问如何在java中从xquery获取json？

EN

XQConnection conn = baseXDataSource.getConnection();
XQPreparedExpression expr = conn.prepareExpression
        ("declare option output:method 'json'; \n" +
                "<json objects=\"json\">\n" +
                "<a>asd</a>\n" +
                "</json>");

XQSequence result1 = expr.executeQuery();
result1.next();
XMLStreamReader result = result1.getSequenceAsStream();
Transformer transformer = TransformerFactory.newInstance().newTransformer();
transformer.setOutputProperty(OutputKeys.INDENT, "yes");
StringWriter stringWriter = new StringWriter();
transformer.transform(new StAXSource(result), new StreamResult(stringWriter));
return stringWriter.toString();

javax.xml.xquery.XQException: The prefix "output" is unknown in this static context.
    net.xqj.basex.bin.I.getNamespaceURI(Unknown Source)

{
  "a":"asd"
}