鼠标控制问题 跑起来白屏什么都不显示

public class App extends JFrame implements KeyListener {
    private JLabel lblNewLabel;
    private String str = "";
    public App() {
        setDefaultCloseOperation(EXIT_ON_CLOSE);
        setLocationRelativeTo(null);
        setSize(400, 300);
        setResizable(false);
        getContentPane().setLayout(null);
        lblNewLabel = new JLabel("");
        lblNewLabel.setBounds(0, 259, 394, 13);
        getContentPane().add(lblNewLabel);
        setVisible(true);
        new Thread() {
            public void run() {
                while (true) {
                    try {
                        Point mainPoint = getLocation();
                        Point point = MouseInfo.getPointerInfo().getLocation();
                        int x = point.x - mainPoint.x;
                        int y = point.y - mainPoint.y;
                        if (x < 0 || x > 400) {
                            sleep(2);
                            continue;
                        }
                        if (y < 0 || y > 300) {
                            sleep(2);
                            continue;
                        }
                        lblNewLabel.setText(str + "x : " + x + " y : " + y);
                        sleep(2);
                    } catch (Exception e) {
                    }
                }
            }
        }.start();
        this.addKeyListener(this);
    }
    public static void main(String[] args) {
        new App();
    }
    public void keyPressed(KeyEvent e) {
        if (e.getKeyCode() == KeyEvent.VK_CONTROL) {
            str = "C ";
            return;
        }
        if (e.getKeyCode() == KeyEvent.VK_SHIFT) {
            str = "S ";
            return;
        }
        str = "D ";
    }
    public void keyReleased(KeyEvent e) {
        str = "U ";
    }
    public void keyTyped(KeyEvent e) {
    }
}

你这个代码我跑起来左下角是有个鼠标的坐标的啊