使用 Content-Type multipart/form-data 的 POST 数据

Here's some sample code.

In short, you'll need to use the mime/multipart package to build the form.

package main


import (
"bytes"
"fmt"
"io"
"mime/multipart"
"net/http"
"net/http/httptest"
"net/http/httputil"
"os"
"strings"
)


func main() {


var client *http.Client
var remoteURL string
{
//setup a mocked http client.
ts := httptest.NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
b, err := httputil.DumpRequest(r, true)
if err != nil {
panic(err)
}
fmt.Printf("%s", b)
}))
defer ts.Close()
client = ts.Client()
remoteURL = ts.URL
}


//prepare the reader instances to encode
values := map[string]io.Reader{
"file":  mustOpen("main.go"), // lets assume its this file
"other": strings.NewReader("hello world!"),
}
err := Upload(client, remoteURL, values)
if err != nil {
panic(err)
}
}


func Upload(client *http.Client, url string, values map[string]io.Reader) (err error) {
// Prepare a form that you will submit to that URL.
var b bytes.Buffer
w := multipart.NewWriter(&b)
for key, r := range values {
var fw io.Writer
if x, ok := r.(io.Closer); ok {
defer x.Close()
}
// Add an image file
if x, ok := r.(*os.File); ok {
if fw, err = w.CreateFormFile(key, x.Name()); err != nil {
return
}
} else {
// Add other fields
if fw, err = w.CreateFormField(key); err != nil {
return
}
}
if _, err = io.Copy(fw, r); err != nil {
return err
}


}
// Don't forget to close the multipart writer.
// If you don't close it, your request will be missing the terminating boundary.
w.Close()


// Now that you have a form, you can submit it to your handler.
req, err := http.NewRequest("POST", url, &b)
if err != nil {
return
}
// Don't forget to set the content type, this will contain the boundary.
req.Header.Set("Content-Type", w.FormDataContentType())


// Submit the request
res, err := client.Do(req)
if err != nil {
return
}


// Check the response
if res.StatusCode != http.StatusOK {
err = fmt.Errorf("bad status: %s", res.Status)
}
return
}


func mustOpen(f string) *os.File {
r, err := os.Open(f)
if err != nil {
panic(err)
}
return r
}

I have found this tutorial very helpful to clarify my confusions about file uploading in Go.

Basically you upload the file via ajax using form-data on a client and use the following small snippet of Go code on the server:

file, handler, err := r.FormFile("img") // img is the key of the form-data
if err != nil {
fmt.Println(err)
return
}
defer file.Close()


fmt.Println("File is good")
fmt.Println(handler.Filename)
fmt.Println()
fmt.Println(handler.Header)




f, err := os.OpenFile(handler.Filename, os.O_WRONLY|os.O_CREATE, 0666)
if err != nil {
fmt.Println(err)
return
}
defer f.Close()
io.Copy(f, file)

Here r is *http.Request. P.S. this just stores the file in the same folder and does not perform any security checks.

After having to decode the accepted answer for this question for use in my unit testing I finally ended up with the follow refactored code:

func createMultipartFormData(t *testing.T, fieldName, fileName string) (bytes.Buffer, *multipart.Writer) {
var b bytes.Buffer
var err error
w := multipart.NewWriter(&b)
var fw io.Writer
file := mustOpen(fileName)
if fw, err = w.CreateFormFile(fieldName, file.Name()); err != nil {
t.Errorf("Error creating writer: %v", err)
}
if _, err = io.Copy(fw, file); err != nil {
t.Errorf("Error with io.Copy: %v", err)
}
w.Close()
return b, w
}


func mustOpen(f string) *os.File {
r, err := os.Open(f)
if err != nil {
pwd, _ := os.Getwd()
fmt.Println("PWD: ", pwd)
panic(err)
}
return r
}

Now it should be pretty easy to use:

    b, w := createMultipartFormData(t, "image","../luke.png")


req, err := http.NewRequest("POST", url, &b)
if err != nil {
return
}
// Don't forget to set the content type, this will contain the boundary.
req.Header.Set("Content-Type", w.FormDataContentType())

Here's a function I've used that uses io.Pipe() to avoid reading in the entire file to memory or needing to manage any buffers. It handles only a single file, but could easily be extended to handle more by adding more parts within the goroutine. The happy path works well. The error paths have not hand much testing.

