Java Swing - 如何center定位在GroupLayout中

import static javax.swing.GroupLayout.Alignment.CENTER;

import java.awt.Container;

import javax.swing.GroupLayout;
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JList;
import javax.swing.JScrollPane;

public class Main extends JFrame {
  public Main() {
    Container pane = getContentPane();
    GroupLayout gl = new GroupLayout(pane);
    pane.setLayout(gl);

    JLabel avLbl = new JLabel("Available");
    JLabel tagsLbl = new JLabel("Tags");
    JLabel selLbl = new JLabel("Selected");

    JButton newBtn = new JButton("New");
    JButton moveBtn = new JButton(">>");
    JButton remBtn = new JButton("Remove");

    JList leftList = new JList();
    JScrollPane spleft = new JScrollPane(leftList);
    JList rightList = new JList();
    JScrollPane spright = new JScrollPane(rightList);

    gl.setAutoCreateGaps(true);
    gl.setAutoCreateContainerGaps(true);

    gl.setHorizontalGroup(gl
        .createParallelGroup(CENTER)
        .addComponent(tagsLbl)
        .addGroup(
            gl.createSequentialGroup()
                .addGroup(
                    gl.createParallelGroup(CENTER).addComponent(avLbl)
                        .addComponent(spleft, 100, 200, Short.MAX_VALUE)
                        .addComponent(newBtn))
                .addComponent(moveBtn)
                .addGroup(
                    gl.createParallelGroup(CENTER).addComponent(selLbl)
                        .addComponent(spright, 100, 200, Short.MAX_VALUE)
                        .addComponent(remBtn))));
    gl.setVerticalGroup(gl
        .createSequentialGroup()
        .addComponent(tagsLbl)
        .addGroup(
            gl.createParallelGroup(CENTER)
                .addGroup(
                    gl.createSequentialGroup().addComponent(avLbl)
                        .addComponent(spleft, 100, 250, Short.MAX_VALUE)
                        .addComponent(newBtn))
                .addComponent(moveBtn)
                .addGroup(
                    gl.createSequentialGroup().addComponent(selLbl)
                        .addComponent(spright, 100, 250, Short.MAX_VALUE)
                        .addComponent(remBtn))));
    pack();
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
  }
  public static void main(String[] args) {
    Main ex = new Main();
    ex.setVisible(true);
  }
}