Automatically Size to Fit Image in a JLabel

Images inside a Java Swing component do not automatically resize when it's larger than the component it resides in, resulting in a clipped image.

Oftentimes, it is convenient to have an image automatically resize to fit inside a component while preserving the aspect ratio. Thumbnail display is one that comes to mind.

It is actually fairly easy to implement this capability by extending JLabel and overriding the paintComponent() method:

 protected void paintComponent(Graphics g)
 {
  ImageIcon icon = (ImageIcon) getIcon();
  int iconWidth = icon.getIconWidth();
  int iconHeight = icon.getIconHeight();
  double iconAspect = (double) iconHeight / iconWidth;

  int w = getWidth();
  int h = getHeight();
  double canvasAspect = (double) h / w;

  int x = 0, y = 0;
  
  // Maintain aspect ratio.
  if(iconAspect < canvasAspect)
  {
   // Drawing space is taller than image.
   y = h;
   h = (int) (w * iconAspect);
   y = (y - h) / 2; // center it along vertical
  }
  else
  {
   // Drawing space is wider than image.
   x = w;
   w = (int) (h / iconAspect);
   x = (x - w) / 2; // center it along horizontal
  }
      
  Image img = icon.getImage();
  g.drawImage(img, x, y, w + x, h + y, 0, 0, iconWidth, iconHeight, null);
 }

Download Source Code

ThumbnailLabel.java