sessionDestroyed 怎样在关闭页面时就调用，求详细方法！！！！！！！

[Ace_m]

public void sessionDestroyed(HttpSessionEvent hse) {
HttpSession session=hse.getSession();
Student student=(Student)session.getAttribute("student");

Set<Student> onlineUsers=(Set<Student>)application.getAttribute("onlineusers");
onlineUsers.remove(student);
application.setAttribute("onlineusers", onlineUsers);

SqlSession sessions = SqlSessionFactoryUtil.getSqlSession();
INumDao idd=sessions.getMapper(INumDao.class);

idd.numjian();
sessions.commit();

}
想在关闭网页时调用写在sessionDestroyed中的方法