Golang net/http Context and Dependency Injection

using-context.go

package main

import (
	"context"
	"net/http"
)

func main() {
	helloHandler := AddMessageMiddleware("Hello!", http.HandlerFunc(RespondWithMessage))
	goodbyeHandler := AddMessageMiddleware("Goodbye!", http.HandlerFunc(RespondWithMessage))
	noMessageHandler := http.HandlerFunc(RespondWithMessage)

	mux := new(http.ServeMux)
	mux.Handle("/hello", helloHandler)
	mux.Handle("/goodbye", goodbyeHandler)
	mux.Handle("/error", noMessageHandler)

	http.ListenAndServe(":8000", mux)
}

const MessageContextKey = "message"

func AddMessageMiddleware(message string, next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		ctx := context.WithValue(r.Context(), MessageContextKey, message)
		next.ServeHTTP(w, r.WithContext(ctx))
	})
}

func RespondWithMessage(w http.ResponseWriter, r *http.Request) {
	message := r.Context().Value(MessageContextKey)
	if message == nil {
		w.WriteHeader(500)
		w.Write([]byte("No message provided."))
	} else {
		w.WriteHeader(200)
		w.Write([]byte(message.(string)))
	}
}