAdded(HttpSessionBindingEvent e) {
    System.out.println("-------------*start added*-----------------------"
        + sessions.size());
    HttpSession session = e.getSession();
    ActionContext ctx = ActionContext.getContext();
    boolean newOne = true;
    String attrName = e.getName();
    // 登录
    if (attrName.equals(Constant.USER_NAME)) {
      // 检查登录人数
      if (sessions.size() >= Constant.USER_LIMIT) {
        newOne = false;
        ctx.put("timeoutMSG", "serverBusy");
      }
      String nowUser = (String) e.getValue();
      // 遍历所有session，检查是否已经登录，若是则提示已经登录
      for (int i = sessions.size() - 1; i >= 0; i--) {
        SessionAndUser tem = sessions.get(i);
        if (tem.getUserName().equals(nowUser)) {
          newOne = false;
          ctx.put("timeoutMSG", "beenLoged");// tem.getSession().invalidate();//
                            // 同账号顶替登录，自动调用remove
          break;
        }
      }
      // 新登录帐号添加进账户维护列表
      if (newOne) {
        SessionAndUser sau = new SessionAndUser();
        sau.setUserName(nowUser);
        sau.setSession(session);
        sau.setSid(session.getId());
        sessions.add(sau);
      }
    }
  }
  public void attributeRemoved(HttpSessionBindingEvent e)
      throws IllegalStateException {
    HttpSession session = e.getSession();
    System.out
        .println("-------------*start Removed*-----------------------"
            + sessions.size());
    if (delS > -1) {
      if (flag) {
        sessions.remove(delS);
        flag = false;
      }
    } else {
      // 登录
      String attrName = e.getName();
      if (attrName.equals(Constant.USER_NAME)) {
        String nowUser = (String) e.getValue();
        // 遍历所有session
        for (int i = sessions.size() - 1; i >= 0; i--) {
          SessionAndUser tem = sessions.get(i);
          if (tem.getUserName().equals(nowUser)) {
            sessions.remove(i);
            break;
          }
        }
      }
    }
  }
  public void attributeReplaced(HttpSessionBindingEvent e) {
    HttpSession session = e.getSession();
    System.out
        .println("-------------*start replace*-----------------------"
            + sessions.size());
    String attrName = e.getName();
    delS = -1;
    // 登录
    if (attrName.equals(Constant.USER_NAME)) {
      // User nowUser = (User) e.getValue();//old value
      String nowUser = (String) session.getAttribute(Constant.USER_NAME);// 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.getUserName().equals(nowUser)
            && !tem.getSid().equals(session.getId())) {
          System.out.println("Remove:invalidate 1!");
          delS = i;
          flag = true;
        } else if (tem.getSid().equals(session.getId())) {
          tem.setUserName(nowUser);
        }
      }
      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
      }
    }
  }
}

session.setAttribute(Constant.USER_NAME, operator.getUsername());
    ActionContext ctx = ActionContext.getContext();
    if("serverBusy".equals(ctx.get("timeoutMSG"))){
      ctx.put("timeoutMSG", "服务器繁忙，请稍后再试");
      return "jump";
    }
    if("beenLoged".equals(ctx.get("timeoutMSG"))){
      ctx.put("timeoutMSG", "此账户在别处登录");
      return "jump";
    }

<%@taglib prefix="s" uri="/struts-tags"%>
<s:property value="#attr.timeoutMSG" />