java – 无头画

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();
}

但是我总是得到空图像,直到我将面板放入一个重量级的组件并在屏幕上显示(参见注释代码).我不想显示它,这个应用程序在服务器上运行.

这是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();
                }                   
            }
        });
    }    
}