Issues with xpath that contained namespaces in Jav

Question

I'm trying to parse an rdfs xml file in order to find all the Classes in an rdfs file.

The xpath: "/rdf:RDF/rdfs:Class" is working in my XML editor. When i insert the xpath in my Java program (i have implemented a dom parser), i get 0 Classes. The following example runs but it outputs 0 classes!

I do:

import java.io.IOException;
import javax.xml.parsers.ParserConfigurationException;
import javax.xml.xpath.XPathExpressionException;
import org.xml.sax.SAXException;

public class Main {
public static void main(String args[]) throws XPathExpressionException, ParserConfigurationException, SAXException, IOException{

    FindClasses FSB = new FindClasses();
    FSB.FindAllClasses("C:\\Workspace\\file.xml"); //rdfs file
     }
}

The class FindClasses is as follows:

import java.io.IOException;
import java.util.Collection;
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.XPathExpression;
import javax.xml.xpath.XPathExpressionException;
import javax.xml.xpath.XPathFactory;
import org.w3c.dom.Document;
import org.w3c.dom.Node;
import org.w3c.dom.NodeList;
import org.xml.sax.SAXException;

public class FindClasses {

public void FindAllClasses(String fileName) throws XPathExpressionException, ParserConfigurationException, SAXException, IOException {


    DocumentBuilderFactory domFactory = DocumentBuilderFactory.newInstance();
    domFactory.setNamespaceAware(true); 
    DocumentBuilder builder = domFactory.newDocumentBuilder();
    Document doc = builder.parse(fileName);

    XPath xpath = XPathFactory.newInstance().newXPath();
    XPathExpression classes_expr = xpath.compile("/rdf:RDF/rdfs:Class");

    Object result = classes_expr.evaluate(doc, XPathConstants.NODESET);
    NodeList classes = (NodeList) result;

    System.out.println("I found : " + classes.getLength() + " classes " );

        }
    }

The rdfs file is:

<?xml version="1.0" encoding="UTF-8"?>
<rdf:RDF xml:lang="en" xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:rdfs="http://www.w3.org/2000/01/rdf-schema#">
        <rdfs:Class rdf:about="Class1">
        </rdfs:Class>
        <rdfs:Class rdf:about="Class2">
        </rdfs:Class>
</rdf:RDF>  

I don't really understand why the xpath returns 0 nodes in that example. It's weird, cause i have implemented other dom parsers as well and they were working fine. Can somebody help me?

Thanks

Answer 1

I visited the following link and i solved my problem: Issues with xpath in java

The problem was that the xpath contained two namespaces (rdf,rdfs) like "/rdf:RDF/rdfs:Class". If the xpath didn't contain any namespace e.g. /RDF/Class , there was not going to be an issue.

So after the line:

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

and before the line:

XPathExpression classes_expr = xpath.compile("/rdf:RDF/rdfs:Class");    

I added the following:

xpath.setNamespaceContext(new NamespaceContext() {
    public String getNamespaceURI(String prefix) {
      switch (prefix) {
        case "rdf": return "http://www.w3.org/1999/02/22-rdf-syntax-ns#";
        case "rdfs" : return "http://www.w3.org/2000/01/rdf-schema#"; 
       }
    return prefix;

    }
    public String getPrefix(String namespace) {
        if (namespace.equals("rdf")) return "rdf";
        else if (namespace.equals("rdfs")) return "rdfs";
        else return null;
       }
    @Override
    public Iterator getPrefixes(String arg0) {
        // TODO Auto-generated method stub
        return null;
    }
    });