【Go】Gmail APIでメールを送信する

Go LangでGmail APIからメールを送ってみるチュートリアルです。

Google API ConsoleからGmail APIを有効化し、認証情報(OAuth2.0 クライアントID)をJSONファイルとしてダウンロード、実行ファイルと同じディレクトリに"credentials.json"として配置する必要があります。

send_mail.go

package main

import (
    "encoding/base64"
    "encoding/json"
    "fmt"
    "io/ioutil"
    "log"
    "net/http"
    "net/url"
    "os"
    "os/user"
    "path/filepath"
    "strings"

    "golang.org/x/net/context"
    "golang.org/x/oauth2"
    "golang.org/x/oauth2/google"
    "google.golang.org/api/gmail/v1"
)

// getClient uses a Context and Config to retrieve a Token
// then generate a Client. It returns the generated Client.
func getClient(ctx context.Context, config *oauth2.Config) *http.Client {
    cacheFile, err := tokenCacheFile()
    if err != nil {
        log.Fatalf("Unable to get path to cached credential file. %v", err)
    }
    tok, err := tokenFromFile(cacheFile)
    if err != nil {
        tok = getTokenFromWeb(config)
        saveToken(cacheFile, tok)
    }
    return config.Client(ctx, tok)
}

// getTokenFromWeb uses Config to request a Token.
// It returns the retrieved Token.
func getTokenFromWeb(config *oauth2.Config) *oauth2.Token {
    authURL := config.AuthCodeURL("state-token", oauth2.AccessTypeOffline)
    fmt.Printf("Go to the following link in your browser then type the "+
        "authorization code: \n%v\n", authURL)

    var code string
    if _, err := fmt.Scan(&code); err != nil {
        log.Fatalf("Unable to read authorization code %v", err)
    }

    tok, err := config.Exchange(oauth2.NoContext, code)
    if err != nil {
        log.Fatalf("Unable to retrieve token from web %v", err)
    }
    return tok
}

// tokenCacheFile generates credential file path/filename.
// It returns the generated credential path/filename.
func tokenCacheFile() (string, error) {
    usr, err := user.Current()
    if err != nil {
        return "", err
    }
    tokenCacheDir := filepath.Join(usr.HomeDir, ".credentials")
    os.MkdirAll(tokenCacheDir, 0700)
    return filepath.Join(tokenCacheDir,
        url.QueryEscape("gmail-go-quickstart.json")), err
}

// tokenFromFile retrieves a Token from a given file path.
// It returns the retrieved Token and any read error encountered.
func tokenFromFile(file string) (*oauth2.Token, error) {
    f, err := os.Open(file)
    if err != nil {
        return nil, err
    }
    t := &oauth2.Token{}
    err = json.NewDecoder(f).Decode(t)
    defer f.Close()
    return t, err
}

// saveToken uses a file path to create a file and store the
// token in it.
func saveToken(file string, token *oauth2.Token) {
    fmt.Printf("Saving credential file to: %s\n", file)
    f, err := os.Create(file)
    if err != nil {
        log.Fatalf("Unable to cache oauth token: %v", err)
    }
    defer f.Close()
    json.NewEncoder(f).Encode(token)
}

func main() {
    ctx := context.Background()

    b, err := ioutil.ReadFile("credentials.json")
    if err != nil {
        log.Fatalf("Unable to read client secret file: %v", err)
    }

    config, err := google.ConfigFromJSON(b, gmail.MailGoogleComScope)
    if err != nil {
        log.Fatalf("Unable to parse client secret file to config: %v", err)
    }
    client := getClient(ctx, config)

    srv, err := gmail.New(client)
    if err != nil {
        log.Fatalf("Unable to retrieve gmail Client %v", err)
    }

    var message gmail.Message

    temp := []byte("From: 'me'\r\n" +
        "reply-to: [e-mail]\r\n" +
        "To:  [e-mail]\r\n" +
        "Subject: Testing Gmail API \r\n" +
        "\r\nHelloooooo!!")

    message.Raw = base64.StdEncoding.EncodeToString(temp)
    message.Raw = strings.Replace(message.Raw, "/", "_", -1)
    message.Raw = strings.Replace(message.Raw, "+", "-", -1)
    message.Raw = strings.Replace(message.Raw, "=", "", -1)

    _, err = srv.Users.Messages.Send("me", &message).Do()
    if err != nil {
        log.Fatalf("Unable to send. %v", err)
    }

}