Stop scroll bar in JScrollPane from updating viewport when dragging knob

Question

Hi,
Does anyone know if it's possible to stop the JScrollPane from updating the viewport whilst dragging the knob of the scrollbar and only update once released.
The problem I have is that the view of the scroll panes viewport is a JList that has an underlying model of a RandomAccessFile which can be very large. So when the user is dragging the scrollbars it will be accessing the file system to retrieve the relevant data for the JList.
I've tried creating my own JScrollBar and adding a AdjustmentListener to ignore events whilst dragging:
class MyAdjustmentListener implements AdjustmentListener {
// This method is called whenever the value of a scrollbar is changed,
// either by the user or programmatically.
public void adjustmentValueChanged(AdjustmentEvent evt) {
Adjustable source = evt.getAdjustable();
// getValueIsAdjusting() returns true if the user is currently
// dragging the scrollbar's knob and has not picked a final value
if (evt.getValueIsAdjusting()) {
// The user is dragging the knob
return;
Looking through the JScrollBar code it has a model that will fire adjustment events anyway, which I suppose are being picked up by the JScrollPane somewhere.
I could use my own JScrollBar and not add it to the scroll pane and process the adjustment events myself and update the JList but I was wondering if there is a better way.
Many Thanks,
Martin.

Answer 1

Two small changes seem to do the trick. You may want to test it thoroughly though ;)import javax.swing.BoundedRangeModel;
import javax.swing.DefaultBoundedRangeModel;
import javax.swing.JFrame;
import javax.swing.JList;
import javax.swing.JScrollBar;
import javax.swing.JScrollPane;
import javax.swing.SwingUtilities;
public class OneStepScroller {
   public static void main(String[] args) {
      SwingUtilities.invokeLater(new Runnable() {
         @Override
         public void run() {
            new OneStepScroller().makeUI();
   public void makeUI() {
      Object[] data = new Object[200];
      for (int i = 0; i < data.length; i++) {
         data[i] = "Item Number " + i;
      JList list = new JList(data);
      JScrollPane scrollPane = new JScrollPane(list);
      BoundedRangeModel model = scrollPane.getVerticalScrollBar().getModel();
      final JScrollBar scrollBar = scrollPane.getVerticalScrollBar();
      scrollBar.setModel(new DefaultBoundedRangeModel(model.getValue(),
            model.getExtent(), model.getMinimum(), model.getMaximum()) {
         int oldValue;
         @Override
         public int getValue() {
            // changed here
            if (!getValueIsAdjusting() && !(oldValue == super.getValue())) {
               oldValue = super.getValue();
               // added this
               fireStateChanged();
            return oldValue;
      JFrame frame = new JFrame("");
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setSize(400, 400);
      frame.setLocationRelativeTo(null);
      frame.add(scrollPane);
      frame.setVisible(true);
}db