【app.use】

　　app.use([path,] function [, function...])

　　Mounting a middleware at a path will cause the middleware function to be executed whenever the base of the requested path matches the path.

　　当basePath匹配时，use监听会被调用。

　　Since path defaults to “/”, middleware mounted without a path will be executed for every request to the app.

　　Middleware functions are executed sequentially, therefore the order of middleware inclusion is important.

参考：http://www.expressjs.com.cn/4x/api.html#app.use

app.use