code: public void shutdown ()
      {
          if (_downers == null) {
              log.warning("Refusing repeat shutdown() request.");
              return;
          }
  
          ObserverList<ShutdownComponent> list = _downers.toObserverList();
          _downers = null;
          list.apply(new ObserverList.ObserverOp<ShutdownComponent>() {
              public boolean apply (ShutdownComponent comp) {
                  log.debug("Shutting down component", "comp", comp);
                  comp.shutdown();
                  return true;
              }
          });
      }
docs: Shuts down all components immediately on the caller's thread.