提供带有自定义状态代码的html文件

I need to have a custom not found html page. Here is what I've tried:

package main

import (
    "net/http"

    "github.com/julienschmidt/httprouter"
)

func main() {
    r := httprouter.New()

    r.NotFound = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        w.WriteHeader(404)
        http.ServeFile(w, r, "files/not-found.html")
    })

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

I have the line w.WriteHeader(404) to make sure the status code is 404, but the code above gives the error:

http: multiple response.WriteHeader calls

Without the line w.WriteHeader(404) there are no errors and the page is shown correctly, but the status code is 200. I want it to be 404.

You can simply just write the contents yourself.

Something like:

r.NotFound = http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        contents, err := ioutil.ReadFile("files/not-found.html")
        if err != nil {
            panic(err) // or do something useful
        }
        w.WriteHeader(404)
        w.Header().Set("Content-Type", "text/html; charset=utf-8")
        w.Write(contents)
    })

David's answer worked, and this is another way.

// other header stuff
w.WriteHeader(http.StatusNotFound)
file, err := os.Open("files/not-found.html")
if err != nil {
    log.Println(err)
    return
}
_, err = io.Copy(w, file)
if err != nil {
    log.Println(err)
}
file.Close() // consider defer ^