JFrame如何添加背景？

运行如下代码后，未能达到预期效果，怎样修改代码可以使背景显示？

static JFrame frame = new JFrame("测试");
    static JPanel panel = new JPanel();
    static JLabel label = null;

    public static void main(String[] args) {

        double width = Toolkit.getDefaultToolkit().getScreenSize().width;
        double height = Toolkit.getDefaultToolkit().getScreenSize().height;

        frame.setUndecorated(true);
        frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
        frame.setBounds(0,0,(int) width, (int) height);
        frame.setVisible(true);

        ImageIcon icon = new ImageIcon(new ImageIcon("./images/StartBackground.png").getImage().getScaledInstance((int) width,
                (int) height, Image.SCALE_DEFAULT));
        label = new JLabel("12121221");
        label.setIcon(icon);
        label.setBackground(new Color(0, 0, 0));
        label.setBounds(0, 0, (int) width, (int) height);
        panel.add(label);

        frame.setContentPane(panel);
    }

label.setBackground(new Color(0, 0, 0));
你这么写只是设置的label的背景