Swing component flickering when updated a lot

Question

I've got a couple thousand lines of code somewhere and I've noticed that my JTextPane flickers when I update it too much.. I wrote a simplified version here:

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

public class Test
{
    static JFrame f;
    static JTextPane a;
    static final String NL = "\n";

    public static void main(String... args)
    {
        EventQueue.invokeLater(new Runnable(){
        public void run()
        {
        f = new JFrame();
        f.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        f.setVisible(true);
        f.setSize(400, 300);
        f.setLocationRelativeTo(null);

        a = new JTextPane();
        f.add(new JScrollPane(a));

        new Thread(new Runnable(){
            public void run()
            {
                int i = 0;
                StringBuffer b = new StringBuffer();
                while(true)
                {
                    b.append(++i+NL);
                    a.setText(b.toString());
                    a.setCaretPosition(b.length());
                    try{Thread.sleep(10);}catch(Exception e){}
                }
            }
        }).start();
        }
        });

    }
}

This is for a terminal (cmd) style GUI component--

I think I've made all the optimizations I could here, including having \n as a final variable so it won't be constructed hundreds of times. Still, the flickering is noticeable and unacceptable. After a few minutes, the component freezes completely. I must update the component very quickly, and the pane must be scrolled to the bottom when updated.

I've been thinking about making my own version of JTextPane from scratch, but I'd like to see if you guys have an easier solution.

Why the curious emphasis on the word "experts"? Did you think you'd get a better answer by insulting us? — Paul Tomblin

Eddie Eddie · Accepted Answer · 2009-03-22T02:54:41

Part of your error is that you are accessing a Swing component from outside the event thread! Yes, setText() is thread-safe, but Swing methods are not Thread-safe unless they are explicitly declared as such. Thus, setCaretPosition() is not Thread-safe and must be accessed from the event thread. This is almost certainly why your application eventually freezes.

NOTE: JTextPane inherits its setText() method from JEditorPane and its setCaretPosition method from JTextComponent, which explains the links in the previous paragraph not going to the JTextPane JavaDoc page.

To be Thread-safe, you really need to at least call setCaretPosition() from within the event thread, which you can do with code like this:

SwingUtilities.invokeAndWait(new Runnable() {
  public void run() {
    a.setText(b.toString());
    a.setCaretPosition(b.length());
  }
}

And since you have to call setCaretPosition() from within the event thread, you might as well also call setText() from the same place.

It's possible that you may not need to manually set the caret position. Check out the section "Caret Changes" in the JavaDoc for JTextComponent.

Finally, you may want to check out a series of two articles:

Swing component flickering when updated a lot

2 Answers