我们用Golang做网站时，必然会有这样的代码：
http.Handle("/", http.FileServer(http.Dir(".")))
注意这里的 http.Handle( 函数的第二个参数，Golang的源码中这个函数的描述如下：
// Handle registers the handler for the given pattern
// in the DefaultServeMux.
// The documentation for ServeMux explains how patterns are matched.
func Handle(pattern string, handler Handler) { DefaultServeMux.Handle(pattern, handler) }
其中第二个参数是一个 interface，必须实现ServeHTTP(ResponseWriter, *Request)方法
// Objects implementing the Handler interface can be
// registered to serve a particular path or subtree
// in the HTTP server.
//
// ServeHTTP should write reply headers and data to the ResponseWriter
// and then return. Returning signals that the request is finished
// and that the HTTP server can move on to the next request on
// the connection.
type Handler interface {
ServeHTTP(ResponseWriter, *Request)
}
如果我们用WebSocket时，又会发现， 我们给http.Handle( 传递的是 websocket.Handler(。
http.Handle("/socket", websocket.Handler(Echo))
websocket.Handler 又是另外一个接口，如下
// Handler is an interface to a WebSocket.
type Handler func(*Conn)
这里为啥会出现接口更换了？ 其实 再往下看可以看到 websocket.Handler 接口多一个公共的方法：ServeHTTP， 即接口 websocket.Handler 实现了 http 的 Handler 接口。
// ServeHTTP implements the http.Handler interface for a Web Socket
func (h Handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
// Handler is an interface to a WebSocket.type Handler func(*Conn)
// ServeHTTP implements the http.Handler interface for a Web Socketfunc (h Handler) ServeHTTP(w http.ResponseWriter, req *http.Request) {
