java - XPath、XML 命名空间和 Java

Question

过去一天我一直在尝试从以下文档中提取一个 XML 节点，但无法掌握 XML 命名空间的细微差别以使其正常工作。

XML 文件太大，无法全部发布，所以这里是我关心的部分:

<?xml version="1.0" encoding="ISO-8859-1" standalone="no"?>
<XFDL xmlns="http://www.PureEdge.com/XFDL/6.5" xmlns:custom="http://www.PureEdge.com/XFDL/Custom" xmlns:designer="http://www.PureEdge.com/Designer/6.1" xmlns:pecs="http://www.PureEdge.com/PECustomerService" xmlns:xfdl="http://www.PureEdge.com/XFDL/6.5">
   <globalpage sid="global">
      <global sid="global">
         <xmlmodel xmlns:xforms="http://www.w3.org/2003/xforms">
            <instances>
               <xforms:instance id="metadata">
                  <form_metadata>
                     <metadataver version="1.0"/>
                     <metadataverdate>
                        <date day="05" month="Jul" year="2005"/>
                     </metadataverdate>
                     <title>
                        <documentnbr number="2062" prefix.army="DA" scope="army" suffix=""/>
                        <longtitle>HAND RECEIPT/ANNEX NUMBER </longtitle>
                     </title>

文档继续并且从头到尾都结构良好。我正在尝试从“documentnbr”标签(从底部开始三个)中提取“number”属性。

我用来执行此操作的代码如下所示:

/***
     * Locates the Document Number information in the file and returns the form number.
     * @return File's self-declared number.
     * @throws InvalidFormException Thrown when XPath cannot find the "documentnbr" element in the file.
     */
    public String getFormNumber() throws InvalidFormException
    {
        try{
            XPath xPath = XPathFactory.newInstance().newXPath();
            xPath.setNamespaceContext(new XFDLNamespaceContext());

            Node result = (Node)xPath.evaluate(QUERY_FORM_NUMBER, doc, XPathConstants.NODE);
            if(result != null) {
                return result.getNodeValue();
            } else {
                throw new InvalidFormException("Unable to identify form.");
            }

        } catch (XPathExpressionException err) {
            throw new InvalidFormException("Unable to find form number in file.");
        }

    }

其中 QUERY_FORM_NUMBER 是我的 XPath 表达式，XFDLNamespaceContext 实现了 NamespaceContext，看起来像这样:

public class XFDLNamespaceContext 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.PureEdge.com/XFDL/6.5";
        else if ("custom".equals(prefix))
            return "http://www.PureEdge.com/XFDL/Custom";
        else if ("designer".equals(prefix)) 
            return "http://www.PureEdge.com/Designer/6.1";
        else if ("pecs".equals(prefix)) 
            return "http://www.PureEdge.com/PECustomerService";
        else if ("xfdl".equals(prefix))
            return "http://www.PureEdge.com/XFDL/6.5";      
        else if ("xforms".equals(prefix)) 
            return "http://www.w3.org/2003/xforms";
        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;
    }

}

我已经尝试了许多不同的 XPath 查询，但我一直觉得这应该有效:

protected static final String QUERY_FORM_NUMBER = 
        "/globalpage/global/xmlmodel/xforms:instances/instance" + 
        "/form_metadata/title/documentnbr[number]";

不幸的是，它不起作用，我不断得到一个 null 返回值。

我读了很多书 here , here , 和 here ，但事实证明，没有任何证据足以帮助我完成这项工作。

我几乎可以肯定，当我弄清楚这一点时，我将面临困境，但对于我所缺少的东西，我真的束手无策。

感谢您通读所有内容，并提前感谢您的帮助。

-安迪

Answer 1

啊哈，我试着调试你的表达式 + 让它工作。你错过了一些事情。这个 XPath 表达式应该这样做:

/XFDL/globalpage/global/xmlmodel/instances/instance/form_metadata/title/documentnbr/@number

1. 您需要包含根元素(在本例中为 XFDL)
2. 出于某种原因，我并没有最终需要在表达式中使用任何命名空间。不知道为什么。如果是这种情况，则永远不会调用 NamespaceContext.getNamespaceURI()。如果我将 instance 替换为 xforms:instance，则 getNamespaceURI() 将被调用一次，并将 xforms 作为输入参数，但程序会抛出异常。
3. 属性值的语法是@attr，而不是[attr]。

我的完整示例代码:

import java.io.File;
import java.io.IOException;
import java.util.Collections;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;

import javax.xml.XMLConstants;
import javax.xml.namespace.NamespaceContext;
import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.xpath.XPath;
import javax.xml.xpath.XPathConstants;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;

import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.xml.sax.SAXException;

public class XPathNamespaceExample {
    static public class MyNamespaceContext implements NamespaceContext {
        final private Map<String, String> prefixMap;
        MyNamespaceContext(Map<String, String> prefixMap)
        {
            if (prefixMap != null)
            {
                this.prefixMap = Collections.unmodifiableMap(new HashMap<String, String>(prefixMap));
            }
            else
            {
                this.prefixMap = Collections.emptyMap();
            }
        }
        public String getPrefix(String namespaceURI) {
            // TODO Auto-generated method stub
            return null;
        }
        public Iterator getPrefixes(String namespaceURI) {
            // TODO Auto-generated method stub
            return null;
        }
        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.PureEdge.com/XFDL/6.5";
                else if ("custom".equals(prefix))
                    return "http://www.PureEdge.com/XFDL/Custom";
                else if ("designer".equals(prefix)) 
                    return "http://www.PureEdge.com/Designer/6.1";
                else if ("pecs".equals(prefix)) 
                    return "http://www.PureEdge.com/PECustomerService";
                else if ("xfdl".equals(prefix))
                    return "http://www.PureEdge.com/XFDL/6.5";      
                else if ("xforms".equals(prefix)) 
                    return "http://www.w3.org/2003/xforms";
                else    
                    return XMLConstants.NULL_NS_URI;
        }


    }

    protected static final String QUERY_FORM_NUMBER = 
        "/XFDL/globalpage/global/xmlmodel/xforms:instances/instance" + 
        "/form_metadata/title/documentnbr[number]";

    public static void main(String[] args) {
        try
        {
            DocumentBuilderFactory dbfac = DocumentBuilderFactory.newInstance();
            DocumentBuilder docBuilder = dbfac.newDocumentBuilder();
            Document doc = docBuilder.parse(new File(args[0]));
            System.out.println(extractNodeValue(doc, "/XFDL/globalpage/@sid"));
            System.out.println(extractNodeValue(doc, "/XFDL/globalpage/global/xmlmodel/instances/instance/@id" ));
            System.out.println(extractNodeValue(doc, "/XFDL/globalpage/global/xmlmodel/instances/instance/form_metadata/title/documentnbr/@number" ));
        } catch (SAXException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        } catch (ParserConfigurationException e) {
            e.printStackTrace();
        }
    }

    private static String extractNodeValue(Document doc, String expression) {
        try{

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

            Node result = (Node)xPath.evaluate(expression, doc, XPathConstants.NODE);
            if(result != null) {
                return result.getNodeValue();
            } else {
                throw new RuntimeException("can't find expression");
            }

        } catch (XPathExpressionException err) {
            throw new RuntimeException(err);
        }
    }
}