Edit: please read the better way here: Better way to Resize Images Using Java!

I am building a Seam application which need to support users uploading images, and then the site displaying them. I wanted to resize big images into something reasonable (say 1024 or 800 px wide) and generate a thumbnail, and I also wanted to standardize on a single image format just to make things easy and consistent.

So I needed method that would take an uploaded image as a byte array (byte[]) and would resize the image (if needed) and convert it to a fixed quality JPG, and give me back a byte array to store in the database. I am using Seam, so I get a handy byte array, but this method should work fine for non-Seam applications as well.

     * This method takes in an image as a byte array (currently supports GIF, JPG, PNG and possibly other formats) and
     * resizes it to have a width no greater than the pMaxWidth parameter in pixels. It converts the image to a standard
     * quality JPG and returns the byte array of that JPG image.
     * @param pImageData
     *                the image data.
     * @param pMaxWidth
     *                the max width in pixels, 0 means do not scale.
     * @return the resized JPG image.
     * @throws IOException
     *                 if the iamge could not be manipulated correctly.
    public byte[] resizeImageAsJPG(byte[] pImageData, int pMaxWidth) throws IOException {
	// Create an ImageIcon from the image data
	ImageIcon imageIcon = new ImageIcon(pImageData);
	int width = imageIcon.getIconWidth();
	int height = imageIcon.getIconHeight();
	mLog.info("imageIcon width: #0  height: #1", width, height);
	// If the image is larger than the max width, we need to resize it
	if (pMaxWidth > 0 && width > pMaxWidth) {
	    // Determine the shrink ratio
	    double ratio = (double) pMaxWidth / imageIcon.getIconWidth();
	    mLog.info("resize ratio: #0", ratio);
	    height = (int) (imageIcon.getIconHeight() * ratio);
	    width = pMaxWidth;
	    mLog.info("imageIcon post scale width: #0  height: #1", width, height);
	// Create a new empty image buffer to "draw" the resized image into
	BufferedImage bufferedResizedImage = new BufferedImage(width, height, BufferedImage.TYPE_INT_RGB);
	// Create a Graphics object to do the "drawing"
	Graphics2D g2d = bufferedResizedImage.createGraphics();
	g2d.setRenderingHint(RenderingHints.KEY_INTERPOLATION, RenderingHints.VALUE_INTERPOLATION_BICUBIC);
	// Draw the resized image
	g2d.drawImage(imageIcon.getImage(), 0, 0, width, height, null);
	// Now our buffered image is ready
	// Encode it as a JPEG
	ByteArrayOutputStream encoderOutputStream = new ByteArrayOutputStream();
	JPEGImageEncoder encoder = JPEGCodec.createJPEGEncoder(encoderOutputStream);
	byte[] resizedImageByteArray = encoderOutputStream.toByteArray();
	return resizedImageByteArray;

In my application I call this method twice, once to convert the uploaded image into a limited size JPG, and then once again to generate a much smaller thumbnail. I store both of these in the database and will use caching at the Apache layer to ensure performance.