Como utilizar JDialog como se fosse JInternalFrame?

import javax.swing.*;
import java.awt.*;
import java.awt.event.*;

public class Principal extends JFrame{
	
	private JDesktopPane desktopPane;
	private JMenuBar menuBar;
	private JMenu menu; 
	private JMenuItem menuItem1;
	private JDialog1 jd1;

	public Principal(){
		super("Principal");
		setExtendedState(MAXIMIZED_BOTH);
		setSize(500,400);
		setLocationRelativeTo(null);
		setVisible(true);
		setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		
		desktopPane = new JDesktopPane();
		menuBar = new JMenuBar();
		setJMenuBar(menuBar);
		menu = new JMenu("Menu");
		
		menuItem1 = new JMenuItem("MenuItem1");
		menuItem1.addActionListener(new ActionListener(){
		public void actionPerformed(ActionEvent e){
			if(jd1 == null){
				jd1 = new JDialog1();
				jd1.setVisible(true);
			}
			else if(!jd1.isVisible()){
				jd1.setVisible(true);
			}
		}});
				
		setContentPane(desktopPane);
		menuBar.add(menu);
		menu.add(menuItem1);
		}
	
	public static void main(String[]args){
		Principal principal = new Principal();
	}
}

import java.awt.*;
import javax.swing.*;

public class JDialog1 extends JDialog{

	public JPanel panel;
	public Container container;
	
	public JDialog1(){
		setTitle("JDialog");
		setModal(true);
		
		panel = new JPanel();
		container = getContentPane();
		container.add(panel);
		
		this.dispose();
	}
}