terminating java applet thread on page unload

I have got a problem here in terminating the threads. The problem is that I am using applet thread to access JS DOM and manipulate innerHTML of a span to right the Thread Name as the innerHTML. It works fine until I keep refreshing the page but if I dont give it a sleep of like 200ms and go to another page which dont have any applet on it, it will give me an error in firebug console:

document.getElementById("userName") is null

What I am suspecting is, although I send an interrupt on stop and I have also tried the same by setting a flag variable of running in run while loop, the thread executes much faster than the time JVM takes to unload it and I get my page changed in browser but applet is still trying to find an element with id userName.

Is that true? Is there any work around of it? Is there any way to know how many threads are running at the moment? Is there any kill all command to send on destroy()?

I have searched a lot, seen all the API of java threads and applets, couldn't get this to work.

Below is the code:

import java.applet.Applet;
import netscape.javascript.*;

//No need to extend JApplet, since we don't add any components;
//we just paint.
public class firstApplet extends Applet {
    /**
     * 
     */
    private static final long serialVersionUID = 1L;
    firstThread first;
    boolean running = false;

    public class firstThread extends Thread{
        JSObject win;
        JSObject doc;
        firstThread second;

        public firstThread(JSObject window){
            this.win = window;
        }
        public void run() {
            try{
                System.out.println("running... ");

                while (!isInterrupted()){

                    doc = (JSObject) win.getMember("document"); 
                    String userName = Thread.currentThread().getName();
                    //String userName = "John Doe";
                    String S = "document.getElementById(\"userName\").innerHTML = \""+userName+"\";";
                    doc.eval(S);
                    Thread.sleep(5);
                }
            }catch(Exception e){
                System.out.println("exception in first thread... ");
                e.printStackTrace();
            }
        }
    }

    public void init() {
        System.out.println("initializing... ");
    }

    public void start() {
        System.开发者_开发技巧out.println("starting... ");
        JSObject window = JSObject.getWindow(this);

        first = new firstThread(window);
        first.start();
        running = true;
    }

    public void stop() {
        first.interrupt();
        first = null;
        System.out.println("stopping... ");
    }

    public void destroy() {
        if(first != null){
            first = null;
        }
        System.out.println("preparing for unloading...");
    }
}

Thanks

Can't you just check if document.getElementById("userName") is null and only try and set it if it's not...?

This problem was solved, using JS variables to control Java applet execution, the complete solution is here: http://ciitronian.com/blog/programming/time-efficiency-controlling-java-applet-from-javascript-especially-if-you-make-round-trips-to-the-web-page-from-applet/