Golang : Force download file example

For this tutorial, we will learn how to create a page that will force download a file to the client. Instead of waiting for the user to click the download link to a file, the server will force download the file with the help of meta refresh.

In case the force download doesn't work... you still can offer a link for the visitor to click on to initiate the download process.

Here you go!

 package main

 import (
 "bytes"
 "fmt"
 "io/ioutil"
 "net/http"
 "strconv"
 "time"
 )

 // change to your target filename
 // that you want to force download to your visitor browser
 var file = "img.pdf"

 func Home(w http.ResponseWriter, r *http.Request) {

 // We will force download after 2 seconds. If you want to increase the delay
 // simply change 2 to whatever number you wanted.

 html := "<html><meta http-equiv='refresh' content='2; url=http://localhost:8080/download' />"
 html = html + "Downloading file now. If the download does not happen automagically. Please "
 html = html + "<a href=" + file + ">click here</a></html>"
 w.Write([]byte(html))
 }

 func ForceDownload(w http.ResponseWriter, r *http.Request) {

 downloadBytes, err := ioutil.ReadFile(file)

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

 // set the default MIME type to send
 mime := http.DetectContentType(downloadBytes)

 fileSize := len(string(downloadBytes))

 // Generate the server headers
 w.Header().Set("Content-Type", mime)
 w.Header().Set("Content-Disposition", "attachment; filename="+file+"")
 w.Header().Set("Expires", "0")
 w.Header().Set("Content-Transfer-Encoding", "binary")
 w.Header().Set("Content-Length", strconv.Itoa(fileSize))
 w.Header().Set("Content-Control", "private, no-transform, no-store, must-revalidate")

 //b := bytes.NewBuffer(downloadBytes)
 //if _, err := b.WriteTo(w); err != nil {
 // fmt.Fprintf(w, "%s", err)
 // }

 // force it down the client's.....
 http.ServeContent(w, r, file, time.Now(), bytes.NewReader(downloadBytes))

 }

 func main() {
 http.HandleFunc("/", Home)
 http.HandleFunc("/download", ForceDownload)

 // SECURITY : Only expose the file permitted for download.
 http.Handle("/"+file, http.FileServer(http.Dir("./")))

 http.ListenAndServe(":8080", nil)
 }

In the code above, change the file img.pdf to whatever filename that you have on the same directory as the program.

Point your browser to localhost:8080 and if everything goes smoothly, you should see the message and the download action is initiated automagically after 2 seconds.

Happy coding!

References:

https://www.socketloop.com/tutorials/golang-how-to-stream-file-to-client-browser-or-write-to-http-responsewriter

https://www.socketloop.com/tutorials/golang-how-to-display-image-file-or-expose-css-js-files-from-localhost

https://www.codeigniter.com/userguide/helpers/downloadhelper.html#force_download

Golang : Force download file example

See also : Golang : How to stream file to client(browser) or write to http.ResponseWriter?

By Adam Ng

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