Menuに半透明の影を付ける

public class CustomPopupMenuUI extends BasicPopupMenuUI {
  public static ComponentUI createUI(JComponent c) {
    return new CustomPopupMenuUI();
  }

  private static boolean isHeavyWeightContainer(Component contents) {
    for (Container p = contents.getParent(); p != null; p = p.getParent()) {
      if ((p instanceof JWindow) || (p instanceof Panel)) {
        return true;
      }
    }
    return false;
  }

  public Popup getPopup(JPopupMenu popup, int x, int y) {
    Popup pp = super.getPopup(popup, x, y);
    JPanel panel = (JPanel) popup.getParent();
    if (isHeavyWeightContainer(panel)) {
      System.out.println("outer");
      Point p = new Point(x, y);
      panel.setBorder(new ShadowBorder(panel, p));
    } else {
      System.out.println("inner");
      panel.setBorder(new ShadowBorderInPanel());
    }
    panel.setOpaque(false);
    return pp;
  }
// ...