Java Swing - 如何布局组件与GridBagLayout到西，东和中心

import java.awt.BorderLayout;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.GridBagConstraints;
import java.awt.GridBagLayout;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;

public class Main extends JFrame {
  public Main() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setLayout(new BorderLayout());

    JPanel upper = new JPanel();
    GridBagLayout gridbag = new GridBagLayout();
    upper.setLayout(gridbag);
    GridBagConstraints gbc = new GridBagConstraints();

    JButton toolbar1 = new JButton("toolbar1");
    JButton toolbar2 = new JButton("toolbar2");
    JButton toolbar3 = new JButton("toolbar3");

    gbc.gridx = 0;
    gbc.gridy = 0;
    gbc.weightx = 1.0;
    gbc.weighty = 1.0;
    gbc.anchor = GridBagConstraints.WEST;
    upper.add(toolbar1, gbc);

    gbc.gridx = 1;
    gbc.anchor = GridBagConstraints.CENTER;
    upper.add(toolbar2, gbc);

    gbc.gridx = 2;
    gbc.anchor = GridBagConstraints.EAST;
    gbc.gridwidth = GridBagConstraints.REMAINDER;
    upper.add(toolbar3, gbc);

    add(upper, BorderLayout.NORTH);

    JPanel something = new JPanel();
    something.setBackground(Color.WHITE);
    something.setPreferredSize(new Dimension(600, 600));
    something.repaint();
    add(something, BorderLayout.CENTER);

    pack();
    setVisible(true);
  }

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