public class test extends JFrame{
    private static final long serialVersionUID = 5699328341463955028L;
    private JScrollPane jsp = null;
    private JPanel jp = null;
    private JPanel jpMainList2 = null;
    private JTextArea jta = null;
    
    
    private JScrollPane getJsp(){
        if(jsp == null){
            jsp = new JScrollPane();
            jsp.setLayout(null);
            jsp.setAutoscrolls(true);
            jsp.setHorizontalScrollBarPolicy(ScrollPaneConstants.HORIZONTAL_SCROLLBAR_ALWAYS);
            jsp.setVerticalScrollBarPolicy(ScrollPaneConstants.VERTICAL_SCROLLBAR_ALWAYS);
            jsp.add(getjp());
            jsp.setSize(500,600);
            jsp.setBackground(Color.red);
        }
        return jsp;
    }
    
    private JPanel getjp(){
        if(jp == null){
            jp = new JPanel();
            jp.setLayout(null);
            jp.add(getjta());
            jp.setLocation(50, 50);
            jp.setSize(1000,1000);
            jp.setBackground(Color.white);
            
        }
        return jp;
    }
    
    private JTextArea getjta(){
        if(jta == null){
            jta = new JTextArea();
            jta.setSize(1500, 1500);
            jta.setBackground(Color.CYAN);
        }
        return jta;
    }
    
    public test(){
        this.setLayout(null);
        this.add(getJsp());
        this.setExtendedState(Frame.MAXIMIZED_BOTH);
        this.setVisible(true);
    }
    
    public static void main(String[] args){
        new test();
    }
}

import java.awt.Dimension;
import java.awt.Toolkit;

import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JScrollPane;

@SuppressWarnings("serial")
public class Test extends JPanel {
    @Override
    public Dimension getPreferredSize() {
        return new Dimension(600, 600); // 这个值只是推荐值，并不一定会使用到
    }

    private static void createGuiAndShow() {
        JFrame frame = new JFrame("");
        
        JPanel panel = new JPanel();
        panel.add(new JButton("....Button...."));
        panel.setPreferredSize(new Dimension(600, 600)); // 这里设置才有用
        JScrollPane scroller = new JScrollPane(panel);
        frame.getContentPane().add(scroller);

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        int sw = Toolkit.getDefaultToolkit().getScreenSize().width;
        int sh = Toolkit.getDefaultToolkit().getScreenSize().height;
        int w = 400;
        int h = 400;
        int x = (sw - w) / 2;
        int y = (sh - h) / 2 - 40;
        x = x > 0 ? x : 0;
        y = y > 0 ? y : 0;
        frame.setBounds(x, y, w, h);
        frame.setVisible(true);
    }

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