code: public void postUnit (Unit unit)
      {
          if (shutdownRequested()) {
              throw new IllegalStateException("Cannot post units to shutdown invoker.");
          }
          // note the time
          unit.queueStamp = System.currentTimeMillis();
          // and append it to the queue
          _queue.append(unit);
      }
docs: Posts a unit to this invoker for subsequent invocation on the invoker's thread.