[ACCEPTED]-Writing image metadata in Java, preferably PNG-metadata

I had to do the the same thing some days 5 ago.. I have not found the exact solution 4 on the internet either but looking at the 3 com.sun.imageio.plugins.png.PNGMetadata class I could achieve some results..

To 2 write a custom metadata to a PNG file:

public byte[] writeCustomData(BufferedImage buffImg, String key, String value) throws Exception {
    ImageWriter writer = ImageIO.getImageWritersByFormatName("png").next();

    ImageWriteParam writeParam = writer.getDefaultWriteParam();
    ImageTypeSpecifier typeSpecifier = ImageTypeSpecifier.createFromBufferedImageType(BufferedImage.TYPE_INT_RGB);

    //adding metadata
    IIOMetadata metadata = writer.getDefaultImageMetadata(typeSpecifier, writeParam);

    IIOMetadataNode textEntry = new IIOMetadataNode("tEXtEntry");
    textEntry.setAttribute("keyword", key);
    textEntry.setAttribute("value", value);

    IIOMetadataNode text = new IIOMetadataNode("tEXt");
    text.appendChild(textEntry);

    IIOMetadataNode root = new IIOMetadataNode("javax_imageio_png_1.0");
    root.appendChild(text);

    metadata.mergeTree("javax_imageio_png_1.0", root);

    //writing the data
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    ImageOutputStream stream = ImageIO.createImageOutputStream(baos);
    writer.setOutput(stream);
    writer.write(metadata, new IIOImage(buffImg, null, metadata), writeParam);
    stream.close();

    return baos.toByteArray();
}

Then, to 1 read the data:

public String readCustomData(byte[] imageData, String key) throws IOException{
    ImageReader imageReader = ImageIO.getImageReadersByFormatName("png").next();

    imageReader.setInput(ImageIO.createImageInputStream(new ByteArrayInputStream(imageData)), true);

    // read metadata of first image
    IIOMetadata metadata = imageReader.getImageMetadata(0);

    //this cast helps getting the contents
    PNGMetadata pngmeta = (PNGMetadata) metadata; 
    NodeList childNodes = pngmeta.getStandardTextNode().getChildNodes();

    for (int i = 0; i < childNodes.getLength(); i++) {
        Node node = childNodes.item(i);
        String keyword = node.getAttributes().getNamedItem("keyword").getNodeValue();
        String value = node.getAttributes().getNamedItem("value").getNodeValue();
        if(key.equals(keyword)){
            return value;
        }
    }
    return null;
}

Java provides the metadata package and the ImageWriter class 3 along with the ImageIO package.

You create your 2 IIOMetadata object, then getImageWriters for your BufferedImage or IIOImage and use them 1 to write the metadata.

To add to other answer, you can also try 4 the PNGJ library, it has full metadata support.

BTW, I 3 don't understand what you are refering to 2 with the "XML-like" structure 1 of metadata.

Using the method from posted by the OP gets 5 most of the way there; the only issue is 4 that PNGMetadata is proprietary and so causes compiler 3 warnings.

There is a method of doing it without 2 using the proprietary API, by searching 1 the metadata tree for tEXtEntry nodes:

private List<Node> findNodesWithName(String name, Node root) {
    List<Node> found = new ArrayList<>();
    Node n = root.getFirstChild();
    while (n != null) {
        if (n.getNodeName().equals(name)) {
            found.add(n);
        }
        found.addAll(findNodesWithName(name, n));
        n = n.getNextSibling();
    }
}

// ...
// To use it:
IIOMetadata metadata = ...;
List<Node> tEXtNodes = findNodesWithName(
        "tEXtEntry",
        metadata.getAsTree(metadata.getNativeMetadataFormatName()));

for (Node n : tEXtNodes) {
    String keyword = node.getAttributes().getNamedItem("keyword");
    String value = node.getAttributes().getNamedItem("value");
    System.out.println("keyword: " + keyword + "; value: " + value);
}