Weird behavior of GridBagLayout

Question

I've discovered recently strange thing in this code:

import java.awt.Color;
import java.awt.Container;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;
import java.io.IOException;

import javax.swing.BorderFactory;
import javax.swing.Box;
import javax.swing.JComponent;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.SwingUtilities;


public class Test {

    public static void main(String [] args) throws IOException {
        final JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel panel = new JPanel();
        panel.setLayout(new GridBagLayout());

        GridBagConstraints gbc = new GridBagConstraints();

        gbc.insets.top = 5;
        gbc.insets.right = 5;

        add(0, 0, 1, 1, 2, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(2, 0, 1, 1, 2, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(0, 1, 1, 1, 2, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(2, 1, 1, 1, 2, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(0, 2, 1, 1, 1, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(1, 2, 1, 1, 2, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(3, 2, 1, 1, 1, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(0, 3, 1, 1, 1, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(1, 3, 1, 1, 2, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(3, 3, 1, 1, 1, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(0, 4, 1, 1, 1, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(1, 4, 1, 1, 2, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(3, 4, 1, 1, 1, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());
        add(0, 5, 1, 1, 4, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, panel, createPanel());

        frame.setContentPane(panel);

        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    private static void add(int gridx, int gridy, int weightx, int weighty, int gridw, int gridh, int fill, int anchor, GridBagConstraints gbc, Container container, JComponent component) {
        gbc.gridx = gridx;
        gbc.gridy = gridy;
        gbc.weightx = weightx;
        gbc.weighty = weighty;
        gbc.gridwidth = gridw;
        gbc.gridheight = gridh;
        gbc.fill = fill;
        gbc.anchor = anchor;

        component.add(Box.createHorizontalStrut(5));
        component.add(new JLabel("(" + gridx + "; " + gridy + ")"));
        component.add(Box.createHorizontalStrut(5));
        component.add(new JLabel("gridwidth = " + gridw));
        component.add(Box.createHorizontalStrut(5));

        container.add(component, gbc);
    }

    private static JPanel createPanel() {
        JPanel panel1 = new JPanel();
        panel1.setBackground(Color.WHITE);
        panel1.setBorder(BorderFactory.createLineBorder(Color.RED));

        return panel1;
    }
}

And how it looks on run:

There are coordinates in brackets and gridwidth of each panel. As you can see from last row, the global gridwidth is 4. Rows 3-5 (y=2, y=3, y=4) in column 1 (x=0) have gridwidth equal to 1. But in first two rows the first column has gridwidth equal to 2, and it supposed to be divided in a middle of frame, but us isn't. Why?

Answer 1

What I found by myself:

public class Test {

    public static void main(String [] args) throws IOException {
        final JFrame frame = new JFrame();
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        JPanel panel = new JPanel();
        panel.setLayout(new BoxLayout(panel, BoxLayout.Y_AXIS));

        GridBagConstraints gbc = new GridBagConstraints();

        gbc.insets.top = 5;
        gbc.insets.right = 5;

        JPanel topPanel = new JPanel();
        topPanel.setLayout(new GridBagLayout());
        JPanel bottomPanel = new JPanel();
        bottomPanel.setLayout(new GridBagLayout());

        add(0, 0, 1, 1, 1, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, topPanel, createPanel());
        add(1, 0, 1, 1, 2, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, topPanel, createPanel());
        add(0, 0, 1, 1, 1, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, bottomPanel, createPanel());
        add(1, 0, 1, 1, 1, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, bottomPanel, createPanel());
        add(2, 0, 1, 1, 1, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, bottomPanel, createPanel());
        add(0, 2, 1, 1, 3, 1, GridBagConstraints.BOTH, GridBagConstraints.CENTER, gbc, bottomPanel, createPanel());

        panel.add(topPanel);
        panel.add(bottomPanel);

        frame.setContentPane(panel);

        SwingUtilities.invokeLater(new Runnable() {
            @Override
            public void run() {
                frame.pack();
                frame.setLocationRelativeTo(null);
                frame.setVisible(true);
            }
        });
    }

    private static void add(int gridx, int gridy, int weightx, int weighty, int gridw, int gridh, int fill, int anchor, GridBagConstraints gbc, Container container, JComponent component) {
        gbc.gridx = gridx;
        gbc.gridy = gridy;
        gbc.weightx = weightx;
        gbc.weighty = weighty;
        gbc.gridwidth = gridw;
        gbc.gridheight = gridh;
        gbc.fill = fill;
        gbc.anchor = anchor;

        component.add(Box.createHorizontalStrut(5));
        component.add(new JLabel("(" + gridx + "; " + gridy + ")"));
        component.add(Box.createHorizontalStrut(5));
        component.add(new JLabel("gridwidth = " + gridw));
        component.add(Box.createHorizontalStrut(5));

        container.add(component, gbc);
    }

    private static JPanel createPanel() {
        JPanel panel1 = new JPanel();
        panel1.setBackground(Color.WHITE);
        panel1.setBorder(BorderFactory.createLineBorder(Color.RED));

        return panel1;
    }
}

Result looks like:

What I've done: divided the content pane into two separate panels - top (with a single row which has two equally divided cells) and bottom (with two rows: one has 3 cells and another has 1 single wide cell). Content pane now has the BoxLayout instead of GBL, and it works as expected. But I'm disappointed about GridBagLayout, it was the perfect layout for me before now.