Java JTextPane add emty space to JScrollPane

Question

I have A JTextPane inside a JScrollPane. What I am trying to achieve is to add an empty space under the JTextPane to allow the user to scroll further down the text pane, so that the last line (Which would be visible at the bottom of the frame) can be scrolled up to the first line position. Hard to explain so here have some pics:

My attempt to solve this was to add a JPanel with Borderlayout.SOUTH under the JTextPane. And update the setPreferredSize(0, frameHeight - (const+ fontHeight))

const = A constant value I got from experimenting

int fontHeight = textPane.getFontMetrics(textPane.getFont()).getHeight();

This works fine until you resize the font... Basically it is very hacky and doesn't seem to be very reliable.

Question: Is there a internal method to add this feature and if not how could I improve the existing solution for more stability?

EDIT----------------- Here the working code:

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Insets;
import java.awt.Rectangle;
import java.awt.event.ComponentAdapter;
import java.awt.event.ComponentEvent;

import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextPane;
import javax.swing.JViewport;
import javax.swing.SwingUtilities;
import javax.swing.border.EmptyBorder;
import javax.swing.text.BadLocationException;


public class Main extends JFrame{
    private JTextPane zNum, tField;
    private JPanel scrollPanel;
    private JScrollPane tScrollPane;
    private Font font = new Font("Source Code Pro", Font.PLAIN, 12);

public Main(){
    super("Test Editor");
    getContentPane().setLayout(new BorderLayout());

    zNum = new JTextPane();
    zNum.setText("1");
    zNum.setEditable(false);
    zNum.setBorder(new EmptyBorder(2, 6, 2, 6));
    zNum.setBackground(Color.GRAY);
    zNum.setFont(font);

    tField = new JTextPane();
    tField.setBorder(new EmptyBorder(2, 6, 2, 6));
    tField.setBackground(Color.LIGHT_GRAY);
    tField.setFont(font);

    scrollPanel = new JPanel();
    scrollPanel.setLayout(new BorderLayout());
    scrollPanel.add(zNum, BorderLayout.LINE_START);
    scrollPanel.add(tField, BorderLayout.CENTER);
    scrollPanel.setBackground(Color.LIGHT_GRAY);

    tScrollPane = new JScrollPane(scrollPanel);
    tScrollPane.setBorder(BorderFactory.createEmptyBorder());
    getContentPane().add(tScrollPane, BorderLayout.CENTER);

    setPreferredSize(new Dimension(300, 200));
    setDefaultCloseOperation(DISPOSE_ON_CLOSE);
    setResizable(true);
    pack();
    setLocationRelativeTo(null);
    setVisible(true);

    // Resize
    this.addComponentListener(new ComponentAdapter() {
        public void componentResized(ComponentEvent event) {
            updateMargin(tField);
        }
    });
}

void updateMargin(JTextPane textPane) {
    JViewport viewport = (JViewport)
        SwingUtilities.getAncestorOfClass(JViewport.class, textPane);

    if (viewport != null) {
        int len = textPane.getDocument().getLength();
        try {
            Rectangle end = textPane.modelToView(len);
            if (end != null) {
                tField.setBorder(new EmptyBorder(0, 0, viewport.getHeight() - (end.height + 4), 0));
            }
        } catch (BadLocationException e) {
            throw new RuntimeException(e);
        }
    }
}

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

Answer 1

You can set the JTextPane's margin, based on the bounds of its enclosing JViewport and the last line of its document:

static void updateMargin(JTextPane textPane) {
    JViewport viewport = (JViewport)
        SwingUtilities.getAncestorOfClass(JViewport.class, textPane);

    if (viewport != null) {
        Insets margin = textPane.getMargin();

        int len = textPane.getDocument().getLength();
        try {
            Rectangle end = textPane.modelToView(len);
            if (end != null) {
                margin.bottom = viewport.getHeight() - end.height;
                textPane.setMargin(margin);
            }
        } catch (BadLocationException e) {
            throw new RuntimeException(e);
        }
    }
}

You'll want to have that method called whenever the JTextPane is resized, when it becomes displayable, and when its text is changed. For good measure, I would also call it if the JTextPane's inherited setPage method is called:

static void configureMargin(final JTextPane textPane) {
    textPane.addPropertyChangeListener("page", new PropertyChangeListener() {
        @Override
        public void propertyChange(PropertyChangeEvent event) {
            updateMargin(textPane);
        }
    });

    textPane.addHierarchyListener(new HierarchyListener() {
        @Override
        public void hierarchyChanged(HierarchyEvent event) {
            long flags = event.getChangeFlags();
            if ((flags & HierarchyEvent.DISPLAYABILITY_CHANGED) != 0) {
                updateMargin(textPane);
            }
        }
    });

    textPane.addComponentListener(new ComponentAdapter() {
        @Override
        public void componentResized(ComponentEvent event) {
            updateMargin(textPane);
        }
    });

    textPane.getDocument().addDocumentListener(new DocumentListener() {
        private void updateTextPane() {
            EventQueue.invokeLater(new Runnable() {
                public void run() {
                    updateMargin(textPane);
                }
            });
        }

        @Override
        public void changedUpdate(DocumentEvent event) {
            updateTextPane();
        }

        @Override
        public void insertUpdate(DocumentEvent event) {
            updateTextPane();
        }

        @Override
        public void removeUpdate(DocumentEvent event) {
            updateTextPane();
        }
    });
}

Answer 2

Well for one, you could use a different Layout Manager. For example, with GridBagLayout you can set padding between components and between the border and the components. I cant confirm that this works though.