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
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 eg /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;
}
});
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