无法使用java在xpath中基于父属性值检索childelement

Question

我正在尝试获取父节点的name元素，其属性值为FieldType my source xml如下所示。

<external xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="Format" type="Format" version="0.0.1" formattype="Format">
    <header>
        <field xsi:type="FieldType">
            <name>Header Length</name>
        </field>
   </header>

1. 下面是我的java代码

   DocumentBuilderFactory domFactory = DocumentBuilderFactory.newInstance();
   DocumentBuilder builder;
   try {
       builder = domFactory.newDocumentBuilder();
       Document document=builder.parse(file);
       XPath xPath =  XPathFactory.newInstance().newXPath();
       xPath.setNamespaceContext(new NamespaceContext() {
   
           public Iterator getPrefixes(String namespaceURI) {
               // TODO Auto-generated method stub
               return null;
           }
   
           public String getPrefix(String namespaceURI) {
               // TODO Auto-generated method stub
               return null;
           }
   
           public String getNamespaceURI(String prefix) {
               // TODO Auto-generated method stub
               if(prefix.equals("xsi")){
                   return "http://www.w3.org/2001/XMLSchema-instance";}else if(prefix.equals("type")){
                   return "Format";}
               return null;}
       });NodeList nodelist1=(NodeList)xPath.compile("//field[@xsi:type='FieldType']//name").evaluate(document,XPathConstants.NODESET);
       System.out.println("node1="+nodelist1.getLength());