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我在使用 Java 中的 Xpath 提取 XML 节点时遇到问题。这是我正在使用的 YouTube 提要的 URL

http://gdata.youtube.com/feeds/api/videos?q=skateboarding+dog&start-index=21&max-results=10&v=2

这是提要的摘录

<?xml version="1.0" encoding="UTF-8" ?> 
- <feed xmlns="http://www.w3.org/2005/Atom" xmlns:app="http://www.w3.org/2007/app" xmlns:media="http://search.yahoo.com/mrss/" xmlns:openSearch="http://a9.com/-/spec/opensearch/1.1/" xmlns:gd="http://schemas.google.com/g/2005" xmlns:yt="http://gdata.youtube.com/schemas/2007" gd:etag="W/"CEANQ3o9fCp7I2A9WhBbFEg."">
  <id>tag:youtube.com,2008:videos</id> 
  <updated>2013-05-13T13:46:32.464Z</updated> 
  <category scheme="http://schemas.google.com/g/2005#kind" term="http://gdata.youtube.com/schemas/2007#video" /> 
  <title>Videos matching: skateboarding dog</title> 
  <logo>http://www.youtube.com/img/pic_youtubelogo_123x63.gif</logo> 
  <link rel="alternate" type="text/html" href="http://www.youtube.com" /> 
  <link rel="http://schemas.google.com/g/2005#feed" type="application/atom+xml" href="http://gdata.youtube.com/feeds/api/videos?v=2" /> 
  <link rel="http://schemas.google.com/g/2005#batch" type="application/atom+xml" href="http://gdata.youtube.com/feeds/api/videos/batch?v=2" /> 
  <link rel="self" type="application/atom+xml" href="http://gdata.youtube.com/feeds/api/videos?q=skateboarding+dog&start-index=21&max-results=10&v=2" /> 
  <link rel="service" type="application/atomsvc+xml" href="http://gdata.youtube.com/feeds/api/videos?alt=atom-service&v=2" /> 
  <link rel="previous" type="application/atom+xml" href="http://gdata.youtube.com/feeds/api/videos?q=skateboarding+dog&start-index=11&max-results=10&v=2" /> 
  <link rel="next" type="application/atom+xml" href="http://gdata.youtube.com/feeds/api/videos?q=skateboarding+dog&start-index=31&max-results=10&v=2" /> 
+ <author>
  <name>YouTube</name> 
  <uri>http://www.youtube.com/</uri> 
  </author>
  <generator version="2.1" uri="http://gdata.youtube.com">YouTube data API</generator> 
  <openSearch:totalResults>277515</openSearch:totalResults>

我创建了一个 NamespaceContext 类

import java.util.Iterator;
import javax.xml.XMLConstants;
import javax.xml.namespace.NamespaceContext;

public class YouTubeNameSpaceContext implements NamespaceContext {

@Override
public String getNamespaceURI(String prefix) {
    if (prefix == null) throw new NullPointerException("Invalid Namespace Prefix");
    else if (prefix.equals(XMLConstants.DEFAULT_NS_PREFIX))
        return "http://www.w3.org/2005/Atom";
    else if ("app".equals(prefix))
        return "http://www.w3.org/2007/app";
    else if ("media".equals(prefix)) 
        return "http://search.yahoo.com/mrss/";
    else if ("openSearch".equals(prefix)) 
        return "http://a9.com/-/spec/opensearch/1.1/";
    else if ("gd".equals(prefix))
        return "http://schemas.google.com/g/2005";      
    else if ("yt".equals(prefix)) 
        return "http://gdata.youtube.com/schemas/2007";
    else    
        return XMLConstants.NULL_NS_URI;
}

@Override
public String getPrefix(String arg0) {
    // TODO Auto-generated method stub
    return null;
}

@Override
public Iterator getPrefixes(String arg0) {
    // TODO Auto-generated method stub
    return null;
}

}

我用来获取 XML 节点的 Java。我试图获得的节点是，但结果始终为“空”:(

XPath xPath = XPathFactory.newInstance().newXPath();
xPath.setNamespaceContext(new YouTubeNameSpaceContext());

String QUERY_FORM_NUMBER =  "//feed/openSearch:totalResults"; 

Node result = (Node)xPath.evaluate(QUERY_FORM_NUMBER, document, XPathConstants.NODE);
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