Creer une BufferedImage sur la fenêtre active

import java.awt.Desktop;
import java.awt.Dimension;
import java.awt.Point;
import java.awt.Rectangle;
import java.awt.Robot;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.image.BufferedImage;
import java.io.File;

import javax.imageio.ImageIO;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JOptionPane;

public class Test
{
    public static void main(String[] args)
    {
        final JFrame frame = new JFrame();
        
        JButton button = new JButton("Capture");
        button.addActionListener(new ActionListener()
        {
            @Override
            public void actionPerformed(ActionEvent ae)
            {
                try
                {
                    Robot robot = new Robot();
                    
                    Dimension size = frame.getSize();
                    Point pos = frame.getLocationOnScreen();
                    
                    BufferedImage img = robot.createScreenCapture(new Rectangle(pos.x, pos.y, size.width, size.height));
                    
                    File tmp = File.createTempFile("awtScreenCapture_", ".png");
                    ImageIO.write(img, "png", tmp);
                    
                    Desktop.getDesktop().open(tmp);
                }
                catch (Exception e)
                {
                    JOptionPane.showMessageDialog(frame, e, e.getClass().getCanonicalName(), JOptionPane.ERROR_MESSAGE);
                }
            }
        });
        
        frame.setSize(200, 200);
        frame.add(button);
        frame.setLocationRelativeTo(null);
        frame.setVisible(true);
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
}