Filter是servlet中很是重要的一个类，它就是一个过滤器，过滤器的意思就是拦截某一些你指定的url地址，然后对立面的内容进行截取，根据你的要求来操作数据。它其实就是一个实现了servlet的Filter接口的类，在web.xml里面需要配置。(Filter servlet is a kind of important, it is a filter, filter means to intercept some of you specify the URL address, and then on the opposite side of the contents of the interception, to operate according to your request data. It is actually a class that implements the Filter interface of servlet, which needs to be configured inside the web.xml.)