import (
"fmt"
"io"
"mime/multipart"
"net/http"
"os"
)


func UploadMultipartFile(client *http.Client, uri, key, path string) (*http.Response, error) {
body, writer := io.Pipe()


req, err := http.NewRequest(http.MethodPost, uri, body)
if err != nil {
return nil, err
}


mwriter := multipart.NewWriter(writer)
req.Header.Add("Content-Type", mwriter.FormDataContentType())


errchan := make(chan error)


go func() {
defer close(errchan)
defer writer.Close()
defer mwriter.Close()


w, err := mwriter.CreateFormFile(key, path)
if err != nil {
errchan <- err
return
}


in, err := os.Open(path)
if err != nil {
errchan <- err
return
}
defer in.Close()


if written, err := io.Copy(w, in); err != nil {
errchan <- fmt.Errorf("error copying %s (%d bytes written): %v", path, written, err)
return
}


if err := mwriter.Close(); err != nil {
errchan <- err
return
}
}()


resp, err := client.Do(req)
merr := <-errchan


if err != nil || merr != nil {
return resp, fmt.Errorf("http error: %v, multipart error: %v", err, merr)
}


return resp, nil
}

To extend on @attila-o answer, here is the code I went with to perform a POST HTTP req in Go with:

• 1 file
• configurable file name (f.Name() didn't work)
• extra form fields.

Curl representation:

curl -X POST \
http://localhost:9091/storage/add \
-H 'content-type: multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW' \
-F owner=0xc916Cfe5c83dD4FC3c3B0Bf2ec2d4e401782875e \
-F password=$PWD \
-F file=@./internal/file_example_JPG_500kB.jpg

Go way:

client := &http.Client{
Timeout: time.Second * 10,
}
req, err := createStoragePostReq(cfg)
res, err := executeStoragePostReq(client, req)




func createStoragePostReq(cfg Config) (*http.Request, error) {
extraFields := map[string]string{
"owner": "0xc916cfe5c83dd4fc3c3b0bf2ec2d4e401782875e",
"password": "pwd",
}


url := fmt.Sprintf("http://localhost:%d%s", cfg.HttpServerConfig().Port(), lethstorage.AddRoute)
b, w, err := createMultipartFormData("file","./internal/file_example_JPG_500kB.jpg", "file_example_JPG_500kB.jpg", extraFields)
if err != nil {
return nil, err
}


req, err := http.NewRequest("POST", url, &b)
if err != nil {
return nil, err
}
req.Header.Set("Content-Type", w.FormDataContentType())


return req, nil
}


func executeStoragePostReq(client *http.Client, req *http.Request) (lethstorage.AddRes, error) {
var addRes lethstorage.AddRes


res, err := client.Do(req)
if err != nil {
return addRes, err
}
defer res.Body.Close()


data, err := ioutil.ReadAll(res.Body)
if err != nil {
return addRes, err
}


err = json.Unmarshal(data, &addRes)
if err != nil {
return addRes, err
}


return addRes, nil
}


func createMultipartFormData(fileFieldName, filePath string, fileName string, extraFormFields map[string]string) (b bytes.Buffer, w *multipart.Writer, err error) {
w = multipart.NewWriter(&b)
var fw io.Writer
file, err := os.Open(filePath)


if fw, err = w.CreateFormFile(fileFieldName, fileName); err != nil {
return
}
if _, err = io.Copy(fw, file); err != nil {
return
}


for k, v := range extraFormFields {
w.WriteField(k, v)
}


w.Close()


return
}

Send file from one service to another:

func UploadFile(network, uri string, f multipart.File, h *multipart.FileHeader) error {


buf := new(bytes.Buffer)
writer := multipart.NewWriter(buf)


part, err := writer.CreateFormFile("file", h.Filename)


if err != nil {
log.Println(err)
return err
}


b, err := ioutil.ReadAll(f)


if err != nil {
log.Println(err)
return err
}


part.Write(b)
writer.Close()


req, _ := http.NewRequest("POST", uri, buf)


req.Header.Add("Content-Type", writer.FormDataContentType())
client := &http.Client{}
resp, err := client.Do(req)


if err != nil {
return err
}
defer resp.Body.Close()


b, _ = ioutil.ReadAll(resp.Body)
if resp.StatusCode >= 400 {
return errors.New(string(b))
}
return nil
}

Here is an option that works for files or strings:

package main


import (
"bytes"
"io"
"mime/multipart"
"os"
"strings"
)


func createForm(form map[string]string) (string, io.Reader, error) {
body := new(bytes.Buffer)
mp := multipart.NewWriter(body)
defer mp.Close()
for key, val := range form {
if strings.HasPrefix(val, "@") {
val = val[1:]
file, err := os.Open(val)
if err != nil { return "", nil, err }
defer file.Close()
part, err := mp.CreateFormFile(key, val)
if err != nil { return "", nil, err }
io.Copy(part, file)
} else {
mp.WriteField(key, val)
}
}
return mp.FormDataContentType(), body, nil
}

Example:

package main
import "net/http"


func main() {
form := map[string]string{"image": "@IMAGEFILE", "key": "KEY"}
ct, body, err := createForm(form)
if err != nil {
panic(err)
}
http.Post("https://stackoverflow.com", ct, body)
}

https://golang.org/pkg/mime/multipart#Writer.WriteField