Thread: problem with invokeLater and repaint

I have a weird problem with my gui. I read on the internet that you should use invokeLater to start swing stuff so I thought I'd give it a try. But when in run the gui with it, it just shows a blank jframe and doesn't even respond when I click the X to close it. So I made a little test program:

package Test;
 
import java.awt.*;
import javax.swing.*;
import java.awt.geom.*;
 
public class TestWindow
{
	private JFrame frame;	
	private JDialog window;
	private TestPanel panel;
 
	public TestWindow()
	{
		frame = new JFrame("test");
		panel = new TestPanel();
		frame.setContentPane(panel);
		frame.setPreferredSize(new Dimension(800,600));
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);		
		frame.pack();
		frame.setVisible(true);		
	}
 
	public void runGUI()
	{
		long lasttime = System.currentTimeMillis();		
		int waittime = 1000;
		while(true)
		{
			long thistime = System.currentTimeMillis();
			if( (thistime - lasttime) >= waittime )
			{				
				System.out.println("checkpoint 1"); //this comes up once a second like it should when i run the program
                                panel.repaint();
				lasttime = System.currentTimeMillis();				
			}
			else
			{
				Thread.yield();
			}
		}		
	}
 
	public static void main(String[] args)
	{
		SwingUtilities.invokeLater( new Runnable()
		{
			public void run()
			{
				TestWindow test = new TestWindow();
				test.runGUI(); //<-- this causes the problem
			}
		});
	}
}
 
class TestPanel extends JPanel
{
	int x;
 
	public TestPanel()
	{
		x = 0;
	}
 
	public void paintComponent(Graphics g)
	{
		System.out.println("checkpoint 2"); //this never comes up
                super.paintComponent(g);
		Graphics2D g2 = (Graphics2D) g;
		x++;
		g2.draw( new Rectangle2D.Double(x,x,50,50) );
	}
}

If I never call runGUI() the program works and the square moves when i resize the window. When I do call runGUI() it gives me the blank window. I have no idea why it's doing this or even what to google for.

package Test;
 
import java.awt.*;
import javax.swing.*;
import java.awt.geom.*;
import java.awt.event.*;
import javax.swing.event.*;
 
public class TestWindow
{
	private JFrame frame;	
	private JDialog window;
	private TestPanel panel;
 
	public TestWindow()
	{
		frame = new JFrame("test");
		panel = new TestPanel();
		frame.setContentPane(panel);
		frame.setPreferredSize(new Dimension(800,600));
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);		
		frame.pack();
		frame.setVisible(true);
		class PaintAction extends AbstractAction
		{
			@Override
			public void actionPerformed(ActionEvent e)
			{
				panel.repaint();
			}
		}
		Timer timer = new Timer(1000, new PaintAction());		
		timer.start();
	}
 
	public static void main(String[] args)
	{
		SwingUtilities.invokeLater( new Runnable()
		{
			public void run()
			{
				TestWindow test = new TestWindow();				
			}
		});
	}
}
 
class TestPanel extends JPanel
{
	int x;
 
	public TestPanel()
	{
		x = 0;
	}
 
	public void paintComponent(Graphics g)
	{		
		super.paintComponent(g);
		Graphics2D g2 = (Graphics2D) g;
		x++;
		g2.draw( new Rectangle2D.Double(x,x,50,50) );
	}
}

The timer does make it work, but it's still weird that the while loop doesn't since it works perfectly if I don't use invokeLater.

It should become less of a mystery if you read up on Swing, the EDT, and how drawing/updating is done. Consider what the while(true) loop is doing as you study.

With the while loop on the EDT it blocks that whole thread preventing updating and it worked before because they were seperate threads then, right?

You've actually created an interesting demo that shows some of the important differences that occur when starting a Swing app the right and wrong ways. You've essentially summarized why the right way performed worse than the wrong way, but I've added some code to yours to enhance the demo. I also moved the runGUI() method to the TestPanel class, because that's where it really belongs. The TestWindow is the container, the TestPanel is the content, and the animation is occurring on the TestPanel, so TestPanel should contain the code that creates the animation.

There are two ways in the main() method to start the app, the right way using invokeLater() and the wrong way which calls the TestWindow() constructor on the main thread. (You can actually do both at once, but it's impossible to tell from which instance the messages are being printed, so there's little value in doing that.) I think you'll be able to verify your statement above by reviewing the messages that are printed while the app is running in the two different cases. Let me know if you have additional questions.

import java.awt.*;
import javax.swing.*;
import java.awt.geom.*;
 
public class TestWindow
{
    private JFrame frame; 
    private TestPanel panel;
 
    public TestWindow()
    {
        frame = new JFrame("test");
        panel = new TestPanel();
        frame.setContentPane(panel);
        frame.setPreferredSize(new Dimension(800,600));
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);  
        frame.pack();
        frame.setVisible(true);
 
        System.out.println( "Swing app is started on EDT: " +
                           SwingUtilities.isEventDispatchThread() );
 
        panel.runGUI();
    }
 
    public static void main(String[] args)
    {
        System.out.println( "Main thread is EDT: " +
                      SwingUtilities.isEventDispatchThread() );
 
        // either comment out this line:
        // new TestWindow();
 
        // or comment out this entire invokeLater() call and then run.
        SwingUtilities.invokeLater( new Runnable()
        {
            public void run()
            {
                new TestWindow();
            }
        });
 
    }
}
 
class TestPanel extends JPanel
{
    int x;
 
    public TestPanel()
    {
        x = 0;
    }
 
    @Override
    public void paintComponent(Graphics g)
    {
        System.out.println("checkpoint 2"); //this never comes up
        System.out.println( "panel's paintComponent() method is on EDT: " +
               SwingUtilities.isEventDispatchThread() );
        super.paintComponent(g);
        Graphics2D g2 = (Graphics2D) g;
        g2.draw( new Rectangle2D.Double(x,x,50,50) );
    }
 
    // note that i moved this to the TestPanel class, because that's where
    // it really belongs. i would also rename it, 'animatePanel()' or similar
    public void runGUI()
    {
        long lasttime = System.currentTimeMillis();  
        int waittime = 1000;
        while(true)
        {
            long thistime = System.currentTimeMillis();
            if( (thistime - lasttime) >= waittime )
            {    
                System.out.println("checkpoint 1"); //this comes up once a second like it should when i run the program
                System.out.println( "while( true ) loop is on EDT: " +
                       SwingUtilities.isEventDispatchThread() );
 
                x++;
                repaint();
                lasttime = System.currentTimeMillis();    
            }
        }  
    }
 
}