@Override
    public void writeExternal( ObjectOutput out ) throws IOException
    {
        super.writeExternal( out );

        final ExternalizableUtil util = ExternalizableUtil.getInstance();
        util.writeSerializableMap( out, nodes );
        util.writeSafeUTF( out, associationPolicy.name() );
        util.writeSerializableCollection( out, associationTrusted );
        util.writeInt( out, maxLeafNodes );
    }
Only those on a whitelist may associate leaf nodes with the collection.
xmppserver/src/main/java/org/jivesoftware/openfire/pubsub/CollectionNode.java