Exception when use getAppletContext().showDocument(url);

Question

I want to open a page using getAppletContext().showDocument(yourURL), but it troughs the following exception:
java.lang.NullPointerException
java.applet.Applet.getAppletContext(Unkonow Source)
This happen when i put the getappletcontext() in a method that I invoke at the end of the proces of my applet..but when I put the getAppletcontext() in the first method of my applet it works......., but because of the flow of my applet I have to put the getAppletContext() at the end...
So my quiestion is: when do I have to use getAppletContext() and why is this exception is happening?

Answer 1

Here is the code: for instance if I put getAppletcontext().showDocument(url) in the init() method or even in the onButtonClick() method it works, but the place where i have to put it is in the onSendData() method, but there doesn't work and happened the exception.....
public class TestApplet2 extends JApplet implements ActionListener {
UCapture active;
JButton button;
JComboBox scanner;
JPanel buttonPanel;
JPanel comboPanel;
JPanel activePanel;
JFrame frame;
int scannerElegido;
public void init() {
try {
System.out.println("begin..........");
active = new UCapture();
Evento appListener = new Evento();
active.add_DUCaptureEventsListener(appListener);
} catch (Exception e) {
System.out.println("error en el constructor........");
e.printStackTrace();
try {
activePanel = new JPanel();
activePanel.setBorder(BorderFactory.createLineBorder(Color.black));
activePanel.add(active);
button = new JButton(" Capture ");
button.setMnemonic(KeyEvent.VK_C);
button.setBorder(BorderFactory.createRaisedBevelBorder());
buttonPanel = new JPanel();
buttonPanel.setBorder(BorderFactory.createLineBorder(Color.black));
buttonPanel.add(button);
Container container = getContentPane();
container.add(active,BorderLayout.CENTER);
container.add(buttonPanel,BorderLayout.PAGE_START);
String[] scanners= new String[numScanner+1];
scanners[0] = "Select Scanner";
for (int i = 1; i <= numScanner; i++) {
scanners[i] = active.getSensorManufacturerTR(i);
scanner = new JComboBox(scanners);
scanner.setSelectedIndex(0);
scanner.setBorder(BorderFactory.createRaisedBevelBorder());
comboPanel = new JPanel();
comboPanel.setBorder(BorderFactory.createLineBorder(Color.black));
comboPanel.add(scanner);
container.add(comboPanel, BorderLayout.PAGE_END);
button.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
onButtonClick();
setVisible(true);
} catch (Exception ex) {
ex.printStackTrace();
public void onButtonClick() {
try {
scannerElegido = scanner.getSelectedIndex();
boolean openSensor = active.openSensor(scannerElegido);
boolean captureFinger = active.captureFinger();
} catch (Exception ex) {
ex.printStackTrace();
public void onSendData(Template template) {
try {
URL urlServlet = new URL("http://localhost:8080/triad/test.do");
URLConnection con = (URLConnection) urlServlet.openConnection();
// con.setDoInput(true);
con.setDoOutput(true);
con.setUseCaches(false);
con.setRequestProperty("Content-Type","application/x-java-serialized-object");
// send data to the servlet
ObjectOutputStream oos = new ObjectOutputStream(con.getOutputStream());
oos.writeObject(template.getTemplate());
oos.flush();
oos.close();
// receive result from servlet
con.getInputStream();
ObjectInputStream inputFromServlet = new ObjectInputStream(con.getInputStream());
Object result = (Object) inputFromServlet.readObject();
inputFromServlet.close();
// HERE IS WHERE THE EXCEPTION OCCURS
URL urlServlet1 = new URL("http://localhost:8080/triad/test.do");
System.out.println(getAppletContext().ShowDocument( urlServlet1);
} catch (Exception ex) {
ex.printStackTrace();
public void stop() {
try{
active.closeSensor();
catch (Exception ex) {
ex.printStackTrace();
public void actionPerformed(ActionEvent event) {
}