Animated Gifからフレーム画像を抽出する

ImageReader reader = null;
Iterator<ImageReader> readers = ImageIO.getImageReaders(imageStream);
while (readers.hasNext()) {
  reader = readers.next();
  String metaFormat = reader.getOriginatingProvider().getNativeImageMetadataFormatName();
  if ("gif".equalsIgnoreCase(reader.getFormatName()) &&
      !"javax_imageio_gif_image_1.0".equals(metaFormat)) {
    continue;
  } else {
    break;
  }
}
reader = Objects.requireNonNull(reader, "Can not read image format!");
boolean isGif = reader.getFormatName().equalsIgnoreCase("gif");
reader.setInput(imageStream, false, !isGif);
List<BufferedImage> list = new ArrayList<>();
for (int i = 0; i < reader.getNumImages(true); i++) {
  IIOImage frame = reader.readAll(i, null);
  list.add((BufferedImage) frame.getRenderedImage());
}
reader.dispose();