Java Swing - 如何防止组件在使用GridBagLayout调整大小时分散

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

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JPanel;
import javax.swing.JPasswordField;
import javax.swing.JSeparator;
import javax.swing.JTextField;

public class Main {

  public static void main(String[] args) {
    JFrame frame = new JFrame("Testing");
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.setLayout(new BorderLayout());
    frame.add(new LoginPanel());
    frame.pack();
    frame.setVisible(true);
  }
}

class LoginPanel extends JPanel {
  JTextField userfield = new JTextField(10);
  JPasswordField passfield = new JPasswordField(10);
  JButton login = new JButton("Login");
  JButton create = new JButton("Create Account");

  public LoginPanel() {
    setLayout(new GridBagLayout());
    GridBagConstraints gbc = new GridBagConstraints();

    gbc.anchor = GridBagConstraints.CENTER;
    gbc.weightx = 1;
    gbc.gridx = 2;
    gbc.anchor = GridBagConstraints.EAST;
    JLabel label = new JLabel("Username: ");
    add(label, gbc);

    gbc.anchor = GridBagConstraints.WEST;
    gbc.gridx = 3;
    gbc.gridwidth = 2;
    add(userfield, gbc);

    gbc.gridy = 1;
    add(passfield, gbc);

    gbc.anchor = GridBagConstraints.EAST;
    gbc.gridx = 2;
    label = new JLabel("Password: ");
    add(label, gbc);

    gbc.fill = GridBagConstraints.HORIZONTAL;
    gbc.gridy = 2;
    gbc.gridx = 1;
    gbc.gridwidth = 5;
    add(new JSeparator(JSeparator.HORIZONTAL), gbc);
  }
}