OAuth Google API for Java unable to impersonate user

I would like to impersonate a user and add files to the users Google Drive on their behalf from a server process. I've setup a service account and can successfully access the Drive as the service account adding and listing files, etc. using the following code:

  /** Global instance of the HTTP transport. */
  private static final HttpTransport HTTP_TRANSPORT = new NetHttpTransport();

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

  public static void main(String[] args) {
    try {
        GoogleCredential credential = 
                new GoogleCredential.Builder().setTransport(HTTP_TRANSPORT)
                  .setJsonFactory(JSON_FACTORY)
                  .setServiceAccountId("XXXXX@developer.gserviceaccount.com")
                  .setServiceAccountScopes(DriveScopes.DRIVE)
                  .setServiceAccountPrivateKeyFromP12File(new File("c:/junk/key.p12"))
                  .build();
        Drive drive = new Drive.Builder(HTTP_TRANSPORT, JSON_FACTORY, credential).build();         
        drive.files().list().execute();
  } catch (Exception e) {
     e.printStackTrace();
  }

This works, however only returns files that are associated to what I assume is associated with the service accounts drive (?).

According to the JavaDoc, GoogleCredential can also be used to impersonate a user by adding the service account users email address as follows:

        GoogleCredential credential = 
                new GoogleCredential.Builder().setTransport(HTTP_TRANSPORT)
                  .setJsonFactory(JSON_FACTORY)
                  .setServiceAccountId("XXXXX@developer.gserviceaccount.com")
                  .setServiceAccountScopes(DriveScopes.DRIVE)
                  .setServiceAccountPrivateKeyFromP12File(new File("c:/junk/key.p12"))
                  .setServiceAccountUser("usera@domain.com") //<-- impersonate user a
                  .build();

However, when executing this code, the following exception is thrown:

com.google.api.client.auth.oauth2.TokenResponseException: 400 Bad Request
{
  "error" : "access_denied"
}
at com.google.api.client.auth.oauth2.TokenResponseException.from(TokenResponseException.java:103)
at com.google.api.client.auth.oauth2.TokenRequest.executeUnparsed(TokenRequest.java:303)
at com.google.api.client.auth.oauth2.TokenRequest.execute(TokenRequest.java:323)
at com.google.api.client.googleapis.auth.oauth2.GoogleCredential.executeRefreshToken(GoogleCredential.java:340)
at com.google.api.client.auth.oauth2.Credential.refreshToken(Credential.java:508)
at com.google.api.client.auth.oauth2.Credential.intercept(Credential.java:260)
at com.google.api.client.http.HttpRequest.execute(HttpRequest.java:796)
at com.google.api.client.googleapis.json.GoogleJsonResponseException.execute(GoogleJsonResponseException.java:198)
at com.google.api.client.googleapis.services.GoogleClient.executeUnparsed(GoogleClient.java:237)
at com.google.api.client.http.json.JsonHttpRequest.executeUnparsed(JsonHttpRequest.java:207)
at com.google.api.services.drive.Drive$Files$List.execute(Drive.java:1071)

Am I missing a step or configuration setting?

Thanks, David

David

I found a similar question as mine: Can a Google Apps Admin manage users files with Drive SDK? to mine which has helped me figure out the answer.

The cPanel documentation is a little misleading as it refers to enabling the consumer key and then adding the domain to the Manage API client access screen. This appears to be valid for the gdata api and not the new Google Drive api. By adding the client id as suggested in the other question and granting access to the Drive scope I'm now able to impersonate a user.

Get your admin to add scopes to xxxxx.apps.googleusercontent.com via admin panel:

I added the following to work on spreadsheets:

https://www.googleapis.com/auth/drive
https://docs.google.com/feeds
https://spreadsheets.google.com/feeds