Java Swing刷新JPanel

JApplet

setEnabled(true)

add(ScrollPane)

add(JScrollPane)

public void actionPerformed(ActionEvent e) {
    String command = e.getActionCommand();

    if (STAMPA_COMMAND.equals(command)) {

        stampa.setEnabled(false);
        JPanel areaPrint = new JPanel();
        JLabel lab = new JLabel(\"Wait Printing...\");
        areaPrint.setBackground(Color.magenta);
        areaPrint.add(lab);
        scrollArea.getViewport().add(areaPrint); // THIS IS THE PROBLEM...THE CHANGE ARE NOT REFRESHED

        try {
            PrintPdf printPDFFile;
            ArrayList assegniDaStampare = new ArrayList();

            for (int i = 0; i < assegni.size(); i++) {
                DatiAssegno datiAss = (DatiAssegno) assegni.get(i);

                if (datiAss != null && datiAss.getStatoAssegno().equals(STATUS_OK)) {
                    printPDFFile = new PrintPdf(\"Stampa Assegni\", datiAss);

                    printPDFFile.print();

                    String servletLocation = \"http://localhost/Servlet\";

                    //         connect to the servlet 

                    URL studentDBservlet = new URL(servletLocation);
                    URLConnection servletConnection = studentDBservlet.openConnection();
                    servletConnection.setUseCaches(false);
                    servletConnection.setDefaultUseCaches(false);
                    servletConnection.setDoInput(true);
                    servletConnection.setDoOutput(true);

                    OutputStream outstream = servletConnection.getOutputStream();
                    ObjectOutputStream oos = new ObjectOutputStream(outstream);
                    oos.writeObject(datiAss.idAssegno);
                    oos.flush();
                    oos.close();

                    ObjectInputStream inputFromServlet = new ObjectInputStream(servletConnection.getInputStream());
                    try {
                        String statusSave = (String) inputFromServlet.readObject();
                    } catch (ClassNotFoundException e4) {
                        e4.printStackTrace();
                    }
                }
            }
            JPanel areaPrint2 = new JPanel();
            JLabel lab2 = new JLabel(\"Print Complete\");
            areaPrint2.setBackground(Color.green);
            areaPrint2.add(lab2);
            scrollArea.getViewport().add(areaPrint2);
        } catch (FileNotFoundException e1) {
            //do something
        } catch (IOException e2) {
            //do something
        } catch (PrinterException e3) {
            //do something
        }
    }
    if (EXIT_COMMAND.equals(command)) {
        JSObject win = JSObject.getWindow(appletParent);
        appletParent.stop();
        appletParent.destroy();
        win.eval(\"self.close()\");
    }
}