Java Swing - 如何在未装饰的JFrame周围绘制边框

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

import javax.swing.BorderFactory;
import javax.swing.JFrame;
import javax.swing.JLabel;

public class Main extends JFrame {
  JLabel label = new JLabel("Welcome!", JLabel.CENTER);

  public Main() {
    setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    setSize(new Dimension(200, 200));
    add(label, BorderLayout.CENTER);
    setUndecorated(true);
    getRootPane().setBorder(
        BorderFactory.createMatteBorder(4, 4, 4, 4, Color.RED));
    setVisible(true);

  }

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

}