how to create middleware with net/http Handle() by implementing own Handler interface [golang]

main.go

package main

import (
	"log"
	"net/http"
)

type myHandler http.HandlerFunc

func (f myHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
	log.Println("do something always")
	f(w, r)
}

func main() {
	http.Handle("/abc", abcMiddleware(doABC))
	http.Handle("/abc/by-pass-abc-middleware", doABC)
	http.Handle("/xyz", xyzMiddleware(doXYZ))
	http.Handle("/xyz/by-pass-xyz-middleware", doXYZ)
	http.ListenAndServe(":8080", nil)
}

var doABC myHandler = func(w http.ResponseWriter, r *http.Request) {
	log.Println("do something every time ABC handler invoked")
	w.WriteHeader(http.StatusOK)
}

func abcMiddleware(next http.Handler) http.Handler {
	log.Println("do something only once on ABC middleware init")
	return myHandler(func(w http.ResponseWriter, r *http.Request) {
		log.Println("do something every time middleware for ABC handler invoked ")
		next.ServeHTTP(w, r)
	})
}

var doXYZ myHandler = func(w http.ResponseWriter, r *http.Request) {
	log.Println("do something every time XYZ handler invoked")
	w.WriteHeader(http.StatusOK)
}

func xyzMiddleware(next http.Handler) http.Handler {
	log.Println("do something only once on XYZ middleware init")
	return myHandler(func(w http.ResponseWriter, r *http.Request) {
		log.Println("do something every time middleware for XYZ handler invoked")
		next.ServeHTTP(w, r)
	})
}

output.log

$ go run main.go
2019/12/31 05:47:47 do something only once on ABC middleware init
2019/12/31 05:47:47 do something only once on XYZ middleware init
>>>
>>> get http://localhost:8080/abc
>>>
2019/12/31 05:47:54 do something always
2019/12/31 05:47:54 do something every time middleware for ABC handler invoked 
2019/12/31 05:47:54 do something always
2019/12/31 05:47:54 do something every time ABC handler invoked
>>>
>>> get http://localhost:8080/abc/by-pass-abc-middleware
>>>
2019/12/31 05:49:21 do something always
2019/12/31 05:49:21 do something every time ABC handler invoked
>>>
>>> get http://localhost:8080/xyz/by-pass-xyz-middleware
>>>
2019/12/31 05:50:53 do something always
2019/12/31 05:50:53 do something every time XYZ handler invoked
>>>
>>> get http://localhost:8080/xyz
>>>
2019/12/31 05:51:49 do something always
2019/12/31 05:51:49 do something every time middleware for XYZ handler invoked
2019/12/31 05:51:49 do something always
2019/12/31 05:51:49 do something every time XYZ handler invoked

Avoid usage of DefaultServeMux in production code due to its security risk

• DefaultServeMux is global variable and if you accidentally import any compromised third-party package, it could potentially register a route in DefaultServeMux and expose to the world some malicious handler
• You can use ServeMux locally scoped (to the func main() in the example above) and returned with http.NewServeMux()