java.lang.Thread uses two global counters:

    /* For autonumbering anonymous threads. */
    private static int threadInitNumber;
    private static synchronized int nextThreadNum() {
        return threadInitNumber++;
    }

    /* For generating thread ID */
    private static long threadSeqNumber;
    private static synchronized long nextThreadID() {
        return ++threadSeqNumber;
    }

both of these should have been updated in Java 5 to take advantage of the j.u.c AtomicInteger/AtomicLong classes to avoid contention on the Thread Class object's monitor.