A simple demonstration of how to write middleware for HTTP handlers in Go.

middleware_demo.go

package main

import (
	"log"
	"net/http"
)

var (
	GREETING = []byte("Hello there!")
)

func Logger(handler http.Handler) http.Handler {
	var wrappedHandler http.HandlerFunc = func(w http.ResponseWriter, r *http.Request) {
		// Before:
		log.Printf("Request to %s initiated", r.URL.Path)

		// Call inner handler (what we are wrapping):
		handler.ServeHTTP(w, r)

		//After:
		log.Printf("Request to %s completed", r.URL.Path)
	}
	return wrappedHandler
}

func Greet(w http.ResponseWriter, r *http.Request) {
	w.Write(GREETING)
}

func main() {
	http.Handle("/", Logger(http.HandlerFunc(Greet)))
	log.Fatalln(http.ListenAndServe(":8000", nil))
}