code: public void onChildThreadLaunchEnd()
    {
      try {
        if (_threadMax <= _threadCount.getAndDecrement()) {
          wake();
        }
  
        wakeIfLowIdle();
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
docs: Callback from the launched thread's run().
  
  Must _not_ be called by any other method, including other spawning.