如何使用驱动器 api java 从谷歌驱动器下载文件?

I have tried some functions of Google drive, but I couldn't able to download the file from Drive.

这是我使用的代码

private static InputStream downloadFile(Drive service, File file) {
    if (file.getDownloadUrl() != null && file.getDownloadUrl().length() > 0) {
      try {
        HttpResponse resp =
            service.getRequestFactory().buildGetRequest(new GenericUrl(file.getDownloadUrl()))
                .execute();
        return resp.getContent();
      } catch (IOException e) {
        // An error occurred.
        e.printStackTrace();
        return null;
      }
    } else {
      // The file doesn't have any content stored on Drive.
      return null;
    }
  }

当我运行脚本时，它显示 file.getDownloadUrl() 为 NULL.

File file1 = service.files().get(fileid).execute();
downloadFile(service,file1);