// 这是监听器代码
public class UserListener implements HttpSessionBindingListener {
    
    private final static String ON_LINE_USERS  = "onLineUsers";
    private final static String CURRENT_USER   = "currentUser";
    private Integer userId;

    // 当有用户登录时，便把该用户信息保存在当前servlet上下文的map中
    public void valueBound(HttpSessionBindingEvent event) {
        User user = (User) event.getSession().getAttribute(CURRENT_USER);
        if (null != user) {
            Map onLineUsers = getMap(event);
            onLineUsers.put(user.getId(), user);
            userId = user.getId();
            event.getSession().getServletContext().setAttribute(ON_LINE_USERS, onLineUsers);
        }
    }

    // 当有用户退出时，便从当前servlet上下文中清除该用户信息
    public void valueUnbound(HttpSessionBindingEvent event) {
        Map onLineUsers = getMap(event);
        onLineUsers.remove(userId);
        event.getSession().getServletContext().setAttribute(ON_LINE_USERS, onLineUsers);
    }

    //得到用来存储用户的map对象
    public Map getMap(HttpSessionBindingEvent event) {
        Map map = (Map) event.getSession().getServletContext().getAttribute(ON_LINE_USERS);
        if (null == map) {
            map = new HashMap();
        }
        return map;
    }
}

// 这是注销的代码（我用了struts1）
public ActionForward quit(ActionMapping mapping, ActionForm form,
            HttpServletRequest request, HttpServletResponse response) {
        
        request.getSession().setAttribute(CURRENT_USER, null);
        request.getSession().removeAttribute(USER_LISTENER);
        return null;
    }