使用带有FlowLayout的getContentpane在布局上应用更改，但它不起作用，为什么？

import javax.swing.*;
import java.awt.*;

public class FlowLayoutExample {
    public static void main(String[] args) {
        JFrame frame = new JFrame("FlowLayout Example");
        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

        Container contentPane = frame.getContentPane();
        FlowLayout flowLayout = new FlowLayout();
        contentPane.setLayout(flowLayout);

        JButton button1 = new JButton("Button 1");
        JButton button2 = new JButton("Button 2");
        contentPane.add(button1);
        contentPane.add(button2);

        // 确保布局更改生效
        contentPane.revalidate();
        contentPane.repaint();

        frame.setSize(400, 300);
        frame.setVisible(true);
    }
}