Headless painting

Question

I would like to paint a JPanel into a BufferedImage in headless mode (no GUI on the screen at all).

final JPanel panel = createPanel();
panel.setSize(panel.getPreferredSize());
panel.validate();

//  JFrame frame = new JFrame();
//  frame.getContentPane().add(panel);
//  frame.pack();
//  frame.setVisible(true);

final BufferedImage image = new BufferedImage(
            panel.getBounds().width, 
            panel.getBounds().height, 
            BufferedImage.TYPE_INT_ARGB
);

final Graphics2D gc = image.createGraphics();
gc.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
gc.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);

try {
    panel.paint(gc);
    ...save the image somewhere...
} finally {
    gc.dispose();
}

But I always get empty image until I put the panel into a heavy-weight component and show it on the screen (see the commented code). I don't want to show it, this application runs on server.

Here is SSCCE:

    public class Example {

    private static JPanel createPanel() {
        final JPanel panel = new JPanel(new GridBagLayout());           
        final JLabel label = new JLabel("Yeah, it's working!", SwingConstants.CENTER);
        label.setFont(new Font("Arial", Font.PLAIN, 12));           
        final GridBagConstraints constraints = new GridBagConstraints();
        constraints.fill = GridBagConstraints.BOTH;
        constraints.weightx = 1;
        constraints.weightx = 1;
        panel.add(label, constraints);          
        return panel;
    }

    public static void main(String[] args) {
        SwingUtilities.invokeLater(new Runnable() {

            @Override
            public void run() {
                final JPanel panel = createPanel();
                panel.setSize(panel.getPreferredSize());
                panel.validate();

    //              JFrame frame = new JFrame();
    //              frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    //              frame.getContentPane().add(panel);
    //              frame.pack();
    //              frame.setVisible(true);

                final BufferedImage image = new BufferedImage(
                        panel.getBounds().width, 
                        panel.getBounds().height, 
                        BufferedImage.TYPE_INT_ARGB
                );    
                final Graphics2D gc = image.createGraphics();
                gc.setRenderingHint(RenderingHints.KEY_ANTIALIASING, RenderingHints.VALUE_ANTIALIAS_ON);
                gc.setRenderingHint(RenderingHints.KEY_TEXT_ANTIALIASING, RenderingHints.VALUE_TEXT_ANTIALIAS_ON);    
                try {
                    panel.paint(gc);
                    ImageIO.write(image, "png", new File("image.png"));
                } catch (IOException e) {
                    e.printStackTrace();
                } finally {
                    gc.dispose();
                }                   
            }
        });
    }    
}

Answer 1

Components have a zero size until the component has been realized so the painting methods don't work.

Check out Screen Image. It will handle this problem for you by invoking doLayout() on the panel to make sure all components have a valid size.