code: public static Object getBlocker(Thread t) {
          if (t == null)
              throw new NullPointerException();
          return U.getObjectVolatile(t, PARKBLOCKER);
      }
docs: Returns the blocker object supplied to the most recent
  invocation of a park method that has not yet unblocked, or null
  if not blocked.  The value returned is just a momentary
  snapshot -- the thread may have since unblocked or blocked on a
  different blocker object.
  
  @param t the thread
  @return the blocker
  @throws NullPointerException if argument is null
  @since 1.6