code: @Override
      public void shutdown() throws JournalException {
          super.testStateChange(State.SHUTDOWN);
          if (super.getState() != State.SHUTDOWN) {
              super.setState(State.SHUTDOWN);
          }
      }
docs: On a request to shut down,
  <ul>
  <li>check that we are in a valid state,</li>
  <li>set the state.</li>
  </ul>
  If we have already shut down, a second call is not an error, but requires
  no action.