Go Web Examples: Middleware (Basic)

This example will show how to create basic logging middleware in Go.

A middleware simply takes a http.HandlerFunc as one of its parameters, wraps it and returns a new http.HandlerFunc for the server to call.

// basic-middleware.go
package main

import (
	"fmt"
	"log"
	"net/http"
)

func logging(f http.HandlerFunc) http.HandlerFunc {
	return func(w http.ResponseWriter, r *http.Request) {
		log.Println(r.URL.Path)
		f(w, r)
	}
}

func foo(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintln(w, "foo")
}

func bar(w http.ResponseWriter, r *http.Request) {
	fmt.Fprintln(w, "bar")
}

func main() {
	http.HandleFunc("/foo", logging(foo))
	http.HandleFunc("/bar", logging(bar))

	http.ListenAndServe(":8080", nil)
}
$ go run basic-middleware.go
2017/02/10 23:59:34 /foo
2017/02/10 23:59:35 /bar
2017/02/10 23:59:36 /foo?bar

$ curl -s http://localhost:8080/foo
$ curl -s http://localhost:8080/bar
$ curl -s http://localhost:8080/foo?bar