JTreeのルートノードアイコンのサイズを変更し、その中央に接続線が描画されるように設定する

int ow = UIManager.getIcon("Tree.openIcon").getIconWidth();
int iw = 32;
int ih = 24;
Icon icon1 = new ColorIcon(Color.GREEN, new Dimension(ow, ih));
Icon icon2 = new ColorIcon(new Color(0x55_00_00_AA, true), new Dimension(iw, ih));
JTree tree = new JTree();
tree.setRowHeight(0);
tree.setBorder(BorderFactory.createEmptyBorder(1, 1 + (iw - ow) / 2, 1, 1));
tree.setCellRenderer(new DefaultTreeCellRenderer() {
  @Override public Component getTreeCellRendererComponent(
      JTree tree, Object value, boolean selected, boolean expanded,
      boolean leaf, int row, boolean hasFocus) {
    JLabel l = (JLabel) super.getTreeCellRendererComponent(
        tree, value, selected, expanded, leaf, row, hasFocus);
    if (value.equals(tree.getModel().getRoot())) {
      l.setIcon(icon1);
      l.setIconTextGap(2 + (iw - icon1.getIconWidth()) / 2);
    }
    return l;
  }
});
LayerUI<JTree> layerUI = new LayerUI<JTree>() {
  @Override public void paint(Graphics g, JComponent c) {
    super.paint(g, c);
    Graphics2D g2 = (Graphics2D) g.create();
    icon2.paintIcon(c, g2, 1, 1);
    g2.dispose();
  }
};