[concurrency-interest] ThreadLocal example bug.

[Pete.Soper at Sun.COM]

Hi Jason,
             For the example code field uniqueId is a "dispenser" of the 
next available id value and that is used once per thread (but is visible 
to all threads) via the overriden initialVlaue method invocation that is 
a side effect of the first invocation of uniqueNum.get. The per-thread 
value is maintained by ThreadLocal code that you can't see and it never 
changes for a given thread. But maybe I'm missing something. Do you have 
a misbehaving test?

Regards,
Pete

  import java.util.concurrent.atomic.AtomicInteger;

  public class UniqueThreadIdGenerator {

      private static final ThreadLocal < Integer > uniqueNum =
          new ThreadLocal < Integer > () {
              final AtomicInteger uniqueId = new AtomicInteger(0);
              @Override protected Integer initialValue() {
                  return uniqueId.getAndIncrement();
              }
          };

      public static int getCurrentThreadId() {
          return uniqueNum.get();
      }
  } // UniqueThreadIdGenerator