Thread Error

Question

Hi Guys,
I have a problem with a thread. I would appreciate any help.
I have an applet (called BaseApplet) that is the basis for all my applets. This BaseApplet is basically an extension of Applet that implements Runnable. A thread runs which simply waits for a given time then aborts (basically a timeout). I am getting the following error, apparently randomly);
java.lang.NullPointerException
     at com/innsite/applet/BaseApplet.run
     at java/lang/Thread.run
The BaseApplet is defined as follows;
public class BaseApplet extends Applet implements Runnableand the code to execute the thread is as follows;
  public void init()
    super.init();
    // start the timeout monitor
    timeout = new Thread(this);
    timeout.setDaemon(true);
    timeout.start();
  }First of all can I ask;
Is the error because some code is trying to execute the thread that is not instantiated? If not what could be causing this error?
I can provide the run() method if required but I have put some System.out's and it doesn't appear to run when I get this error. The application does time out as required. I have searched the application and cannot find any other call to start this thread.
Sorry to be so vague. The application is large/commercial so I cannot realistically post all of it.
Has anyone experienced problems with threads being executed?
Thanks in advance.
Slacks

Answer 1

I have a little example. Running this eample if you press 'Destroy Thread' then close the the applet you get a similar error. This error gives the line number etc.
import java.applet.*;
import java.awt.*;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.net.MalformedURLException;
import java.net.URL;
public class BaseApplet extends Applet implements Runnable {
     protected static BaseApplet instance;
     private transient Thread timeout = null;
     private transient Thread timeout1 = null;
     public BaseApplet() {
          super();
          instance = this;
     public void init() {
          super.init();
          // start the timeout monitor
          timeout = new Thread(this);
          timeout.setDaemon(true);
          timeout.start();
          timeout1 = new Thread(this);
          timeout1.setDaemon(true);
          timeout1.start();
          Panel p = new Panel();
          Button b1 = new Button("Reset");
          b1.addActionListener(new ActionListener() {
               public void actionPerformed(ActionEvent arg0) {
                    System.out.println("Timeout Reset : ");
                    resetTimeout();
          Button b2 = new Button("Distroy thread");
          b2.addActionListener(new ActionListener() {
               public void actionPerformed(ActionEvent arg0) {
                    System.out.println("Destroy Thread : ");
                    timeout=null;
                    timeout1=null;
          p.add(b1);
          p.add(b2);
          add(p);
     public void run() {
          int delay = 30000;
          boolean stopped = false;
          URL exitPage = null;
          try {
               exitPage = new URL("http://127.0.0.1/homepage.html");
          } catch (MalformedURLException e1) {
               e1.printStackTrace();
          while (!stopped) {
               try {
                    synchronized (timeout) {
                         // wait for n milliseconds or until interrupted
                         timeout.wait(delay);
                         System.out.println("Timeout!!!!");
                         stopped = true;
                         getAppletContext().showDocument(exitPage, "_self");
               } catch (InterruptedException e) {
                    // Invoked when interrupt() is called on the timeout thread.
                    // Means that the timeout whould either reset or stop.
          timeout = null;
     public final static void resetTimeout() {
          if ((instance != null) && (instance.timeout != null))
               instance.timeout.interrupt();