JPEG metadata always read-only

Greetings. I’m trying to read and modify metadata of JPEG images. Let’s look at the following (heavily simplified) code:

try (final Closeable imageInputStream = ImageIO.createImageInputStream(new File("D:\\testImage.jpg"))) {
  final ImageReader reader = ImageIO.getImageReaders(imageInputStream).next();
  reader.setInput(imageInputStream);

  final IIOImage imageAndMeta = reader.readAll(0, reader.getDefaultReadParam());
  reader.dispose();

  final IIOMetadata metadata = imageAndMeta.getMetadata();

  final Node root = new IIOMetadataNode("javax_imageio_jpeg_image_1.0");
  root.appendChild(new IIOMetadataNode("JPEGvariety"));
  root.appendChild(new IIOMetadataNode("markerSequence"));
  //root.appendChild(...somethingElse...)

  metadata.mergeTree("javax_imageio_jpeg_image_1.0", root);
} catch (final Exception error) {
  error.printStackTrace();
}

It works fine without TwelveMonkeys, but throws an error (only) when TwelveMonkeys is installed:

java.lang.IllegalStateException: Metadata is read-only
	at com.twelvemonkeys.imageio.AbstractMetadata.assertMutable(AbstractMetadata.java:117) ~[imageio-core-3.8.1.jar:3.8.1]
	at com.twelvemonkeys.imageio.AbstractMetadata.mergeTree(AbstractMetadata.java:96) ~[imageio-core-3.8.1.jar:3.8.1]
	...

It seems that the metadata is always read-only and thus mergeTree function does not work. I’ve looked into this with debugger and it seems that in TwelveMonkeys’ JPEGImage10Metadata.java isReadOnly() always returns true while in the original com.sun.imageio.plugins.jpeg.JPEGMetadata it’s always false. Any idea why is this and how could I get around this?

[davidekholm]

Hi Harald. I’ve updated to v3.8.3 and the “Invalid DHT node” error is indeed gone now. Thank you.

[haraldk]

No, it gives you a mutable JPEG metadata, so you can copy whatever you like (what this issue is about).

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Copying only Exif is (probably) possible, but tedious…

To copy only the Exif segment, you need to look at the "markerSequence", get all the "unknown" nodes, and get the ones with "MarkerTag" value "225" (APP1/0xFFE1). There might be other APP1 markers beside Exif, so you probably need to look into the userObject, which for all "unknown" segments will contain a byte array. For Exif, this array will start with the string Exif\0\0 (and be followed by the TIFF structure).

Sadly, there is a bug in the JDK metadata, that inserts multiple "unknown" markers when you merge… 😕 Workaround for this is to merge the nodes first, then use setFromTree. Again, unnecessary hard work for something that should be simple.

I’d love to fix it, but proposing PRs to OpenJDK often takes several times that of a workaround… If only there was more time.