JSplitPaneのDividerを円形半透明のつまみに変更して中央に配置する

class DividerLocationDragLayerUI extends LayerUI<JSplitPane> {
  private static final int R = 25;
  private final Point startPt = new Point();
  private final Cursor dc = Cursor.getDefaultCursor();
  private final Cursor wc = Cursor.getPredefinedCursor(Cursor.W_RESIZE_CURSOR);
  private final Ellipse2D thumb = new Ellipse2D.Double();
  private int dividerLocation;
  private boolean isDragging;
  private boolean isEnter;

  @Override public void installUI(JComponent c) {
    super.installUI(c);
    if (c instanceof JLayer) {
      ((JLayer<?>) c).setLayerEventMask(AWTEvent.MOUSE_EVENT_MASK
          | AWTEvent.MOUSE_MOTION_EVENT_MASK);
    }
  }

  @Override public void uninstallUI(JComponent c) {
    if (c instanceof JLayer) {
      ((JLayer<?>) c).setLayerEventMask(0);
    }
    super.uninstallUI(c);
  }

  @Override public void paint(Graphics g, JComponent c) {
    super.paint(g, c);
    if ((isEnter || isDragging) && c instanceof JLayer) {
      updateThumbLocation(((JLayer<?>) c).getView(), thumb);
      Graphics2D g2 = (Graphics2D) g.create();
      g2.setRenderingHint(RenderingHints.KEY_ANTIALIASING,
                          RenderingHints.VALUE_ANTIALIAS_ON);
      g2.setPaint(new Color(255, 100, 100, 100));
      g2.fill(thumb);
      g2.dispose();
    }
  }

  @Override protected void processMouseEvent(
      MouseEvent e, JLayer<? extends JSplitPane> l) {
    JSplitPane splitPane = l.getView();
    switch (e.getID()) {
    case MouseEvent.MOUSE_ENTERED:
      isEnter = true;
      break;
    case MouseEvent.MOUSE_EXITED:
      isEnter = false;
      break;
    case MouseEvent.MOUSE_RELEASED:
      isDragging = false;
      break;
    case MouseEvent.MOUSE_PRESSED:
      Component c = e.getComponent();
      if (isDraggableComponent(splitPane, c)) {
        Point pt = SwingUtilities.convertPoint(c, e.getPoint(), splitPane);
        isDragging = thumb.contains(pt);
        startPt.setLocation(SwingUtilities.convertPoint(c, e.getPoint(), splitPane));
        dividerLocation = splitPane.getDividerLocation();
      }
      break;
    default:
      break;
    }
    splitPane.repaint();
  }

  @Override protected void processMouseMotionEvent(
      MouseEvent e, JLayer<? extends JSplitPane> l) {
    JSplitPane splitPane = l.getView();
    Component c = e.getComponent();
    Point pt = SwingUtilities.convertPoint(c, e.getPoint(), splitPane);
    if (e.getID() == MouseEvent.MOUSE_MOVED) {
      splitPane.setCursor(thumb.contains(e.getPoint()) ? wc : dc);
    } else if (isDragging && isDraggableComponent(splitPane, c)
               && e.getID() == MouseEvent.MOUSE_DRAGGED) {
      int delta = splitPane.getOrientation() == JSplitPane.HORIZONTAL_SPLIT
          ? pt.x - startPt.x : pt.y - startPt.y;
      splitPane.setDividerLocation(Math.max(0, dividerLocation + delta));
    }
  }

  private static boolean isDraggableComponent(JSplitPane splitPane, Component c) {
    return Objects.equals(splitPane, c)
      || Objects.equals(splitPane, SwingUtilities.getUnwrappedParent(c));
  }

  private static void updateThumbLocation(Component c, Ellipse2D thumb) {
    if (c instanceof JSplitPane) {
      JSplitPane splitPane = (JSplitPane) c;
      int pos = splitPane.getDividerLocation();
      if (splitPane.getOrientation() == JSplitPane.HORIZONTAL_SPLIT) {
        thumb.setFrame(pos - R, splitPane.getHeight() / 2 - R, R + R, R + R);
      } else {
        thumb.setFrame(splitPane.getWidth() / 2 - R, pos - R, R + R, R + R);
      }
    }
  }
}