Java – headless painting

I want to draw JPanel as buffered image in headless mode (there is no GUI on the screen)

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 images until I put the panel into a heavyweight component and display it on the screen (see comment code) I don't want to display it. This application runs on the server

This 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,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();
                }                   
            }
        });
    }    
}

Solution

The size of the component is zero until the component has been implemented, so the drawing method does not work

View screen image It will solve this problem for you by calling dolayout () on the panel to ensure that all components have a valid size