How to increase jpeg image ppi or dpi in java?

I am creating a java program which I need to increase input JPEG image's PPI/DPI and save it as JPEG.

Anyone can give me some hits on if this is workable?

You can do it like in this answer, except you have to use "jpeg" as the format name and you need to implement the setDPI method to work with JPEG-specific metadata.

public static final String DENSITY_UNITS_NO_UNITS = "00";
public static final String DENSITY_UNITS_PIXELS_PER_INCH = "01";
public static final String DENSITY_UNITS_PIXELS_PER_CM = "02";

private BufferedImage gridImage;

private void saveGridImage(File output) throws IOException {
    output.delete();

    final String formatName = "jpeg";

    for (Iterator<ImageWriter> iw = ImageIO.getImageWritersByFormatName(formatName); iw.hasNext();) {
        ImageWriter writer = iw.next();
        ImageWriteParam writeParam = writer.getDefaultWriteParam();
        ImageTypeSpecifier typeSpecifier = ImageTypeSpecifier.createFromBufferedImageType(BufferedImage.TYPE_INT_RGB);
        IIOMetadata metadata = writer.getDefaultImageMetadata(typeSpecifier, writeParam);
        if (metadata.isReadOnly() || !metadata.isStandardMetadataFormatSupported()) {
            continue;
        }

        setDPI(metadata);

        final ImageOutputStream stream = ImageIO.createImageOutputStream(output);
        try {
            writer.setOutput(stream);
            writer.write(metadata, new IIOImage(gridImage, null, metadata), writeParam);
        } finally {
            stream.close();
        }
        break;
    }
}

private static void setDPI(IIOMetadata metadata) throws IIOInvalidTreeException {
    String metadataFormat = "javax_imageio_jpeg_image_1.0";
    IIOMetadataNode root = new IIOMetadataNode(metadataFormat);
    IIOMetadataNode jpegVariety = new IIOMetadataNode("JPEGvariety");
    IIOMetadataNode markerSequence = new IIOMetadataNode("markerSequence");

    IIOMetadataNode app0JFIF = new IIOMetadataNode("app0JFIF");
    app0JFIF.setAttribute("majorVersion", "1");
    app0JFIF.setAttribute("minorVersion", "2");
    app0JFIF.setAttribute("thumbWidth", "0");
    app0JFIF.setAttribute("thumbHeight", "0");
    app0JFIF.setAttribute("resUnits", DENSITY_UNITS_PIXELS_PER_INCH);
    app0JFIF.setAttribute("Xdensity", String.valueOf(300));
    app0JFIF.setAttribute("Ydensity", String.valueOf(300));

    root.appendChild(jpegVariety);
    root.appendChild(markerSequence);
    jpegVariety.appendChild(app0JFIF);

    metadata.mergeTree(metadataFormat, root);
}

This code sets the PPI to 300x300 but you may want to make this variable.