Go sample code of net/http middleware.

middleware.go

package main

// http://www.alexedwards.net/blog/making-and-using-middleware

import (
  "errors"
  "fmt"
  "html"
  "log"
  "net/http"
  "regexp"
)

type WrapperWriter struct {
  http.ResponseWriter
  Body string
}

func NewWrapperWriter(w http.ResponseWriter) *WrapperWriter {
  wrapper := new(WrapperWriter)
  wrapper.ResponseWriter = w
  return wrapper
}

func (ww *WrapperWriter) Write(p []byte) (n int, err error) {
  ww.Body += string(p)
  return len(p), nil
}

func (ww *WrapperWriter) Flush() (n int, err error) {
  if ww.Body == "" {
    return 0, errors.New("no body")
  }
  return ww.ResponseWriter.Write([]byte(ww.Body))
}

func middleware(next http.Handler) http.Handler {
  return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
    if r.URL.Path == "/redirect" {
      http.Redirect(w, r, "/redirected", 302)
      return
    }
    if r.URL.Path != "/redirected" {
      r.URL.Path += "/rewritten"
    }

    wrapper := NewWrapperWriter(w)
    defer wrapper.Flush()
    
    next.ServeHTTP(wrapper, r)

    fmt.Fprintf(wrapper, "\ninserted by middleware")

    wrapper.Body = regexp.MustCompile(`Hello,`).ReplaceAllString(wrapper.Body, "こんにちは、")
  })
}

func handler(w http.ResponseWriter, r *http.Request) {
  fmt.Fprintf(w, "Hello, %q", html.EscapeString(r.URL.Path))
}

func main() {
  topHandler := http.HandlerFunc(handler)
  http.Handle("/", middleware(topHandler))
  log.Fatal(http.ListenAndServe(":5000", nil))
}