Why does the JTable header disappear when capturing an image in a JOptionPane?

Why does the JTable header not appear in the image?

When capturing an image of tabular data using the Java swing library, there may be an issue where the table header does not appear in the resulting image. This occurs because when the table is added to a JScrollPane and shown in an option pane, the table header is no longer part of the tree hierarchy when the image is captured.

The solution is to add the table header back to the hierarchy before capturing the image. This can be done using the addNotify() method, as shown below:

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

JOptionPane.showMessageDialog(null, p);
table.addNotify();
p.doLayout();
BufferedImage bi = new BufferedImage(p.getWidth(), p.getHeight(), BufferedImage.TYPE_INT_RGB);

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

By adding the table header back to the hierarchy using addNotify() and doing a layout, the header will be visible when the image is captured.

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