Java / Swing : JTextArea in a JScrollPane, how to prevent auto-scroll?

here's a runnable piece of code that shows what my "problem" is.

I've got a JTextArea wrapped in a JScrollPane. When I change the text of the JTextArea, the JScrollPane scrolls automatically to the end of the text and I don't want that.

Here are my requirements:

• the application should not scroll vertically automatically but...
• the user should be able to scroll vertically
• the user should not be able to scroll horizontally
• the application should never scroll horizontally
• the JTextArea must not be editable

(so even if there's more text than what can fit horizontally, neither the application nor the user should be able to scroll horizontally. While vertically, only the user should be able to scroll.)

I don't know how to "fix" this: should this be fixed using JTextArea or JScrollPane methods?

Note that AFAICT this is not a duplicate at all of: JTextPane prevents scrolling in the parent JScrollPane

Here's the kinda funny example, every 200 ms it puts new text in the JTextArea and you can see the JScrollPane always scrolling automatically to the end of the text.

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

public final class TextInScrollPane extends JFrame {

    private static final Random r = new Random( 42 );

    public static void main( final String[] args ) {
        final JFrame f = new JFrame();
        f.setDefaultCloseOperation( EXIT_ON_CLOSE );
        f.setLayout(new BorderLayout());
        final JTextArea jta = new JTextArea( "Some text", 30, 30 );
        jta.setEditable( false );   // This must not be editable
        final JScrollPane jsp = new JScrollPane( jta );
        jsp.setHorizontalScrollBarPolicy( JScrollPane.HORIZONTAL_SCROLLBAR_NEVER );
        jsp.setVerticalScrollBarPolicy( JScrollPane.VERTICAL_SCROLLBAR_ALWAYS );
        f.add( jsp, BorderLayout.CENTER );
        f.pack();
        f.setLocationRelativeTo( null );
        f.setVisible(true);

        final Thread t = new Thread( new Runnable() {
            public void run() {
                while ( true ) { 
                    try {Thread.sleep( 200 );} catch ( InterruptedException e ) {}
                    final StringBuilder 开发者_如何学运维sb = new StringBuilder();
                    for (int i = 0; i < 50 + r.nextInt( 75 ); i++) {
                        for (int j = 0; j < r.nextInt(120); j++) {
                            sb.append( (char) 'a' + r.nextInt(26) );
                        }
                        sb.append( '\n' );
                    }
                    SwingUtilities.invokeLater( new Runnable() {
                        public void run() {
                            jta.setText( sb.toString() );
                        }
                    } );
                }
            }
        });
        t.start();
    }

}

How to set AUTO-SCROLLING of JTextArea in Java GUI?

http://download.oracle.com/javase/1.5.0/docs/api/javax/swing/text/DefaultCaret.html#NEVER_UPDATE

Try:

JTextArea textArea = new JTextArea();
DefaultCaret caret = (DefaultCaret)textArea.getCaret();
caret.setUpdatePolicy(DefaultCaret.NEVER_UPDATE);

This should prevent the caret from automatically making the document scroll to the bottom.

Answering my own question: I'm not exactly sure this is the best way to solve my issue, but setting the JTextArea's caret using setCaretPosition(0) seems to work fine:

jta.setText( sb.toString() );
jta.jta.setCaretPosition( 0 );

You have run into a very strange behaviour in the implementation of the Document classes. I use a DefaultStyledDocument in a JTextPane inside a JScrollPane.

Now, here is the wierd thing. If I update the document on the EventQueue (like you do by scheduling a runnable to run later) the scroll pane automatically scrolls to the end.

However, the document classes claim to be thread safe and actually updatable from another thread. If I make sure to update on another thread than the EventQueue everything works fine but the scroll pane does NOT scroll to the end.

I have no explanation as to why this is so, I haven't looked in the Swing source. I have been exploiting this "feature" since 2006 and it has been consistent so far :-)