我正在嘗試從 XmlString 構(gòu)造 XML 對象，但 getUnmarshaller(element) 返回 null ：這是代碼：     try {        InitializationService.initialize();        DocumentBuilderFactory documentBuilderFactory = DocumentBuilderFactory.newInstance();        DocumentBuilder docBuilder = documentBuilderFactory.newDocumentBuilder();        Document document = docBuilder.parse(new ByteArrayInputStream(xmlString.trim().getBytes()));        Element element = document.getDocumentElement();        UnmarshallerFactory unmarshallerFactory = XMLObjectProviderRegistrySupport.getUnmarshallerFactory();        org.opensaml.core.xml.io.Unmarshaller unmarshaller = unmarshallerFactory              .getUnmarshaller(element); //This is coming out be null        System.out.println(unmarshaller);     } catch (Exception e) {        e.printStackTrace();     }這是 XMLStringstatic String xmlString ="<?xml version=\"1.0\"?>" +          "  <!DOCTYPE address" +          "  [" +          "     <!ELEMENT address (buildingnumber, street, city, state, zip)>" +          "     <!ATTLIST address xmlns CDATA #IMPLIED>" +          "     <!ELEMENT buildingnumber (#PCDATA)>" +          "     <!ELEMENT street (#PCDATA)>" +          "     <!ELEMENT city (#PCDATA)>" +          "     <!ELEMENT state (#PCDATA)>" +          "     <!ELEMENT zip (#PCDATA)>" +          "  ]>" +          "" +          "  <address>" +          "    <buildingnumber> 29 </buildingnumber>" +          "    <street> South Street</street>" +          "    <city>Vancouver</city>" +          "" +          "    <state>BC</state>" +          "    <zip>V6V 4U7</zip>" +          "  </address>";任何幫助，將不勝感激。