Why is my JPanel inside a JScrollPane not scrolling?

Question

I have a JPanel (yellow) placed in JScrollPane.

When I enter some text in JTextPane, it resizes, but the vertical scrollbar is still not active. yelowPanel.getSize() returns the same value it was before.`(You can see it on redPanel).

So how can I refresh yellowPanel? I want to scroll panel vertically. I tried to:

panelCreating.revalidate();
panelCreating.invalidate();
panelCreating.repaint();

Works only panelCreating.setPreferredSize(new Dimension(333, 777)); but I don't know what size to set. It depends on content.

There is a small example:

package swingtest;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.awt.Insets;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import javax.swing.BorderFactory;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;
import javax.swing.JTextPane;

public class SwingTest extends JFrame {

    public SwingTest() {
        initComponents();
    }

    public static void main(String[] args) {
        java.awt.EventQueue.invokeLater(new Runnable() {

            @Override
            public void run() {
                new SwingTest().setVisible(true);
            }
        });
    }
    private JPanel panelCenter, panelCreating;
    private JScrollPane scrollPaneCreating, scrollPaneCenter;
    private JTextPane textPane1, textPane2;
    private JButton button1;

    private void initComponents() {
        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setMinimumSize(new Dimension(300, 300));

        panelCreating = new JPanel();
        panelCreating.setMinimumSize(new Dimension(160, 200));
        panelCreating.setPreferredSize(new Dimension(160, 200));
        scrollPaneCreating = new JScrollPane(panelCreating,
            JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, 
            JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);

        textPane1 = new JTextPane();
        textPane1.setText("a\na");
        textPane2 = new JTextPane();
        textPane2.setText("b\nb");
        button1 = new JButton("+++");

        panelCenter = new JPanel();
        panelCenter.setBackground(Color.blue);
        scrollPaneCenter = new JScrollPane(panelCenter);



        // ----------------- Left Panel Init -----------------------

        panelCreating.setLayout(new GridBagLayout());
        panelCreating.setBackground(Color.ORANGE);
        panelCreating.setBorder(BorderFactory.createEmptyBorder(8, 8, 8, 8));

        GridBagConstraints c = new GridBagConstraints();
        c.insets = new Insets(0, 0, 4, 4);
        c.anchor = GridBagConstraints.FIRST_LINE_START;
        c.weightx = c.weighty = 0;

        c.gridx = 0;
        c.gridy = GridBagConstraints.RELATIVE;
        c.gridwidth = GridBagConstraints.REMAINDER;
        c.gridheight = 1;
        c.fill = GridBagConstraints.BOTH;
        panelCreating.add(textPane1, c);

        button1.addActionListener(new ActionListener() {

            int height = 50;

            @Override
            public void actionPerformed(ActionEvent e) {
                textPane1.setText(textPane1.getText() + "\na");
                textPane1.setPreferredSize(new Dimension(150, height));
                textPane2.setText(textPane2.getText() + "\nb");
                textPane2.setPreferredSize(new Dimension(150, height));
                height += 30;

                panelCreating.revalidate();
                panelCreating.repaint();
                scrollPaneCreating.revalidate();
            }
        });
        panelCreating.add(button1, c);

        panelCreating.add(textPane2, c);

        // -------------------------------------------------------

        getContentPane().setLayout(new GridBagLayout());
        c = new GridBagConstraints();

        c.ipadx = c.ipady = 0;
        c.insets = new Insets(0, 0, 0, 0);
        c.weighty = 0;
        c.gridheight = 1;
        c.gridx = 0;
        c.gridy = 1;
        c.gridwidth = 1;
        c.weightx = 0;
        c.fill = GridBagConstraints.BOTH;
        getContentPane().add(scrollPaneCreating, c);


        c.gridx = 1;
        c.gridy = 1;
        c.fill = GridBagConstraints.BOTH;
        c.weightx = 1;
        c.weighty = 1;
        getContentPane().add(scrollPaneCenter, c);

    }
}

Yellow panel also uses GridBagLayout. Sorry for my English

Answer 1

Instead of setting a preferred size on panelCreating, set it on scrollPaneCreating. And don't set a preferred size on the text components, they will grow as you add new lines of text to them. The idea is to have the panel inside the scroll pane grow as large as it needs to, and just restrict the size of the scroll pane itself.

    // [...]

    panelCreating = new JPanel();
    //panelCreating.setMinimumSize(new Dimension(160, 200));
    //panelCreating.setPreferredSize(new Dimension(160, 200));
    scrollPaneCreating = new JScrollPane(panelCreating,
        JScrollPane.VERTICAL_SCROLLBAR_ALWAYS, 
        JScrollPane.HORIZONTAL_SCROLLBAR_AS_NEEDED);
    scrollPaneCreating.setMinimumSize(new Dimension(160, 200));
    scrollPaneCreating.setPreferredSize(new Dimension(160, 200));

    // [...]

        @Override
        public void actionPerformed(ActionEvent e) {
            textPane1.setText(textPane1.getText() + "\na");
            //textPane1.setPreferredSize(new Dimension(150, height));
            textPane2.setText(textPane2.getText() + "\nb");
            //textPane2.setPreferredSize(new Dimension(150, height));
            //height += 30;

            // This isn't necessary either
            //panelCreating.revalidate();
            //panelCreating.repaint();
            //scrollPaneCreating.revalidate();
        }

---

Edited to add: another alternative is to set sizes on the JViewport that is attached to the scroll pane. The viewport is where the content is displayed. You can sort of think of the scroll pane as being composed of the viewport plus scrollbars. If the scroll pane is set to a fixed size, then the viewport size is determined by subtracting the scroll bar size. But if the viewport is set to a fixed size, then the scroll pane size is determined by adding the scroll bar size to the viewport size. Setting a fixed size on the viewport is preferable if you want to precisely control how much content should be displayed on screen, because scroll bar sizes can vary by operating system.

scrollPaneCreating.getViewport().setMinimumSize(new Dimension(160, 200));
scrollPaneCreating.getViewport().setPreferredSize(new Dimension(160, 200));