BufferedImage to ServletOutputStream: Black Box

[Greg Donahue]

Hi,

I'm writing a servlet that accepts the following params thru the URL:

1) a filename for an image sitting on my server, and
2) a pair of X and Y dimensions.

The servlet is supposed to scale the image appropriately and send the result to the response OutputStream. My strategy is: take the image file, load it into a BufferedImage, modify it appropriately, and send it from there to an OutputStream. Here's the code that is supposed to accomplish this:

 public void doGet(HttpServletRequest req, HttpServletResponse res)
  throws IOException, ServletException {
   try {
 
    //--Nab the filename
    String ifilename = getImageFileName(req);
    
    //--Load the BufferedImage
    Image img = Toolkit.getDefaultToolkit().getImage(ifilename);
    MediaTracker mt = new MediaTracker(new Canvas());
    mt.addImage(img, 0);
    mt.waitForAll();
    BufferedImage bi = new BufferedImage(img.getWidth(null), img.getHeight(null), BufferedImage.TYPE_INT_RGB);
 
    //--Alter the image as necessary
    alterImage(bi, req);
    
    //--Send the data to response
    res.setContentType("image/jpeg");
    OutputStream os = res.getOutputStream();
    ImageIO.write(bi, "jpg", os);
    os.close();
 
 } catch (Exception e) { res.sendError(500); }
}

...that makes sense, right? Currently, getImageFileName() returns the name of a JPEG which I know is valid, and alterImage() does nothing. But here's the thing: when I run this, the image produced is a big black box, and not the expected image at all, and the box has the same dimensions as the expected image.

What am I doing wrong?

[Greg Donahue]

Aha. You have to *display* the image first.

 AffineTransform tx = new AffineTransform();
 Graphics2D g2d = bi.createGraphics();
 g2d.drawImage(img, tx, null);
 g2d.dispose();

...right after the BufferedImage initialization. Kind of makes sense, and then it kind of doesn't, since you aren't really displaying the image anywhere...