@Override
    public String string() throws IOException {
        XContentBuilder builder = XContentFactory.jsonBuilder();
        toXContent(builder);
        builder.close();
        return builder.string();
    }
@see io.apiman.manager.api.es.util.QueryBuilder#string()
manager/api/es/src/main/java/io/apiman/manager/api/es/util/AbstractQueryBuilder.java