/**
     * Makes available the permit for the given thread, if it
     * was not already available.  If the thread was blocked on
     * {@code park} then it will unblock.  Otherwise, its next call
     * to {@code park} is guaranteed not to block. This operation
     * is not guaranteed to have any effect at all if the given
     * thread has not been started.
     *
     * @param thread the thread to unpark, or {@code null}, in which case
     *        this operation has no effect
     */

    public static void unpark(Thread thread) {

  {- -------------------------------------------
  (1) sun.misc.Unsafe.unpark() を呼び出すだけ.
      ---------------------------------------- -}

            if (thread != null)
                unsafe.unpark(thread);
        }