Java Swing - 如何扩展JLabel来创建可移动背景，抓取并拖动图像滚动标签

import java.awt.Container;
import java.awt.Cursor;
import java.awt.Dimension;
import java.awt.Point;
import java.awt.event.MouseEvent;
import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.awt.event.WindowListener;
import java.net.URL;

import javax.swing.ImageIcon;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JScrollPane;
import javax.swing.JViewport;
import javax.swing.event.MouseInputAdapter;

public class Main extends JFrame {
    public Main() throws Exception{
        super();
        ImageIcon ii = new ImageIcon(new URL("http://www.w3cschool.cn/style/download.png"));
        GrabAndScrollLabel label = new GrabAndScrollLabel(ii);
        label.setPreferredSize(new Dimension(300,300));
        
        JScrollPane jsp = new JScrollPane(label);
        getContentPane().add(jsp);
        setSize(300, 250);
        setVisible(true);

        WindowListener l = new WindowAdapter() {
            public void windowClosing(WindowEvent e) {
                System.exit(0);
            }
        };
        addWindowListener(l);
    }

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

class GrabAndScrollLabel extends JLabel {
    public GrabAndScrollLabel(ImageIcon i) {
        super(i);

        MouseInputAdapter mia = new MouseInputAdapter() {
            int xDiff, yDiff;

            boolean isDragging;

            Container c;

            public void mouseDragged(MouseEvent e) {
                c = GrabAndScrollLabel.this.getParent();
                if (c instanceof JViewport) {
                    JViewport jv = (JViewport) c;
                    Point p = jv.getViewPosition();
                    int newX = p.x - (e.getX() - xDiff);
                    int newY = p.y - (e.getY() - yDiff);

                    int maxX = GrabAndScrollLabel.this.getWidth()
                            - jv.getWidth();
                    int maxY = GrabAndScrollLabel.this.getHeight()
                            - jv.getHeight();
                    if (newX < 0)
                        newX = 0;
                    if (newX > maxX)
                        newX = maxX;
                    if (newY < 0)
                        newY = 0;
                    if (newY > maxY)
                        newY = maxY;

                    jv.setViewPosition(new Point(newX, newY));
                }
            }

            public void mousePressed(MouseEvent e) {
                setCursor(Cursor.getPredefinedCursor(Cursor.MOVE_CURSOR));
                xDiff = e.getX();
                yDiff = e.getY();
            }

            public void mouseReleased(MouseEvent e) {
                setCursor(Cursor.getPredefinedCursor(Cursor.DEFAULT_CURSOR));
            }
        };
        addMouseMotionListener(mia);
        addMouseListener(mia);
    }
}