为什么将表捕获为图像时 JTable 标头会消失？

    table.addNotify();
    p.doLayout();
    BufferedImage bi = new BufferedImage(p.getWidth() + 100,
            p.getHeight() + 100, BufferedImage.TYPE_INT_RGB);

    Graphics g = bi.createGraphics();
    p.paint(g);
    g.dispose();

import javax.swing.*;
import java.awt.*;
import java.awt.image.BufferedImage;

import javax.imageio.ImageIO;
import java.io.File;

class TableImage {

    String[] columns = {"Name", "Age", "GPA", "Pass"};
    /** Any resemblance to persons living or dead is purely incidental. */
    Object[][] data = {
        {"André", new Integer(23), new Double(47.64), new Boolean(false)},
        {"Jeanie", new Integer(23), new Double(84.81), new Boolean(true)},
        {"Roberto", new Integer(22), new Double(78.23), new Boolean(true)}
    };

    TableImage() {
    }

    public JTable getTable() {
        JTable table = new JTable(data, columns);
        table.setGridColor(new Color(115,52,158));
        table.setRowMargin(5);
        table.setShowGrid(true);

        return table;
    }

    /** Method courtesy of camickr.
    https://stackoverflow.com/questions/7369814/why-does-the-jtable-header-not-appear-in-the-image/7375655#7375655
    Requires ScreenImage class available from..
    http://tips4java.wordpress.com/2008/10/13/screen-image/ */
    public BufferedImage getImage1(JTable table) {
        JScrollPane scroll = new JScrollPane(table);

        scroll.setColumnHeaderView(table.getTableHeader());
        table.setPreferredScrollableViewportSize(table.getPreferredSize());

        JPanel p = new JPanel(new BorderLayout());
        p.add(scroll, BorderLayout.CENTER);

        BufferedImage bi = ScreenImage.createImage(p);
        return bi;
    }

    /** Method courtesy of kleopatra.
    https://stackoverflow.com/questions/7369814/why-does-the-jtable-header-not-appear-in-the-image/7372045#7372045 */
    public BufferedImage getImage2(JTable table) {
        JScrollPane scroll = new JScrollPane(table);

        table.setPreferredScrollableViewportSize(table.getPreferredSize());

        JPanel p = new JPanel(new BorderLayout());
        p.add(scroll, BorderLayout.CENTER);

        // without having been shown, fake a all-ready
        p.addNotify();

        // manually size to pref
        p.setSize(p.getPreferredSize());

        // validate to force recursive doLayout of children
        p.validate();

        BufferedImage bi = new BufferedImage(p.getWidth(), p.getHeight(), BufferedImage.TYPE_INT_RGB);

        Graphics g = bi.createGraphics();
        p.paint(g);
        g.dispose();

        return bi;
    }

    public void writeImage(BufferedImage image, String name) throws Exception {
        ImageIO.write(image,"png",new File(name + ".png"));
    }

    public static void main(String[] args) throws Exception {
        UIManager.setLookAndFeel("com.sun.java.swing.plaf.nimbus.NimbusLookAndFeel");
        TableImage ti = new TableImage();
        JTable table;
        BufferedImage bi;

        table = ti.getTable();
        bi = ti.getImage1(table);
        ti.writeImage(bi, "1");
        JOptionPane.showMessageDialog(null, new JLabel(new ImageIcon(bi)));

        table = ti.getTable();
        bi = ti.getImage2(table);
        ti.writeImage(bi, "2");
        JOptionPane.showMessageDialog(null, new JLabel(new ImageIcon(bi)));
    }
}