Go quickstart

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. This quickstart uses a simplified authentication approach that is appropriate for a testing environment. For a production environment, we recommend learning about authentication and authorization before choosing the access credentials that are appropriate for your app.

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

Latest version of Go.
Latest version of Git.
A Google Cloud project.

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 turn them on in a Google Cloud project. You can turn on one or more APIs in a single Google Cloud project.

In the Google Cloud console, enable the Gmail API.

Enable the API

Configure the OAuth consent screen

If you're using a new Google Cloud project to complete this quickstart, configure the OAuth consent screen and add yourself as a test user. If you've already completed this step for your Cloud project, skip to the next section.

In the Google Cloud console, go to Menu > APIs & Services > OAuth consent screen.
Go to OAuth consent screen
For User type select Internal, then click Create.
Complete the app registration form, then click Save and Continue.
For now, you can skip adding scopes and click Save and Continue. In the future, when you create an app for use outside of your Google Workspace organization, you must change the User type to External, and then, add the authorization scopes that your app requires.
Review your app registration summary. To make changes, click Edit. If the app registration looks OK, click Back to Dashboard.

Authorize credentials for a desktop application

To authenticate end users 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.

In the Google Cloud console, go to Menu > APIs & Services > Credentials.
Go to Credentials
Click Create Credentials > OAuth client ID.
Click Application type > Desktop app.
In the Name field, type a name for the credential. This name is only shown in the Google Cloud console.
Click Create. The OAuth client created screen appears, showing your new Client ID and Client secret.
Click OK. The newly created credential appears under OAuth 2.0 Client IDs.
Save the downloaded JSON file as credentials.json, and move the file to your working directory.

Prepare the workspace

Create a working directory:
```
mkdir quickstart
```
Change to the working directory:
```
cd quickstart
```
Initialize the new module:
```
go mod init quickstart
```

Get the Gmail API Go client library and OAuth2.0 package:

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

Set up the sample

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

In the file, paste the following code:

gmail/quickstart/quickstart.go

View on GitHub

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

In your working directory, build and run the sample:
```
go run quickstart.go
```

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, sign in when prompted. If you're signed in to multiple accounts, select one account to use for authorization.
2. Click Accept.
Your Go application runs and calls the Gmail API.

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