package main

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

func postFile(filename string, target_url string) (*http.Response, error) {
    body_buf := bytes.NewBufferString("")
    body_writer := multipart.NewWriter(body_buf)

    // use the body_writer to write the Part headers to the buffer
    _, err := body_writer.CreateFormFile("userfile", filename)
    if err != nil {
        fmt.Println("error writing to buffer")
        return nil, err
    }

    // the file data will be the second part of the body
    fh, err := os.Open(filename)
    if err != nil {
        fmt.Println("error opening file")
        return nil, err
    }
    // need to know the boundary to properly close the part myself.
    boundary := body_writer.Boundary()
    //close_string := fmt.Sprintf("\r\n--%s--\r\n", boundary)
    close_buf := bytes.NewBufferString(fmt.Sprintf("\r\n--%s--\r\n", boundary))

    // use multi-reader to defer the reading of the file data until
    // writing to the socket buffer.
    request_reader := io.MultiReader(body_buf, fh, close_buf)
    fi, err := fh.Stat()
    if err != nil {
        fmt.Printf("Error Stating file: %s", filename)
        return nil, err
    }
    req, err := http.NewRequest("POST", target_url, request_reader)
    if err != nil {
        return nil, err
    }

    // Set headers for multipart, and Content Length
    req.Header.Add("Content-Type", "multipart/form-data; boundary="+boundary)
    req.ContentLength = fi.Size() + int64(body_buf.Len()) + int64(close_buf.Len())

    return http.DefaultClient.Do(req)
}

// sample usage
func main() {
    target_url := "http://localhost:8086/upload"
    filename := "/Users/wei/Downloads/21dian_1.9_10"
    postFile(filename, target_url)
}
参考文章 https://matt.aimonetti.net/posts/2013/07/01/golang-multipart-file-upload-example/ 25

package main

import (
    "bytes"
    "fmt"
    "log"
    "mime/multipart"
    "net/http"
)

// Creates a new file upload http request with optional extra params
func newMultipartRequest(url string, params map[string]string) (*http.Request, error) {
    body := &bytes.Buffer{}
    writer := multipart.NewWriter(body)
    for key, val := range params {
        _ = writer.WriteField(key, val)
    }
    writer.Close()
    return http.NewRequest("POST", url, body)
}

func main() {
    extraParams := map[string]string{
        "title":       "My Document",
        "author":      "zieckey",
        "description": "A document with all the Go programming language secrets",
    }
    request, err := newMultipartRequest("http://127.0.0.1:8091/echo", extraParams)
    if err != nil {
        log.Fatal(err)
    }
    client := &http.Client{}
    resp, err := client.Do(request)
    if err != nil {
        log.Fatal(err)
    } else {
        body := &bytes.Buffer{}
        _, err := body.ReadFrom(resp.Body)
        if err != nil {
            log.Fatal(err)
        }
        resp.Body.Close()
        fmt.Println(resp.StatusCode)
        fmt.Println(resp.Header)
        fmt.Println(body)
    }
}
multipart 的例子

package main

import (
    "bytes"
    "fmt"
    "io"
    "log"
    "mime/multipart"
    "net/http"
    "os"
    "path/filepath"
)

// Creates a new file upload http request with optional extra params
func newfileUploadRequest(uri string, params map[string]string, paramName, path string) (*http.Request, error) {
    file, err := os.Open(path)
    if err != nil {
        return nil, err
    }
    defer file.Close()

    body := &bytes.Buffer{}
    writer := multipart.NewWriter(body)
    part, err := writer.CreateFormFile(paramName, filepath.Base(path))
    if err != nil {
        return nil, err
    }
    _, err = io.Copy(part, file)

    for key, val := range params {
        _ = writer.WriteField(key, val)
    }
    err = writer.Close()
    if err != nil {
        return nil, err
    }

    request, err := http.NewRequest("POST", uri, body)
    request.Header.Add("Content-Type", writer.FormDataContentType())
    return request, err
}

func main() {
    path, _ := os.Getwd()
    path += "/test.pdf"
    extraParams := map[string]string{
        "title":       "My Document",
        "author":      "Matt Aimonetti",
        "description": "A document with all the Go programming language secrets",
    }
    request, err := newfileUploadRequest("http://localhost:8086/upload", extraParams, "userfile", "/Users/wei/Downloads/21dian_1.9_10")
    if err != nil {
        log.Fatal(err)
    }
    client := &http.Client{}
    resp, err := client.Do(request)
    if err != nil {
        log.Fatal(err)
    } else {
        body := &bytes.Buffer{}
        _, err := body.ReadFrom(resp.Body)
        if err != nil {
            log.Fatal(err)
        }
        resp.Body.Close()
        fmt.Println(resp.StatusCode)
        fmt.Println(resp.Header)

        fmt.Println(body)
    }
}