如何在java eclipse中向JPanel添加背景(URL)

import java.awt.Graphics;
import java.awt.Image;
import java.io.IOException;
import java.net.URL;
import javax.imageio.ImageIO;
import javax.swing.ImageIcon;
import javax.swing.JPanel;

class CustomPanel extends JPanel {
    private Image backgroundImage;

    public CustomPanel(URL imageUrl) {
        try {
            backgroundImage = ImageIO.read(imageUrl);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    @Override
    protected void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(backgroundImage, 0, 0, getWidth(), getHeight(), this);
    }
}

public class MainClass {
    public static void main(String[] args) {
        JFrame frame = new JFrame("Background Panel");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setSize(400, 300);

        URL imageUrl = MainClass.class.getResource("background.jpg");
        CustomPanel panel = new CustomPanel(imageUrl);
        frame.add(panel);

        frame.setVisible(true);
    }
}