App Engine和云存储如何通过程序以公共链接的方式保存文件

我写一个函数来创建缩略图，但是缩略图没有公开链接，即使原始文件具有的权限User- allUsers-Reader和公众联系https://storage.googleapis.com/{buckets}/{filename}的所有用户访问的文件。

如何将此权限添加到缩略图中。

public static void thumbnailImage(String filename, int width, int height) throws IOException{
    GcsService gcsService = GcsServiceFactory.createGcsService(new RetryParams.Builder()
              .initialRetryDelayMillis(10)
              .retryMaxAttempts(10)
              .totalRetryPeriodMillis(15000)
              .build());
    AppIdentityService appIdentity = AppIdentityServiceFactory.getAppIdentityService();
    ImagesService imagesService = ImagesServiceFactory.getImagesService();
    // Make an image from a Cloud Storage object, and transform it.
    BlobstoreService blobstoreService = BlobstoreServiceFactory.getBlobstoreService();
    BlobKey blobKey = blobstoreService.createGsBlobKey("/gs/" + appIdentity.getDefaultGcsBucketName() +"/"+ filename);

    Image thumb = null;
    try{
        Image blobImage = ImagesServiceFactory.makeImageFromBlob(blobKey);          
        Transform resize = ImagesServiceFactory.makeResize(width, height);
        thumb = imagesService.applyTransform(resize, blobImage);

    }catch(Exception e){
        e.printStackTrace();
    }
    String extension = Files.getFileExtension(filename);
    filename = stripExtension(filename);
    filename = String.format("%1$s_%2$s.%3$s", filename, thumbnail, extension);

    if(thumb!=null)
    // Write the transformed image back to a Cloud Storage object.
    gcsService.createOrReplace(
        new GcsFilename(appIdentity.getDefaultGcsBucketName(), filename),
        new GcsFileOptions.Builder().mimeType("image/jpeg").build(),
        ByteBuffer.wrap(thumb.getImageData()));
}