监听哪些东西：
1request
2session
3application
a.监听对象的创建和销毁
request:ServletRequestListener
session:HttpSessionListener
context:ServletContextListener
b.监听对象中属性的变更:add,remove,replace
request:ServletRequestAttributeListener
session:HttpSessionAttributeListener
context:ServletContextAttributeListener
c.Session相关
绑定和解绑:HttpSessionBindingListener
钝化和活化[配置tomcat中的context.xml、实现Serializable接口]:HttpSessionActivationListener
