How to download file in browser from Go server

Question

My code get file from remote url and download file in browser:

func Index(w http.ResponseWriter, r *http.Request) {
    url := "http://upload.wikimedia.org/wikipedia/en/b/bc/Wiki.png"

    ...

    resp, err := client.Get(url)
    if err != nil {
        fmt.Println(err)
    }
    defer resp.Body.Close()

    body, err := ioutil.ReadAll(resp.Body)
    if err != nil {
        fmt.Println(err)
    }

    fmt.Println(len(body))
    //download the file in browser

}

func main() {
    http.HandleFunc("/", Index)
    err := http.ListenAndServe(":8000", nil)

    if err != nil {
        fmt.Println(err)
    }
}

code: http://play.golang.org/p/x-EyR2zFjv

Get file is ok, but how to downloaded it in browser?

Answer 1

To make the browser open the download dialog, add a Content-Disposition and Content-Type headers to the response:

w.Header().Set("Content-Disposition", "attachment; filename=WHATEVER_YOU_WANT")
w.Header().Set("Content-Type", r.Header.Get("Content-Type"))

Do this BEFORE sending the content to the client. You might also want to copy the Content-Length header of the response to the client, to show proper progress.

To stream the response body to the client without fully loading it into memory (for big files this is important) - simply copy the body reader to the response writer:

io.Copy(w, resp.Body)

io.Copy is a nice little function that take a reader interface and writer interface, reads data from one and writes it to the other. Very useful for this kind of stuff!

I've modified your code to do this: http://play.golang.org/p/v9IAu2Xu3_