Example of chaining middlware in Go

main.go

package main

import (
	"io"
	"log"
	"mime"
	"net/http"
	"os"

	"github.com/goji/httpauth"
	"github.com/gorilla/handlers"
)

func main() {
	logFile, err := os.OpenFile("server.log", os.O_WRONLY|os.O_CREATE|os.O_APPEND, 0664)
	if err != nil {
		log.Fatal(err)
	}

	loggingHandler := newLoggingHandler(logFile)
	authHandler := httpauth.SimpleBasicAuth("alice", "pa$$word")

	mux := http.NewServeMux()

	finalHandler := http.HandlerFunc(final)
	mux.Handle("/", loggingHandler(authHandler(enforceJSONHandler(finalHandler))))

	log.Println("Listening on :3000...")
	err = http.ListenAndServe(":3000", mux)
	log.Fatal(err)
}

func enforceJSONHandler(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		contentType := r.Header.Get("Content-Type")

		if contentType != "" {
			mt, _, err := mime.ParseMediaType(contentType)
			if err != nil {
				http.Error(w, "Malformed Content-Type header", http.StatusBadRequest)
				return
			}

			if mt != "application/json" {
				http.Error(w, "Content-Type header must be application/json", http.StatusUnsupportedMediaType)
				return
			}
		}

		next.ServeHTTP(w, r)
	})
}

func newLoggingHandler(dst io.Writer) func(http.Handler) http.Handler {
	return func(h http.Handler) http.Handler {
		return handlers.LoggingHandler(dst, h)
	}
}

func final(w http.ResponseWriter, r *http.Request) {
	w.Write([]byte("OK"))
}

On line 48

else {
    http.Error(w, "Content-Type header must be provided", http.StatusBadRequest)
    return
}