401 response when athenticating via Service Account and OAuth2 to GoogleDrive - google-drive-api

Question

I seem to have hit a wall trying to get a Java application to list of files stored in my GoogleDrive folder. The
I have a project setup in the Developer Console.
I have enabled for that project the Drive API
I have create Auth credentials for a Service Account - and am using the P12 key produced.
My code looks like this :
import com.google.api.client.auth.oauth2.Credential;
import com.google.api.client.auth.oauth2.CredentialRefreshListener;
import com.google.api.client.auth.oauth2.TokenErrorResponse;
import com.google.api.client.auth.oauth2.TokenResponse;
import com.google.api.client.googleapis.auth.oauth2.GoogleCredential;
import com.google.api.client.http.HttpTransport;
import com.google.api.client.http.javanet.NetHttpTransport;
import com.google.api.client.json.JsonFactory;
import com.google.api.client.json.jackson2.JacksonFactory;
import com.google.api.services.drive.Drive;
import com.google.api.services.drive.DriveScopes;
import com.google.api.services.drive.model.File;
import com.google.api.services.drive.model.FileList;
import java.io.IOException;
import java.security.GeneralSecurityException;
import java.util.Collections;
import java.util.List;
public class GoogleDriveTest {
/** Global instance of the JSON factory. */
private static final JsonFactory JSON_FACTORY = new JacksonFactory() ;
/** instance of the HTTP transport. */
private static final HttpTransport HTTP_TRANSPORT = new NetHttpTransport() ;
/** Global instance of the Google Drive - the first connect will initialize */
private static Drive drive;
/*
* instance variables
*/
private String accessToken ;
public static void main(String[] args) throws GeneralSecurityException, IOException {
String CLIENTID = "XXXXXXXXXXX-XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX#developer.gserviceaccount.com" ;
String CLIENTUSER = "XXXXXXXX#gmail.com" ;
String P2FILE = "random/src/main/resources/test.p12" ;
try {
GoogleCredential credential = new GoogleCredential.Builder().setTransport(HTTP_TRANSPORT)
.setJsonFactory(JSON_FACTORY)
.setServiceAccountId(CLIENTID)
.setServiceAccountScopes(Collections.singleton(DriveScopes.DRIVE_FILE))
.setServiceAccountPrivateKeyFromP12File(new java.io.File(P2FILE))
.setServiceAccountUser(CLIENTUSER)
.addRefreshListener(new CredentialRefreshListener() {
#Override
public void onTokenResponse(
Credential credential,
TokenResponse tokenResponse) throws IOException {
String token = tokenResponse.getAccessToken();
System.out.println("GOOGLEDRIVE token refreshed " + tokenResponse.getTokenType() + " successfully");
}
#Override
public void onTokenErrorResponse(
Credential credential,
TokenErrorResponse tokenErrorResponse) throws IOException {
System.out.println("GOOGLEDRIVE token refresh failure : " + tokenErrorResponse);
}
})
.build();
/*
* set up the global Drive instance
*/
drive = new Drive.Builder(HTTP_TRANSPORT, JSON_FACTORY, credential).setApplicationName("API Project").build();
/*
* output results
*/
Drive.Files.List fileRequest = drive.files().list() ;
FileList afiles = fileRequest.execute() ;
List<File> bFiles = afiles.getItems() ;
for(File file : bFiles) {
System.out.println(file.getTitle()) ;
}
}
catch (GeneralSecurityException | IOException e) {
e.printStackTrace();
}
}
}
But the output I see is this :
GOOGLEDRIVE token refresh failure : null
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 com.google.api.client.auth.oauth2.Credential.intercept(Credential.java:217)
at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:859)
at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:419)
at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.executeUnparsed(AbstractGoogleClientRequest.java:352)
at com.google.api.client.googleapis.services.AbstractGoogleClientRequest.execute(AbstractGoogleClientRequest.java:469)
at GoogleDriveTest.main(GoogleDriveTest.java:81)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:483)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:134)
Process finished with exit code 0
Can anyone provide some pointers on Service Account access to Google Drive and what I might be doing wrong here ?
As the Service Account is created under my Google Account (XXXXXXXX#gmail.com) I assumed it would have access to the Drive once the API was enabled.
I have seen other posts about granting access to Service Accounts to a domain account via an admin console - but my account is a plain / standard google account. I'm not even sure what a domain account is !
All help would be greatly appreciated.