servlet过滤器Java Web三大组件之一，它与Servlet很相似，但过滤器与Servlet功能不同，过滤器是用来拦截请求的，而不是处理请求的。
当用户请求某个Servlet时，会先执行部署在这个请求上的Filter，如果Filter“放行”，那么会继承执行用户请求的Servlet；如果Filter不“放行”，那么就不会执行用户请求的Servlet。(Servlet filter is one of the three main components of Java Web. It is very similar to Servlet, but filters are different from Servlet functions. Filters are used to intercept requests instead of processing requests.
When a user requests a Servlet, it will execute the Filter deployed on the request first. If Filter is released, it will inherit the Servlet that the user requests. If Filter does not "release", then the user request Servlet will not be executed.)