https://blog.csdn.net/a_flying_bird/article/details/52462001

package main 

import (
    "fmt"
    "log"
    "net/http"
)

type MyServeMux struct {
    http.ServeMux
    id int
}

type MessageHandler struct {
    msg string 
}

func (m *MessageHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
    fmt.Fprint(w, m.msg)
}

func main() {
    mux := new (MyServeMux)
    handler1 := &MessageHandler{"The first handler."}
    mux.Handle("/first",  handler1)
}