作用
1,监听Web应用程序中ServletContext、HttpSession和ServletRequest等域对象的创建和销毁过程
2,监听ServletContext、HttpSession和ServletRequest等域对象属性的修改
3,感知绑定到HttpSession域中某个对象的状态
监听器
1, ServletContextListener,ServletRequestListener,HttpSessionListener
2, ServletContextAttributeListener, ServletRequestAttributeListener, HttpSessionAttributeListener
3, HttpSessionBindingListener, HttpSessionActivationListener