过滤器,就是用来过滤Http请求的,减少通用重复的代码
http://penghuaiyi.iteye.com/blog/1297438
http://blog.youkuaiyun.com/hyeveryone/article/details/7314675
http://www.w3cschool.cc/jsp/jsp-writing-filters.html
http://www.cnblogs.com/draem0507/archive/2012/11/09/2762613.html