JTextArea within a JScrollPane

Question

Hi!
I've searched through the Project Swing archive and found lots of questions regarding how you ensure that when text is entered into a JTextArea control (via the append() method) the JScrollPane automatically scrolls to the position where the text was appended.
Unfortunately I haven't yet found a satisfactory solution to this issue.
I have a JTextArea control within a JScrollPane control. I start a seperate thread in order to perform some processing. This operation being run within this thread needs to provide progress information to the user and I present this progress information via the JTextArera control. Although the information is displayed within the JTextArea the JScrollPane unfortunately doesn't scroll down so that the last message entered into the JTextArea is visible. The user needs to scroll down manually in order to see the messages.
I have read that if you invoke JTextArea.setText() or JTextArea.append() from within a thread otherthan the event dispatching thread then the JScrollPane will have no knowledge that text as been added. The article suggested using SwingUtilities.invokeLater(Runnable run) to ensure that the call to JTextArea.setText() or JTextArea.append() is performed within the event dispatching thread. I did create a Runnable class whose sole purpose was to call JTextArea.append() with message information but still the JScrollPane fails to automatically scroll as successive lines of text are being added to the JTextArea control.
Can somebody please shed some light on my problem?

Answer 1

Here's a simple program that works for me. Note, if you comment out the setCaretPosition() method it stops working.
import java.awt.*;
import javax.swing.*;
import javax.swing.text.*;
public class TestTextArea extends JFrame
     JTextArea textArea;
public TestTextArea()
     JPanel panel = new JPanel();
     setContentPane( panel );
     panel.setPreferredSize( new Dimension( 200, 200 ) );
          textArea = new JTextArea( "one two", 3, 15 );
          JScrollPane scrollPane = new JScrollPane( textArea );
          panel.add( scrollPane );
          JTextArea textArea2= new JTextArea( "abcd", 3, 15 );
          textArea2.setPreferredSize( new Dimension( 50, 10 ) );
          panel.add( textArea2 );
     public void updateTextArea2()
          int line = 0;
          while ( true )
               textArea.append( "\nline: " + ++line );
               textArea.setCaretPosition( textArea.getText().length() );
               try
                    Thread.sleep(1000);
               catch (Exception e) {}
public static void main(String[] args)
TestTextArea frame = new TestTextArea();
frame.setDefaultCloseOperation( EXIT_ON_CLOSE );
frame.pack();
frame.setVisible(true);
          frame.updateTextArea2();