笔记：
监听三个域对象的属性变更的监听器
王类监听器
ServletContextAttributeListener
监听ServletContext对象中的属性变更(属性添加、移除、替换)的监听
器
attributeAdded (ServletContextAttributeEvent event)
attributeRemoved (ServletContextAttributeEvent event)
attributeReplaced (ServletContextAttributeEvent event)
HttpSessionAttributeListener
监听HttpSession对象中的属性变更(属性添加、移除、替换)的监听器
attributeAdded (HttpSessionBindingEvent event)
attributeRemoved (Ht tpSessionBindingEvent event)
attributeReplaced (Ht tpSessionBindingEvent event)
ServletRequestAttributeListener
监听ServletRequest对象中的属性变更(属性添加、移除、替换)的监听
器
attr ibuteAdded (ServletRequestAttributeEvent srae)
attributeRemoved (ServletRequestAttributeEvent srae)
attributeReplaced (ServletRequestAttributeEvent srae)
