Java Google Contacts API Access Service Account Authentication

Question

I'm trying to access Googles Contacts API but my attempt failed already on getting authorized. From other (web) languages i'm used to the APIConsole and the public API-key (authorization).

GoogleCredential credential = new GoogleCredential().setAccessToken("<<PublicAPIKey>>");
System.out.println(credential.refreshToken());          // false

This way I'm not able to refresh the token and be unsure about using the public-key as accesstoken ... Instead I tried over a service account :

private static final String USER_ACCOUNT_EMAIL = "xy@gmail.com";
private static final String SERVICE_ACCOUNT_EMAIL = "xy@developer.gserviceaccount.com";
private static final String SERVICE_ACCOUNT_PKCS12_FILE_PATH = "xy.p12";

public App() {
    Set<String> scopes = new HashSet<String>();
    scopes.add("https://www.google.com/m8/feeds");

    try {
        GoogleCredential credential = new GoogleCredential.Builder()
            .setTransport(new NetHttpTransport())
            .setJsonFactory(new JacksonFactory())
            .setServiceAccountId(SERVICE_ACCOUNT_EMAIL)
            .setServiceAccountScopes(scopes)
            .setServiceAccountUser(USER_ACCOUNT_EMAIL)
            .setServiceAccountPrivateKeyFromP12File(new java.io.File(SERVICE_ACCOUNT_PKCS12_FILE_PATH))
            .build();

        System.out.println(credential.refreshToken());
        //System.out.println(credential.getAccessToken());
    } catch (GeneralSecurityException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

Here my exception:

com.google.api.client.auth.oauth2.TokenResponseException: 401 Unauthorized
        at com.google.api.client.auth.oauth2.TokenResponseException.from(TokenResponseException.java:105)
        at com.google.api.client.auth.oauth2.TokenRequest.executeUnparsed(TokenRequest.java:287)
        at com.google.api.client.auth.oauth2.TokenRequest.execute(TokenRequest.java:307)
        at com.google.api.client.googleapis.auth.oauth2.GoogleCredential.executeRefreshToken(GoogleCredential.java:384)
        at com.google.api.client.auth.oauth2.Credential.refreshToken(Credential.java:489)
        at App.<init>(App.java:50)
        at App.main(App.java:29)

Thanks for a hint!

Answer 1

Without calling setServiceAccountUser() my code just worked perfectly fine. But you just will have an impersonated account (service_account_mail) not your personal contacts.

Another possible source for a "401 Unauthorized" exception is leaving the credential.refreshToken() away. The call is necessary to write the access-code into the reference.

Below the finished class:

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.jackson2.JacksonFactory;
import com.google.gdata.client.contacts.ContactsService;

import java.io.File;
import java.io.IOException;
import java.security.GeneralSecurityException;
import java.util.Collections;

public class Connector {

  private static ContactsService contactService = null;
  private static HttpTransport httpTransport;

  private static final String APPLICATION_NAME = "Your-App/1.0";
  private static final String SERVICE_ACCOUNT_EMAIL = "xy@developer.gserviceaccount.com";

  private Connector() {
    // explicit private no-args constructor
  }

  public static ContactsService getInstance() {
    if (contactService == null) {
      try {
        contactService = connect();

      } catch (GeneralSecurityException | IOException e) {
        e.printStackTrace();
      }
    }

    return contactService;
  }

  private static ContactsService connect() throws GeneralSecurityException, IOException {
    httpTransport = GoogleNetHttpTransport.newTrustedTransport();

    // @formatter:off
    GoogleCredential credential = new GoogleCredential.Builder()
                                            .setTransport(httpTransport)
                                            .setJsonFactory(JacksonFactory.getDefaultInstance())
                                            .setServiceAccountId(SERVICE_ACCOUNT_EMAIL)
                                            .setServiceAccountScopes(Collections.singleton("https://www.google.com/m8/feeds"))
                                            .setServiceAccountPrivateKeyFromP12File(new File("key.p12"))
                                            .build();
    // @formatter:on

    if (!credential.refreshToken()) {
      throw new RuntimeException("Failed OAuth to refresh the token");
    }

    ContactsService myService = new ContactsService(APPLICATION_NAME);
    myService.setOAuth2Credentials(credential);
    return myService;
  }

}