Go quickstart

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Quickstarts explain how to set up and run an app that calls a Google Workspace API.

Google Workspace quickstarts use the API client libraries to handle some details of the authentication and authorization flow. We recommend that you use the client libraries for your own apps. Before you can run the sample app, each quickstart requires that you turn on authentication and authorization. If you're unfamiliar with authentication and authorization for Google Workspace APIs, read the Authentication and authorization overview.

Create a Go command-line application that makes requests to the Gmail API.

Objectives

  • Set up your environment.
  • Set up the sample.
  • Run the sample.

Prerequisites

  • A Google account with Gmail enabled.

Set up your environment

To complete this quickstart, set up your environment.

Enable the API

Before using Google APIs, you need to enable them in a Google Cloud project. You can enable one or more APIs in a single Google Cloud project.

Authorize credentials for a desktop application

To authenticate as an end user and access user data in your app, you need to create one or more OAuth 2.0 Client IDs. A client ID is used to identify a single app to Google's OAuth servers. If your app runs on multiple platforms, you must create a separate client ID for each platform.
  1. In the Google Cloud console, go to the Credentials page.

    Go to Credentials

  2. Click Create Credentials > OAuth client ID.
  3. Click Application type > Desktop app.
  4. In the Name field, type a name for the credential. This name is only shown in the Google Cloud console.
  5. Click Create. The OAuth client created screen appears, showing your new Client ID and Client secret.
  6. Click OK. The newly created credential appears under OAuth 2.0 Client IDs.
Make a note of these credentials because you need them later in this quickstart.

Prepare the workspace

  1. Create a working directory:

    mkdir quickstart
    
  2. Change to the working directory:

    cd quickstart
    
  3. Initialize the new module:

    go mod init quickstart
    
  4. Get the Gmail API Go client library and OAuth2 package:

    go get google.golang.org/api/gmail/v1
    go get golang.org/x/oauth2/google
    

Set up the sample

  1. In your working directory, create a file named quickstart.go.

  2. In the file, paste the following code:

    gmail/quickstart/quickstart.go
    package main
    
    import (
    	"context"
    	"encoding/json"
    	"fmt"
    	"log"
    	"net/http"
    	"os"
    
    	"golang.org/x/oauth2"
    	"golang.org/x/oauth2/google"
    	"google.golang.org/api/gmail/v1"
    	"google.golang.org/api/option"
    )
    
    // Retrieve a token, saves the token, then returns the generated client.
    func getClient(config *oauth2.Config) *http.Client {
    	// The file token.json stores the user's access and refresh tokens, and is
    	// created automatically when the authorization flow completes for the first
    	// time.
    	tokFile := "token.json"
    	tok, err := tokenFromFile(tokFile)
    	if err != nil {
    		tok = getTokenFromWeb(config)
    		saveToken(tokFile, tok)
    	}
    	return config.Client(context.Background(), tok)
    }
    
    // Request a token from the web, then 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 authCode string
    	if _, err := fmt.Scan(&authCode); err != nil {
    		log.Fatalf("Unable to read authorization code: %v", err)
    	}
    
    	tok, err := config.Exchange(context.TODO(), authCode)
    	if err != nil {
    		log.Fatalf("Unable to retrieve token from web: %v", err)
    	}
    	return tok
    }
    
    // Retrieves a token from a local file.
    func tokenFromFile(file string) (*oauth2.Token, error) {
    	f, err := os.Open(file)
    	if err != nil {
    		return nil, err
    	}
    	defer f.Close()
    	tok := &oauth2.Token{}
    	err = json.NewDecoder(f).Decode(tok)
    	return tok, err
    }
    
    // Saves a token to a file path.
    func saveToken(path string, token *oauth2.Token) {
    	fmt.Printf("Saving credential file to: %s\n", path)
    	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
    	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 := os.ReadFile("credentials.json")
    	if err != nil {
    		log.Fatalf("Unable to read client secret file: %v", err)
    	}
    
    	// If modifying these scopes, delete your previously saved token.json.
    	config, err := google.ConfigFromJSON(b, gmail.GmailReadonlyScope)
    	if err != nil {
    		log.Fatalf("Unable to parse client secret file to config: %v", err)
    	}
    	client := getClient(config)
    
    	srv, err := gmail.NewService(ctx, option.WithHTTPClient(client))
    	if err != nil {
    		log.Fatalf("Unable to retrieve Gmail client: %v", err)
    	}
    
    	user := "me"
    	r, err := srv.Users.Labels.List(user).Do()
    	if err != nil {
    		log.Fatalf("Unable to retrieve labels: %v", err)
    	}
    	if len(r.Labels) == 0 {
    		fmt.Println("No labels found.")
    		return
    	}
    	fmt.Println("Labels:")
    	for _, l := range r.Labels {
    		fmt.Printf("- %s\n", l.Name)
    	}
    }
    

Run the sample

  1. In your working directory, build and run the sample:

    go run quickstart.go
    
  2. In your browser, go to http://localhost:8000.

  3. The first time you run the sample, it prompts you to authorize access:

    1. If you're not already signed in to your Google Account, you're prompted to sign in. If you're signed in to multiple accounts, select one account to use for authorization.
    2. Click Accept.
    3. Copy the code from the browser, paste it into the command-line prompt, and press Enter.

    Authorization information is stored in the file system, so the next time you run the sample code, you aren't prompted for authorization.

You have successfully created your first Go application that makes requests to the Gmail API.

Next steps