code: public <T extends Closeable> T addCloseableInstance(T o)
    {
      addHandler(new CloseableHandler(o));
      return o;
    }
docs: Adds a Closeable instance to the lifecycle at {@link Stage#NORMAL} stage, doesn't try to call any "start" method on
  it, use {@link #addStartCloseInstance(Object)} instead if you need the latter behaviour.