ThreadLocal and the API doc

Question

Hi,
I'm trying to understand ThreadLocal. The API javadoc provides the following example of a use of a ThreadLocal at http://java.sun.com/javase/6/docs/api/ :
import java.util.concurrent.atomic.AtomicInteger;
public class UniqueThreadIdGenerator {
     private static final AtomicInteger uniqueId = new AtomicInteger(0);
     private static final ThreadLocal < Integer > uniqueNum =
         new ThreadLocal < Integer > () {
             @Override protected Integer initialValue() {
                 return uniqueId.getAndIncrement();
     public static int getCurrentThreadId() {
         return uniqueId.get();
} // UniqueThreadIdGeneratorHowever I'm not sure I get this right. As far as I can tell, uniqueId is not thread-local, and will return basically the number of threads (minus 1) that have been created at any point in time, for any thread (rather than a unique ID for each thread). Shouldn't it be
public static int getCurrentThreadId() {
         return uniqueNum .get();
     }(ie uniqueNum rather than uniqueID)? I can't imagine this error not having been spotted earlier, but I've looked online and can't find anything about it. Is there some understanding I'm missing?
Thanks,
Lis

Answer 1

Ok I'm not a big thread pro (if that wasn't clear already ; ) ), so I tested this best I could. I copied the test class of the API and renamed to ThreadLocalTest. I then ran the following main:
public class ThreadLocalTestrun
     public static void main(String[] args)
          System.out.println("at the start - " + ThreadLocalTest.getCurrentThreadId() + " - " + Thread.currentThread().getName());
          new MyThread("NewThread-1").start();
          new MyThread("NewThread-2").start();
          System.out.println("in the end - " + ThreadLocalTest.getCurrentThreadId() + " - " + Thread.currentThread().getName());
     public static class MyThread
          extends Thread
          String message;
          MyThread(String name)
               super(name);
          @Override
          public void run()
               boolean hasRun = false;
               while (!hasRun)
                    System.out.println("Current Thread Id: " + ThreadLocalTest.getCurrentThreadId() + " (name '" + Thread.currentThread().getName() + "')");
                    hasRun = true;
}If I run the code with the uniqueId call, I get the following output:
at the start - 0 - main
in the end - 0 - main
Current Thread Id: 0 (name 'NewThread-1')
Current Thread Id: 0 (name 'NewThread-2')(and I don't understand why uniqueId hasn't been incremented except perhaps that uniqueNum never gets instantiated because it's not called?)
I then ran it with uniqueNum instead and got this:
at the start - 0 - main
in the end - 0 - main
Current Thread Id: 2 (name 'NewThread-1')
Current Thread Id: 1 (name 'NewThread-2')which looks a lot more like assigning a unique Id to a thread.
Is there something more subtle at play here which I'm not getting? Why would the int returned by uniqueId be thread-local, if uniqueId itself isn't?