主要是使用JPanel的setPreferredSize方法进行首选大小的设置，同时保证宽高大于JScrollPane的宽高

import java.awt.Dimension;
import java.awt.FlowLayout;

import javax.swing.*;

public class JScrollPaneAndJPanel extends JFrame {
public JScrollPaneAndJPanel() {
super("TestJScrollPane");
this.setLayout(null);
this.setBounds(200, 200, 300, 300);
JPanel panel = new JPanel();
panel.setPreferredSize(new Dimension(200,100));//主要是这句代码，设置panel的首选大小，同时保证宽高大于JScrollPane的宽高，这样下面的JScrollPane才会出现滚动条
JButton button1  = new JButton("1");
panel.add(button1);
JButton button2  = new JButton("2");
panel.add(button2);
JButton button3  = new JButton("3");
panel.add(button3);
JButton button4  = new JButton("4");
panel.add(button4);
JButton button5  = new JButton("5");
panel.add(button5);
JButton button6  = new JButton("6");
panel.add(button6);
JButton button7  = new JButton("7");
panel.add(button7);
JScrollPane scrollPane = new JScrollPane(panel);
scrollPane.setBounds(10, 10, 175, 70);
this.getContentPane().add(scrollPane);
this.setVisible(true);
this.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

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