getHeight() 和 getWidth() 都返回 0

我试图在带有线条的 JPanel 中创建一个网格，为此，我绘制了均匀间隔的水平和垂直线，直到到达 JPanel 的末尾。我使用了一个名为 Drawing 的类，它扩展了 JPanel 并且是我添加到窗口的对象。下面是我的代码。

public final class Drawing extends JPanel {

    public Drawing() {
        super();
        setBackground(new Color(255, 255, 255));
        setBorder(BorderFactory.createLineBorder(new Color(0, 0, 0)));

        GroupLayout workspacePanelLayout = new GroupLayout(this);
        setLayout(workspacePanelLayout);
        workspacePanelLayout.setHorizontalGroup(workspacePanelLayout.createParallelGroup(GroupLayout.Alignment.LEADING).addGap(0, 343, Short.MAX_VALUE));
        workspacePanelLayout.setVerticalGroup(workspacePanelLayout.createParallelGroup(GroupLayout.Alignment.LEADING).addGap(0, 400, Short.MAX_VALUE));

        initWorkSpace();
    }

    private static class Line {

        final int x1;
        final int y1;
        final int x2;
        final int y2;
        final Color color;

        public Line(int x1, int y1, int x2, int y2, Color color) {
            this.x1 = x1;
            this.y1 = y1;
            this.x2 = x2;
            this.y2 = y2;
            this.color = color;
        }
    }

    private final LinkedList<Line> lines = new LinkedList<>();

    public void addLine(int x1, int x2, int x3, int x4) {
        addLine(x1, x2, x3, x4, Color.black);
    }

    public void addLine(int x1, int x2, int x3, int x4, Color color) {
        lines.add(new Line(x1, x2, x3, x4, color));
        repaint();
    }

    public void clearLines() {
        lines.clear();
        repaint();
    }

    @Override
    private void paintComponent(Graphics g) {
        super.paintComponent(g);
        for (Line line : lines) {
            g.setColor(line.color);
            g.drawLine(line.x1, line.y1, line.x2, line.y2);
        }
    }

    public void initWorkSpace() {
        int x = 0;
        int y = 0;
        while (x < this.getWidth()) {
            addLine(x, 0, x, this.getHeight(), new Color(153, 153, 153));
            x += getSpacing();
        }
        while (y < this.getHeight()) {
            addLine(0, y, this.getWidth(), y, new Color(153, 153, 153));
            y += getSpacing();
        }
    }
}

问题是 'this.getHeight()' 和 'this.getWidth()' 都返回 0，因此不会绘制网格。绘制线条效果很好，只是面板显然没有尺寸。我该如何解决这个问题。