如何实现时钟更新？

本想用swing做一个电子时钟（可以实时更新的那种），可是做出来却不能实时更新，求大佬指点
代码如下：

//省略import
public class DateDemo extends JFrame {
    private static final long serialVersionUID = -8501458415481947988L;
    private JPanel contentPane;
    private JPanel panel1;
    private JPanel panel2;
    private String tf;
    private String tt;
    private JLabel label1;
    private JLabel label2;
    public static void main(String[] args) {
        try {
            UIManager.setLookAndFeel("javax.swing.plaf.nimbus.NimbusLookAndFeel");
        } catch (Throwable e) {
            e.printStackTrace();
        }
        EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    DateDemo frame = new DateDemo();
                    frame.setVisible(true);
                }catch(Exception e) {
                    e.printStackTrace();
                }
            }
        });
    }
    public DateDemo() {
        addWindowListener(new WindowAdapter() {
            @Override
            public void windowClosing(WindowEvent e) {
                do_this_windowClosing(e);
            }
        });
        setTitle("数字时钟");
        setDefaultCloseOperation(JFrame.DO_NOTHING_ON_CLOSE);
        setBounds(100,100,275,125);
        contentPane = new JPanel();
        contentPane.setBorder(new EmptyBorder(5,5,5,5));
        setContentPane(contentPane);
        contentPane.setLayout(new GridLayout(2,1,5,5));
        panel1 = new JPanel();
        label1 = new JLabel("年-月-日");
        panel1.add(label1);
        contentPane.add(panel1);
        panel2 = new JPanel();
        label2 = new JLabel("时：分：秒");
        panel2.add(label2);
        contentPane.add(panel2);
        gan();
    }
    protected void do_this_windowClosing(WindowEvent e) {
        JOptionPane.showMessageDialog(this,"确认关闭？",null,JOptionPane.CLOSED_OPTION);
        System.exit(0);
    }
    protected void gan() {
        Date date = new Date();
        tf = String.format("%tF",date);
        tt = String.format("%tT",date);
        label1.setText("年-月-日 =>    " + tf);
        label2.setText("时：分：秒 =>    " + tt);
    }
}

在gan（）方法时用while（true）时它又显示不出窗体
求助！