使用服务帐户访问Google Drive API

Question

我正在尝试使用服务帐户访问Google Drive API。 我已经设置了服务帐户来访问Google Directory API，我认为我可以使用相同的服务帐户。 这是我的代码，它几乎是不言自明的。

 private static List<File> retrieveAllFiles(Drive service) throws IOException {
    List<File> result = new ArrayList<File>();
    Files.List request = service.files().list();

    do {
        FileList files = request.execute();

        result.addAll(files.getItems());
        request.setPageToken(files.getNextPageToken());
    } while (request.getPageToken() != null &&
             request.getPageToken().length() > 0);
     return result;
  }

public static Drive getDriveService(){
    GoogleDriectoryService GDservice = new GoogleDriectoryService();
    java.io.File serviceAccountP12Certificate = GDservice.loadF("resources/APIProject-62d885cdf671.p12");
     HttpTransport httpTransport = new NetHttpTransport();//?
     JacksonFactory jsonFactory = new JacksonFactory();//?
     GoogleCredential credential = new GoogleCredential.Builder()
      .setTransport(httpTransport)
      .setJsonFactory(jsonFactory)
      .setServiceAccountId(serviceAccountEmailAddress)
      .setServiceAccountScopes(Arrays.asList(DriveScopes.DRIVE))
      .setServiceAccountUser(serviceAccountUser)
      .setServiceAccountPrivateKeyFromP12File(serviceAccountP12Certificate)
      .build();
      Drive service = new Drive.Builder(httpTransport, jsonFactory, credential)
          .setApplicationName("radiantlogic-vds").build();
     return service;
}   

public static void main(String[] args){
        Drive service = getDriveService();
        List<File> files = retrieveAllFiles(service);
        System.out.println(files);
}

但是，我在request.execute()行收到“com.google.api.client.auth.oauth2.TokenResponseException：401 Unauthorized”。 任何帮助表示赞赏。