@Override
        public void writeExternal(ObjectOutput out) throws IOException {
            ExternalizableUtil.getInstance().writeSerializable(out, (DefaultElement) element);
            ExternalizableUtil.getInstance().writeExternalizableCollection(out, nodes);
        }
When the local node is joining or leaving a cluster, {@link org.jivesoftware.util.cache.CacheFactory} will swap
the implementation used to instantiate caches. This causes the cache content to be 'reset': it will no longer
contain the data that's provided by the local node. This method restores data that's provided by the local node
in the cache. It is expected to be invoked right after joining ({@link #joinedCluster()} or leaving
({@link #leftCluster()} a cluster.
xmppserver/src/main/java/org/jivesoftware/openfire/disco/IQDiscoItemsHandler.java