How to export image without showing it in Java Swing

I have probelm with code below (it's simplified test version):

import javax.imageio.ImageIO;
import javax.swing.*;
import java.awt.*;
import java.awt.image.BufferedImage;
import java.io.File;

public class Example {
    JFrame frame;
    JPanel background;
    JPanel top;
    JPanel center;
    JPanel bottom;

    public Example() {
        frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(1016, 639);
        frame.setResizable(false);

        background = new JPanel();
        background.setLayout(new BoxLayout(background, BoxLayout.Y_AXIS));
        background.setPreferredSize(new Dimension(1000, 600));

        top = new JPanel();
        top.setPreferredSize(new Dimension(1000, 200));
        top.setBackground(Color.BLACK);

        center = new JPanel();
        center.setPreferredSize(new Dimension(1000, 200));
        center.setBackground(Color.WHITE);

        bottom = new JPanel();
        bottom.setPreferredSize(new Dimension(1000, 200));
        bottom.setBackground(Color.BLUE);

        background.add(top);
        background.add(center);
        background.add(bottom);

        frame.add(background);

        frame.setVisible(true);
    }


   public void getImage() {
        BufferedImage img = new BufferedImage(1000, 600, BufferedImage.TYPE_INT_RGB);
        Graphics2D g2d = img.createGraphics();
        background.paint(g2d);
        g2d.dispose();
        try {
            ImageIO.write(img, "png", new File("frame_2.png"));
        } catch(Exception e) {
            e.printStackTrace();
        }
    }

    public static void main(String... args) {
        Example ex = new Example();
        ex.getImage();
    }
 }

All I want to do is to export JPanel background as png file without showing the frame. Problem is, that now it works, but if I comment line:

frame.setVisible(true);

all I get is png file all black, with the right size.

I thing components are not properly painted if setVisible is false, I looked on the internet, but with my short experience with swing I found nothing that works. Maybe someone here could help?

SOLUTION

Use functions pack() to create components with sizes and dispose() to finish swing's loop.

public void getImage() {
    frame.pack();
    BufferedImage img = new BufferedImage(1000, 600, BufferedImage.TYPE_INT_RGB);
    Graphics2D g2d = img.createGraphics();
    background.paint(g2d);
    g2d.dispose();
    try {
        ImageIO.write(img, "png", new File("frame_2.png"));
    } catch(Exception e) {
        e.printStackTrace();
    }
    frame.dispose();
}