How to authenticate a service account without download for the Google Cloud Java SDK (not on App Engine)

I am trying to create a service account key programmatically in Java with the Google Cloud SDK, for an application not running on App/Compute engine. This question is similar to mine, but it is running on App engine, so I cannot use the same code as it uses classes from the App Engine API.

The relevant code is below. My issue is that AppIdentityCredential is part of the AppEngine API and thus cannot be used here. What can I pass in as a parameter instead? The third parameter in the new Builder() method takes in an HttpRequestInitializer, but I don't understand what implementation of this interface I should pass in. Any help is appreciated.

import com.google.api.services.iam.v1.Iam;
import com.google.api.services.iam.v1.model.CreateServiceAccountKeyRequest;
import com.google.api.services.iam.v1.model.ServiceAccountKey;

AppIdentityCredential credential = new AppIdentityCredential(
            Arrays.asList("https://www.googleapis.com/auth/cloud-platform"));
Iam iam = new Iam.Builder(httpTransport, JSON_FACTORY,credential)
         .setApplicationName(APPLICATION_NAME).build();
ServiceAccountKey key = iam.projects().serviceAccounts().keys()
     .create(SERVICE_ACCOUNT_RESOURCE_NAME, new CreateServiceAccountKeyRequest()).execute();

You can use Application Default Credentials which will allow you to use the same code to fetch the credentials based on the environment where the application is running.

For example, it lets you use your gcloud account credentials when you're developing on your system. When the code runs on Google Compute Engine or Google App Engine, the code will automatically use the associated service account credentials for authentication in the APIs. You can also override it using GOOGLE_APPLICATION_CREDENTIALS environment variable if required to load the credentials from a JSON file instead.

Here is a working example which creates a new key for an existing service account and prints it.

import com.google.api.client.googleapis.auth.oauth2.GoogleCredential;
import com.google.api.client.googleapis.javanet.GoogleNetHttpTransport;
import com.google.api.client.http.HttpTransport;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.json.jackson2.JacksonFactory;
import com.google.api.services.iam.v1.Iam;
import com.google.api.services.iam.v1.IamScopes;
import com.google.api.services.iam.v1.model.CreateServiceAccountKeyRequest;
import com.google.api.services.iam.v1.model.ServiceAccountKey;
import java.io.IOException;
import java.security.GeneralSecurityException;
import java.util.ArrayList;
import java.util.List;

public class IamDemo {
  /** Name of the application. */
  private static final String APPLICATION_NAME = "IamDemoJava";

  /** Project Name. */
  private static final String PROJECT_NAME = "MY_PROJECT_NAME";

  /** Name of the service account to create a new key for. */
  private static final String SERVICE_ACCOUNT_NAME = "dummy-sa";

  /** Full email address of the service account. */
  private static final String SERVICE_ACCOUNT_EMAIL =
      SERVICE_ACCOUNT_NAME + "@" + PROJECT_NAME + ".iam.gserviceaccount.com";

  /** Full service account resource string expected by the IAM API. */
  private static final String SERVICE_ACCOUNT_RESOURCE_NAME =
      "projects/" + PROJECT_NAME + "/serviceAccounts/" + SERVICE_ACCOUNT_EMAIL;

  /** Global instance of the HTTP transport. */
  private static HttpTransport httpTransport;

  /** Global instance of the JSON factory. */
  private static final JsonFactory JSON_FACTORY = JacksonFactory.getDefaultInstance();

  public static void main() throws IOException, GeneralSecurityException {
    Iam iam = initIam();
    ServiceAccountKey key = createServiceAccountKey(iam);

    // Print the key
    System.out.println(key.toString());
  }

  private static Iam initIam() throws IOException, GeneralSecurityException {
    httpTransport = GoogleNetHttpTransport.newTrustedTransport();

    // Authenticate using Google Application Default Credentials.
    GoogleCredential credential = GoogleCredential.getApplicationDefault();


    if (credential.createScopedRequired()) {
      List<String> scopes = new ArrayList<>();
      // Enable full Cloud Platform scope.
      scopes.add(IamScopes.CLOUD_PLATFORM);
      credential = credential.createScoped(scopes);
    }

    // Create IAM API object associated with the authenticated transport.
    return new Iam.Builder(httpTransport, JSON_FACTORY, credential)
        .setApplicationName(APPLICATION_NAME)
        .build();
  }

  private static ServiceAccountKey createServiceAccountKey(Iam iam)
      throws IOException, GeneralSecurityException {
    CreateServiceAccountKeyRequest request = new CreateServiceAccountKeyRequest();

    // Customize the request parameters if needed

    return iam.projects()
        .serviceAccounts()
        .keys()
        .create(SERVICE_ACCOUNT_RESOURCE_NAME, request)
        .execute();
  }
}