How do you create a thumbnail image out of a JPEG in Java?

Image img = ImageIO.read(new File("test.jpg")).getScaledInstance(100, 100, BufferedImage.SCALE_SMOOTH);

This will create a 100×100 pixels thumbnail as an Image object. If you want to write it back to disk simply convert the code to this:

BufferedImage img = new BufferedImage(100, 100, BufferedImage.TYPE_INT_RGB);
img.createGraphics().drawImage(ImageIO.read(new File("test.jpg")).getScaledInstance(100, 100, Image.SCALE_SMOOTH),0,0,null);
ImageIO.write(img, "jpg", new File("test_thumb.jpg"));

Also if you are concerned about speed issues (the method described above is rather slow if you want to scale many images) use these methods and the following declaration :

private BufferedImage scale(BufferedImage source,double ratio) {
  int w = (int) (source.getWidth() * ratio);
  int h = (int) (source.getHeight() * ratio);
  BufferedImage bi = getCompatibleImage(w, h);
  Graphics2D g2d = bi.createGraphics();
  double xScale = (double) w / source.getWidth();
  double yScale = (double) h / source.getHeight();
  AffineTransform at = AffineTransform.getScaleInstance(xScale,yScale);
  g2d.drawRenderedImage(source, at);
  g2d.dispose();
  return bi;
}

private BufferedImage getCompatibleImage(int w, int h) {
  GraphicsEnvironment ge = GraphicsEnvironment.getLocalGraphicsEnvironment();
  GraphicsDevice gd = ge.getDefaultScreenDevice();
  GraphicsConfiguration gc = gd.getDefaultConfiguration();
  BufferedImage image = gc.createCompatibleImage(w, h);
  return image;
}

And then call :

BufferedImage scaled = scale(img,0.5);

where 0.5 is the scale ratio and img is a BufferedImage containing the normal-sized image.