如何使用Java中的XPath访问OWL文档？

我实际上是想获得这个元素：/ rdf：RDF / owl：Ontology / rdfs：label,我这样做了：

DocumentBuilder builder = builderfactory.newDocumentBuilder();
    Document xmlDocument = builder.parse(
            new File(XpathMain.class.getResource("person.xml").getFile()));

    XPathFactory factory = javax.xml.xpath.XPathFactory.newInstance();
    XPath xPath = factory.newXPath();
    XPathExpression xPathExpression = xPath.compile("/rdf:RDF/owl:Ontology/rdfs:label/text()");
    String nameOfTheBook = xPathExpression.evaluate(xmlDocument,XPathConstants.STRING).toString();

我也试过这样只提取rdfs：label元素：

XPathExpression xPathExpression = xPath.compile("//rdfs:label");        
 NodeList nodes = (NodeList) xPathExpression.evaluate(xmlDocument,XPathConstants.NODESET);

但是这个节点列表是空的.

请让我知道我哪里出错了.我正在使用Java XPath API.