2 Synchronized JScrollPanes inside JSplitPane not working properly

Question

I have an issue with Swing. I have placed two JScrollPane inside a JSplitPane horizontally and have synchronized the JScrollPanes. Inside each of the JScrollPane I have placed one JPanel. The synchronization i.e. (scrolling a JScrollPane also scrolls the other JScrollPane) between the two JScrollPane works normally. But when I drag the JSplitPane divider towards left/right till some part of a JPanel becomes hidden then horizontal scroll bar is shown but in disabled form. I am not able to scroll to see the hidden part of JPanel.

Here is the code:

import java.awt.BorderLayout;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JSplitPane;

public class SplitPaneTest extends JFrame {

    public SplitPaneTest() {

        setTitle( "Splits" );
        setDefaultCloseOperation( EXIT_ON_CLOSE );
        setSize( 400, 400 );

        JPanel panel1 = new JPanel();
        panel1.setLayout(new BorderLayout());

        panel1.add( new JLabel( "Left panel!" ) );
        JScrollPane scrollPane1 = new JScrollPane(panel1);

        JPanel panel2 = new JPanel();
        panel2.setLayout(new BorderLayout());

        panel2.add( new JLabel( "Right Panel" ) );
        JScrollPane scrollPane2 = new JScrollPane(panel2);

        scrollPane2.getVerticalScrollBar().setModel(scrollPane1.getVerticalScrollBar().getModel());
        scrollPane2.getHorizontalScrollBar().setModel(scrollPane1.getHorizontalScrollBar().getModel());

        JSplitPane splitPane = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT, scrollPane1,
                scrollPane2);
        splitPane.setResizeWeight(0.5);
        add(splitPane);

        setVisible( true );

    }

    public static void main( String[] args ) {
        new SplitPaneTest();
    }

}

Answer 1

Scrollbars are displayed automatically when the preferred size of the component is greater than the size of the scroll pane.

You can't share the models because the state of each scrollbar will be different depending on how much space is allocation to each scroll pane. The only time they will be in sync is when the divider of the split pane is in the middle.

So you need to listen for changes in the scrollbar of each scroll pane and than attempt to adjust the scrollbar from the other scroll pane.

Something like:

import java.awt.BorderLayout;
import javax.swing.*;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JSplitPane;
import java.awt.event.*;

public class SplitPaneTest2 extends JFrame {

    public SplitPaneTest2() {

        setTitle( "Splits" );
        setDefaultCloseOperation( EXIT_ON_CLOSE );
        setSize( 400, 400 );

        JPanel panel1 = new JPanel();
        panel1.setLayout(new BorderLayout());

        panel1.add( new JLabel( "Left panel!11111111111111111111" ) );
        JScrollPane scrollPane1 = new JScrollPane(panel1);

        JPanel panel2 = new JPanel();
        panel2.setLayout(new BorderLayout());

        panel2.add( new JLabel( "Right Panel11111111111111111111" ) );
        JScrollPane scrollPane2 = new JScrollPane(panel2);

//        scrollPane2.getVerticalScrollBar().setModel(scrollPane1.getVerticalScrollBar().getModel());
//        scrollPane2.getHorizontalScrollBar().setModel(scrollPane1.getHorizontalScrollBar().getModel());
        new ScrollBarSynchronizer(scrollPane1.getHorizontalScrollBar(), scrollPane2.getHorizontalScrollBar());

        JSplitPane splitPane = new JSplitPane(JSplitPane.HORIZONTAL_SPLIT, scrollPane1,
                scrollPane2);
        splitPane.setResizeWeight(0.5);
        add(splitPane);

        setVisible( true );

    }

    public static void main( String[] args ) {
        new SplitPaneTest2();
    }

    static class ScrollBarSynchronizer implements AdjustmentListener
    {
        JScrollBar[] scrollBars;

        public ScrollBarSynchronizer(JScrollBar... scrollBars)
        {
            this.scrollBars = scrollBars;

            for (JScrollBar scrollBar: scrollBars)
                scrollBar.addAdjustmentListener( this );
        }

        @Override
        public void adjustmentValueChanged(AdjustmentEvent e)
        {
            JScrollBar source = (JScrollBar)e.getSource();
            int value = e.getValue();

            for (JScrollBar scrollBar: scrollBars)
            {
                if (scrollBar != source)
                {
                    scrollBar.removeAdjustmentListener( this );
                    scrollBar.setValue( value );
                    scrollBar.addAdjustmentListener( this );
                }
            }
        }
    }

}

You can also create a separate synchronizer for the vertical scrollbars.