使用JScrollpane时遇到问题

我正在学习swing，用以下代码运行后，IScrollpane的左侧滚动框不见了

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

public class guu extends JFrame {
    static int gap = 50;
    public guu() {
        this.setTitle("layout optimisation");
        this.setResizable(false);
        this.setVisible(true);
        this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        this.setBounds(400, 100, 900, 600);
        this.setLayout(null);

        Container container = this.getContentPane();
        JPanel panel = new JPanel();
        panel.setBounds(5,5,400,1000);
        panel.setLayout(null);

        for(int i = 0; i<15; i++){
            Label label = new Label("chuxianba!" + i);
            label.setBounds(20,20+i*gap,100,40);
            panel.add(label);
            Label label1 = new Label("chuxianba!" + i);
            label.setBounds(20,20+i*gap,100,40);
            panel.add(label1);
        }
        JScrollPane scrollPane = new JScrollPane(panel);
        scrollPane.setBounds(5,5,500,500);
        container.add(scrollPane);

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

结果如下

11行后面加上 panel.setPreferredSize(new Dimension(1000,1000));

    JPanel panel = new JPanel();
    panel.setBounds(5,5,400,1000);
    panel.setPreferredSize(new Dimension(1000,1000));
    panel.setLayout(null);