XPath, XML Namespaces and Java

Question

I've spent the past day attempting to extract a one XML node out of the following document and am unable to grasp the nuances of XML Namespaces to make it work.

The XML file is to large to post in total so here is the portion that concerns me:

<?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>

The document continues and is well formed all the way down. I am attempting to extract the "number" attribute from the "documentnbr" tag (three from the bottom).

The code that I'm using to do this looks like this:

/***
     * 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.");
        }

    }

Where QUERY_FORM_NUMBER is my XPath expression, and XFDLNamespaceContext implements NamespaceContext and looks like this:

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;
    }

}

I've tried many different XPath queries but I keep feeling like this should work:

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

Unfortunately it does not work and I continually get a null return.

I've done a fair amount of reading here, here, and here, but nothing has proved sufficiently illuminating to help me get this working.

I'm almost positive that I'm going to face-palm when I figure this out but I'm really at wit's end as to what I'm missing.

Thank you for reading through all of this and thanks in advance for the help.

-Andy

Answer 1

Aha, I tried to debug your expression + got it to work. You missed a few things. This XPath expression should do it:

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

1. You need to include the root element (XFDL in this case)
2. I didn't end up needing to use any namespaces in the expression for some reason. Not sure why. If this is the case, then the NamespaceContext.getNamespaceURI() never gets called. If I replace instance with xforms:instance then getNamespaceURI() gets called once with xforms as the input argument, but the program throws an exception.
3. The syntax for attribute values is @attr, not [attr].

My complete sample code:

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);
        }
    }
}

Answer 2

SAX (alternative to XPath) version:

SAXParser saxParser = SAXParserFactory.newInstance().newSAXParser();
final String[] number = new String[1];
DefaultHandler handler = new DefaultHandler()
{           
    @Override
    public void startElement(String uri, String localName, String qName,
    Attributes attributes) throws SAXException
    {
        if (qName.equals("documentnbr"))
            number[0] = attributes.getValue("number");
    }
};
saxParser.parse("input.xml", handler);
System.out.println(number[0]);

I see it's more complicated to use XPath with namespaces as it should be (my opinion). Here is my (simple) code:

XPath xpath = XPathFactory.newInstance().newXPath();

NamespaceContextMap contextMap = new NamespaceContextMap();
contextMap.put("custom", "http://www.PureEdge.com/XFDL/Custom");
contextMap.put("designer", "http://www.PureEdge.com/Designer/6.1");
contextMap.put("pecs", "http://www.PureEdge.com/PECustomerService");
contextMap.put("xfdl", "http://www.PureEdge.com/XFDL/6.5");
contextMap.put("xforms", "http://www.w3.org/2003/xforms");
contextMap.put("", "http://www.PureEdge.com/XFDL/6.5");

xpath.setNamespaceContext(contextMap);
String expression = "//:documentnbr/@number";
InputSource inputSource = new InputSource("input.xml");
String number;
number = (String) xpath.evaluate(expression, inputSource, XPathConstants.STRING);
System.out.println(number);

You can get NamespaceContextMap class (not mine) from here (GPL license). There is also 6376058 bug.

Answer 3

Have a look at the XPathAPI library. It is a simpler way to use XPath without messing with the low-level Java API, especially when dealing with namespaces.

The code to get the number attribute would be:

String num = XPathAPI.selectSingleNodeAsString(doc, '//documentnbr/@number');

Namespaces are automatically extracted from the root node (doc in this case). In case you need to explicitly define additional namespaces you can use this:

Map<String, String> nsMap = new HashMap<String, String>();
nsMap.put("xforms", "http://www.w3.org/2003/xforms");

String num =
    XPathAPI.selectSingleNodeAsString(doc, '//documentnbr/@number', nsMap);

(Disclaimer: I'm the author of the library.)