how to create middleware with net/http HandleFunc() [golang]

main.go

package main

import (
	"log"
	"net/http"
)

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

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

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

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

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

output.log

$ go run ./m1/*.go
2019/12/31 05:54:33 do something only once on ABC middleware init
2019/12/31 05:54:33 do something only once on XYZ middleware init
>>>
>>> get http://localhost:8080/abc
>>>
2019/12/31 05:55:34 do something every time middleware for ABC handler invoked 
2019/12/31 05:55:34 do something every time ABC handler invoked
>>>
>>> get http://localhost:8080/abc/by-pass-abc-middleware
>>>
2019/12/31 05:56:28 do something every time ABC handler invoked
>>>
>>> get http://localhost:8080/xyz
>>>
2019/12/31 05:57:21 do something every time middleware for XYZ handler invoked 
2019/12/31 05:57:21 do something every time XYZ handler invoked
>>>
>>> get http://localhost:8080/xyz/by-pass-xyz-middleware
>>>
2019/12/31 05:58:02 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()