JButtonのホバーエフェクトを円放射状グラデーションで表現する

protected void update() {
  if (!getBounds().equals(base)) {
    base = getBounds();
    int w = getWidth();
    int h = getHeight();
    if (w > 0 && h > 0) {
      buf = new BufferedImage(w, h, BufferedImage.TYPE_INT_ARGB);
    }
    shape = new RoundRectangle2D.Double(
      0, 0, w - 1, h - 1, ARC_WIDTH, ARC_HEIGHT);
  }
  if (buf == null) {
    return;
  }

  Graphics2D g2 = buf.createGraphics();
  g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                      RenderingHints.VALUE_ANTIALIAS_ON);
  Color c1 = new Color(0x00_F7_23_59, true);
  Color c2 = new Color(0x64_44_05_F7, true);

  g2.setComposite(AlphaComposite.Clear);
  g2.fillRect(0, 0, getWidth(), getHeight());

  g2.setComposite(AlphaComposite.Src);
  g2.setPaint(new Color(getModel().isArmed() ? 0xFF_AA_AA : 0xF7_23_59));
  g2.fill(shape);

  if (radius > 0) {
    int cx = pt.x - radius;
    int cy = pt.y - radius;
    int r2 = radius + radius;
    float[] dist = { 0f, 1f };
    Color[] colors = { c2, c1 };
    g2.setPaint(new RadialGradientPaint(pt, r2, dist, colors));
    g2.setComposite(AlphaComposite.SrcAtop);
    // g2.setClip(shape);
    g2.fill(new Ellipse2D.Double(cx, cy, r2, r2));
  }
  g2.dispose();
}