Java 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 Java command-line application that makes requests to the Postmaster Tools API.


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


  • 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 Postmaster Tools API.

    Enable the API

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.

  1. In the Google Cloud console, go to Menu > APIs & Services > OAuth consent screen.

    Go to OAuth consent screen

  2. For User type select Internal, then click Create.
  3. Complete the app registration form, then click Save and Continue.
  4. 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.

  5. 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.
  1. In the Google Cloud console, go to Menu > APIs & Services > Credentials.

    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.
  7. Save the downloaded JSON file as credentials.json, and move the file to your working directory.

Prepare the workspace

  1. In your working directory, create a new project structure:

    gradle init --type basic
    mkdir -p src/main/java src/main/resources 
  2. In the src/main/resources/ directory, copy the credentials.json file that you previously downloaded.

  3. Open the default build.gradle file and replace its contents with the following code:

      apply plugin: 'java'
    apply plugin: 'application'
    mainClassName = 'GmailPostmasterToolsQuickstart'
    sourceCompatibility = 1.8
    targetCompatibility = 1.8
    version = '1.0'
    repositories {
    dependencies {
        implementation ''
        implementation ''
        implementation ''

Set up the sample

  1. In the src/main/java/ directory, create a new Java file with a name that matches the mainClassName value in your build.gradle file.

  2. Include the following code in your new Java file:

    import java.util.Collections;
    import java.util.List;
    public class PostmasterToolsApiQuickStart {
        private static final String APPLICATION_NAME = "PostmasterTools API Java Quickstart";
        private static final JsonFactory JSON_FACTORY = JacksonFactory.getDefaultInstance();
        private static final String TOKENS_DIRECTORY_PATH = "tokens";
         * Global instance of the scopes required by this quickstart.
         * If modifying these scopes, delete your previously saved tokens/ folder.
        private static final List<String> SCOPES = Collections.singletonList("");
        private static final String CREDENTIALS_FILE_PATH = "/credentials.json";
         * Creates an authorized Credential object.
         * @param HTTP_TRANSPORT The network HTTP Transport.
         * @return An authorized Credential object.
         * @throws IOException If the credentials.json file cannot be found.
        private static Credential getCredentials(final NetHttpTransport HTTP_TRANSPORT) throws IOException {
            // Load client secrets.
            InputStream in = PostmasterToolsApiQuickStart.class.getResourceAsStream(CREDENTIALS_FILE_PATH);
            if (in == null) {
                throw new FileNotFoundException("Resource not found: " + CREDENTIALS_FILE_PATH);
            GoogleClientSecrets clientSecrets = GoogleClientSecrets.load(JSON_FACTORY, new InputStreamReader(in));
            // Build flow and trigger user authorization request.
            GoogleAuthorizationCodeFlow flow = new GoogleAuthorizationCodeFlow.Builder(
                    HTTP_TRANSPORT, JSON_FACTORY, clientSecrets, SCOPES)
                    .setDataStoreFactory(new FileDataStoreFactory(new
            LocalServerReceiver receiver = new LocalServerReceiver.Builder().setPort(8891).build();
            return new AuthorizationCodeInstalledApp(flow, receiver).authorize("user");
        public static void main(String... args) throws IOException, GeneralSecurityException {
            // Build a new authorized API client service.
            final NetHttpTransport HTTP_TRANSPORT = GoogleNetHttpTransport.newTrustedTransport();
            PostmasterTools postmasterTools = new PostmasterTools.Builder(HTTP_TRANSPORT, JSON_FACTORY, getCredentials(HTTP_TRANSPORT))
            List<Domain> domains =;
            if (domains == null || domains.isEmpty()) {
                System.out.println("No domains found!");
            } else {
                for (Domain domain : domains) {

Run the sample

  1. Run the sample:

    gradle run
  1. 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 Java application runs and calls the Postmaster Tools API.

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

Next steps