How to automatically scroll text in JScrollPane

Question

Hi there
I have a very simple question...
How to automatically scroll text in JScrollPane?
Text in the TextArea is constantly getting updated... but the scroll bars dont' move as the text changes. Instead I ahve to scroll and see the changes everytime. How can I make the viewport or the scrollpane to show the latest content?
Thanks in advance.
Dexter

Answer 1

This question is asked daily (it seems) on the forum. Hopefully the TextAreaScroll class will explain whats going on:
**  Short answer is to use the following after the append:
**  textArea.setCaretPosition(textArea.getDocument().getLength()
**  However, if you really want to know what is going on, then I have
**  I have observed the following behaviour in JDK1.4.2
**  JTextArea will scroll automatically when text is appended, if:
**  a) the caret is at the end of the text area, and
**  b) the append is done in the event thread
**  Note: Initializing a text area at creation time by any of the following
**  aproaches will cause the caret to be positioned at the start and therefore
**  scrolling will not happen automatically:
**  a) JTextArea textArea = new JTextArea("Initial text", ...);
**  b) textArea.setText("Initial text");
**  c) textArea.read(someFile, null);
**  The append method can be forced to execute in the Event thread by using
**  SwingUtilities.invokeLater();
**  Alternatively you can force a scroll by repositioning the caret.
import java.awt.*;
import java.awt.event.*;
import java.util.Date;
import javax.swing.*;
import javax.swing.text.*;
public class TextAreaScroll
     public static void main(String[] args)
          final JTextArea textAreaWest = new JTextArea(10, 25);
          JScrollPane scrollPaneWest = new JScrollPane( textAreaWest );
          final JTextArea textAreaEast = new JTextArea(10, 25);
          JScrollPane scrollPaneEast = new JScrollPane( textAreaEast );
          JFrame frame = new JFrame();
          frame.setDefaultCloseOperation( JFrame.EXIT_ON_CLOSE );
          frame.getContentPane().add(scrollPaneWest, BorderLayout.WEST);
          frame.getContentPane().add(scrollPaneEast, BorderLayout.EAST);
          frame.pack();
          frame.setVisible(true);
          //  The West text area will be updated by a Timer. Timer code is
          //  executed in the Event thread so it will scroll correctly.
          new Timer(1000, new ActionListener()
               public void actionPerformed(ActionEvent e)
                    if (textAreaWest.getDocument().getLength() == 0)
                         textAreaWest.append("West will scroll correctly");
                    textAreaWest.append( "\n" + new Date().toString());
          }).start();
          //  The East text area is not updated in the Event thread.
          //  It will not scroll correctly.
          while (true)
               try
                    if (textAreaEast.getDocument().getLength() == 0)
                         textAreaEast.append("East will not scroll correctly");
                    textAreaEast.append( "\n" + new Date().toString() );
                    //  Using this method causes the text area to scroll because
                    //  this method will invoke SwingUtilities.invokeLater(...)
                    textAreaEast.setCaretPosition( textAreaEast.getDocument().getLength() );
                    //  Using SwingUtilities.invokeLater causes the code to execute
                    //  on the Event thread
                    //  (comment all the above lines before testing)
                    SwingUtilities.invokeLater( new Runnable()
                         public void run()
                              textAreaEast.append( "\n" + new Date().toString() );
                    Thread.sleep(1000);
               catch (InterruptedException ie) {}
}