XPath and namespaces

28 Jul, 2012 · by admin · Read in about 2 min · (347 Words)
java xml xpath

Let’s consider the following XML

<?xml version="1.0" encoding="UTF-8"?>
<notes>
   <note>
      <heading>Remember the milk</heading>
      <body>Don't forget me this weekend!</body>
   </note>
   <note>
      <heading>Call Anna</heading>
      <body>Don't forget me this weekend!</body>
   </note>
   <note>
      <heading>Feed the trolls</heading>
      <body>Don't forget me this weekend!</body>
   </note>
   <note>
      <heading>Hooray!</heading>
      <body>Don't forget me this weekend!</body>
   </note>
</notes>

And the following Java program:

NodeList notes = (NodeList) xpath.evaluate("//note/heading/text()",
    builder.parse(ColladaXPath.class.getResourceAsStream("notes.xml")),
    XPathConstants.NODESET);

for (int i = 0; i < notes.getLength(); i++) {
    System.out.println(notes.item(i).getNodeValue());
}

Everything works fine until one adds the infamous xmlns attribute to the root element. At this point, XPath won’t be able to find anything without a prefix, so we’ll have to change the XPath expression to something like

//n:note/n:heading/text()

Note that the n: prefix does not need to appear in the source XML. It is used on the XPath side and mapped to a xmlns with a subclass of javax.xml.namespace.NamespaceContext:

class NotesNamespaceCtx implements NamespaceContext {
    public String getNamespaceURI(String prefix) {
        return "foobarbaz";
    }

    public String getPrefix(String namespaceURI) {
        return "n";
    }

    public Iterator<?> getPrefixes(String namespaceURI) {
        return null;
    }
}

The String returned by getNamespaceURI() must be the value of xmlnls. In this case it’s xmlns="foobarbaz". Here is a full implementation (it needs Guava to be compiled and run)

package com.zybnet;

import java.util.Iterator;

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.XPathFactory;

import org.w3c.dom.NodeList;

import com.google.common.collect.BiMap;
import com.google.common.collect.HashBiMap;

public class ColladaXPath {

    public static void main(String[] args) throws Exception {
        NodeList notes = (NodeList) xpath.evaluate("//n:note/n:heading/text()",
                builder.parse(ColladaXPath.class.getResourceAsStream("notes.xml")),
                XPathConstants.NODESET);

        for (int i = 0; i < notes.getLength(); i++) {
            System.out.println(notes.item(i).getNodeValue());
        }

    }

    static XPath xpath;
    static DocumentBuilder builder;

    static {
        DocumentBuilderFactory domFactory = DocumentBuilderFactory.newInstance();
        domFactory.setNamespaceAware(true); // !IMPORTANT

        try {
            builder = domFactory.newDocumentBuilder();
        } catch (ParserConfigurationException e) {
            e.printStackTrace();
        }

        xpath = XPathFactory.newInstance().newXPath();
        NamespaceCtxImpl ns = new NamespaceCtxImpl();
        ns.register("n", "foobarbaz");
        xpath.setNamespaceContext(ns);
    }

    private static class NamespaceCtxImpl implements NamespaceContext {
        private BiMap<String, String> xmlns = HashBiMap.create();

        public void register(String prefix, String uri) {
            xmlns.put(prefix, uri);
        }

        @Override
        public String getNamespaceURI(String prefix) {
            return xmlns.get(prefix);
        }

        @Override
        public String getPrefix(String namespaceURI) {
            return xmlns.inverse().get(namespaceURI);
        }

        @Override
        public Iterator<String> getPrefixes(String namespaceURI) {
            return xmlns.keySet().iterator();
        }
    }
}