Filter的用途

拦截请求 过滤响应

应用场景：权限检查 日志记录 事务管理 资源准备

With filter, developer can put abstracted common pre-code or post-code in the filter and need not to put interface of the common pre-code or post-code in caller of it. It's extra good to the target resources where the common pre-code/post-code or the interface of it can not be written in.  such as image, audio, video，html, etc.