Embed Resources in Go

The go:embed feature was introduce in Go 1.16. It lets you embed resources into the compiled application. Prior to version 1.16, developers uses external tooling to do this. With go:embed you can embed images, webpages, email templates, predefined SQL statements or an entire directory. That’s neat!

Usage Examples#

Embedding a text file in a string#

//go:embed version.txt
var version string

Embedding a binary file#

//go:embed product-catalog.pdf
var catalog []byte

Embedding a directory filesystem#

//go:embed template/*
var templateFS fs.FS

Embedded resources in httprouter#

Project Structure

go-embed-httprouter 
├─static/ 
│ └─img/ 
│ 
├─cat1.jpg 
│ ├─cat2.jpg 
│ └─cat3.jpg 
└─main.go

main.go

package main

import (
 "embed"
 "fmt"
 "github.com/julienschmidt/httprouter"
 "io/fs"
 "log"
 "net/http"
 "time"
)

//go:embed static/*
var staticFS embed.FS

func main() {
 f := fs.FS(staticFS)
 v, _ := fs.Sub(f, "static")

 router := httprouter.New()
 router.ServeFiles(fmt.Sprintf("%s", "/static/*filepath"), http.FS(v))
 router.HandlerFunc(http.MethodGet, "/", func(w http.ResponseWriter, r *http.Request) {
  http.Redirect(w, r, "/static/img/cat1.jpg", http.StatusTemporaryRedirect)
 })

 port := "8080"
 httpServer := &http.Server{
  Addr:         ":" + port,
  Handler:      router,
  ReadTimeout:  120 * time.Second,
  WriteTimeout: 120 * time.Second,
 }

 log.Printf("Server running at http://localhost:%s/\n", port)
 log.Fatal(httpServer.ListenAndServe())
}

Verifying the embedded files#

One way to verify for the embedded files inside the compiled application is by a hex editor. Using the hex editor, find the marker for the JPEG File Interchange Format ( JFIF). In this case, we will look for 3 jpeg images.

1st jpeg at Offset: 002A:E7CA

2nd jpeg at Offset: 002C:A6EA

3rd jpeg at Offset: 002F:06EA

Source Code#

https://github.com/johnpili/go-embed-httprouter

Conclusion#

The ability to embed all resources inside the compiled binary application is a powerful feature. It makes the application portable and easier to deploy. With go:embed, you don’t need an installer or script to unpack and use your resources.

https://www.w3.org/Graphics/JPEG/jfif3.pdf

https://pkg.go.dev/embed

Originally published at https://johnpili.com on December 18, 2022.