code: protected void checkShutdownInterval ()
      {
          // queue up a shutdown interval, unless we've already got one.
          long idlePeriod = idleUnloadPeriod();
          if (idlePeriod > 0L && _shutdownInterval == null) {
              (_shutdownInterval = _omgr.newInterval(new Runnable() {
                  public void run () {
                      log.debug("Unloading idle place '" + where() + "'.");
                      shutdown();
                  }
              })).schedule(idlePeriod);
          }
      }
docs: Called on startup and when the place is empty.