如何打印单个JPanel的内容？

import javax.print.Doc;
import javax.print.DocFlavor;
import javax.print.DocPrintJob;
import javax.print.PrintService;
import javax.print.PrintServiceLookup;
import javax.print.SimpleDoc;
import javax.print.attribute.HashPrintRequestAttributeSet;
import javax.print.attribute.PrintRequestAttributeSet;
import javax.print.attribute.standard.MediaSizeName;
import javax.print.attribute.standard.OrientationRequested;
import javax.swing.JPanel;

import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.print.PageFormat;
import java.awt.print.Printable;
import java.awt.print.PrinterException;
import java.awt.print.PrinterJob;

public class JPanelPrinting {

    public static void printJPanel(JPanel panel) {
        PrinterJob printJob = PrinterJob.getPrinterJob();
        PageFormat pageFormat = printJob.defaultPage();
        PrintRequestAttributeSet pras = new HashPrintRequestAttributeSet();
        pras.add(MediaSizeName.ISO_A4);
        pras.add(OrientationRequested.PORTRAIT);
        JPanelPrintable printable = new JPanelPrintable(panel);
        printJob.setPrintable(printable, pageFormat);
        if (printJob.printDialog(pras)) {
            try {
                printJob.print(pras);
            } catch (PrinterException e) {
                e.printStackTrace();
            }
        }
    }

    private static class JPanelPrintable implements Printable {

        private JPanel panel;

        public JPanelPrintable(JPanel panel) {
            this.panel = panel;
        }

        @Override
        public int print(Graphics g, PageFormat pageFormat, int pageIndex) {
            if (pageIndex > 0) {
                return NO_SUCH_PAGE;
            }
            Graphics2D g2 = (Graphics2D) g;
            g2.translate(pageFormat.getImageableX(), pageFormat.getImageableY());
            double width = pageFormat.getImageableWidth();
            double height = pageFormat.getImageableHeight();
            panel.setSize((int) width, (int) height);
            panel.printAll(g2);
            return PAGE_EXISTS;
        }
    }
}