Golang 标准库net/http自定义404页面

Golang 标准库net/http自定义404页面，通过中间件的形式来实现。

type hijack404 struct {
	http.ResponseWriter
	R         *http.Request
	Handle404 func(w http.ResponseWriter, r *http.Request) bool
}

// WriteHeader WriteHeader
func (h *hijack404) WriteHeader(code int) {
	if 404 == code && h.Handle404(h.ResponseWriter, h.R) {
		panic(h)
	}
	h.ResponseWriter.WriteHeader(code)
}

// Middle404Handler Middle404Handler
func Middle404Handler(handler http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		hijack := &hijack404{w, r, Page404Handler}
		defer func() {
			if p := recover(); p != nil {
				if p == hijack {
					return
				}
				panic(p)
			}
		}()
		handler.ServeHTTP(hijack, r)
	})
}

// Page404Handler Page404Handler
func Page404Handler(w http.ResponseWriter, r *http.Request) bool {
	ctx, err := ioutil.ReadFile("404.html")
	if err != nil {
		log.Printf("file not found %s", err.Error())
		return false
	}
	if _, ok := w.Header()["Content-Length"]; ok {
		w.Header().Set("Content-Length", strconv.Itoa(len(string(ctx))))
	} else {
		w.Header().Add("Content-Length", strconv.Itoa(len(string(ctx))))
	}
	if _, ok := w.Header()["Content-Type"]; ok {
		w.Header().Set("Content-Type", "text/html; charset=utf-8")
	} else {
		w.Header().Add("Content-Type", "text/html; charset=utf-8")
	}
	w.WriteHeader(404)
	w.Write(ctx)
	return true
}

其它实现方式参考：https://stackoverflow.com/questions/31535569/golang-how-to-read-response-body-of-reverseproxy