xml find attribute ændre værdi og gem

Jeg ville laese XML ind i et W3C DOM document, finde attributten med XPath, aendre den og gemme dokumentet igen.

Eksempel:

package may;

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;

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.DOMImplementation;
import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.bootstrap.DOMImplementationRegistry;
import org.w3c.dom.ls.DOMImplementationLS;
import org.w3c.dom.ls.LSOutput;
import org.w3c.dom.ls.LSSerializer;
import org.xml.sax.SAXException;

public class XmlMod {
    public static void createFile(String fnm) throws IOException {
        PrintWriter pw = new PrintWriter(new FileWriter(fnm));
        pw.println("<?xml version=\"1.0\" encoding=\"UTF-8\"?>");
        pw.println("<data>");
        pw.println("    <x a=\"177\"/>");
        pw.println("</data>");
        pw.close();
    }
    public static void displayFile(String fnm) throws IOException {
        BufferedReader br = new BufferedReader(new FileReader(fnm));
        String line;
        while((line = br.readLine()) != null) {
            System.out.println(line);
        }
        br.close();
    }
    private static void changeFile(String oldfnm, int oldattr, String newfnm, int newattr) throws ParserConfigurationException, SAXException, IOException, ClassCastException, ClassNotFoundException, InstantiationException, IllegalAccessException, XPathExpressionException {
        // read
        DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
        DocumentBuilder db = dbf.newDocumentBuilder();
        Document doc = db.parse(oldfnm);
        // change
        XPath xpath = XPathFactory.newInstance().newXPath();
        Element x = (Element)xpath.evaluate("/data/x[@a=" + oldattr + "]", doc.getDocumentElement(), XPathConstants.NODE);
        x.setAttribute("a", Integer.toString(newattr));
        // write
        DOMImplementation impl = DOMImplementationRegistry.newInstance().getDOMImplementation("XML 3.0");
        DOMImplementationLS feature = (DOMImplementationLS)impl.getFeature("LS","3.0");
        LSSerializer ser = feature.createLSSerializer();
        LSOutput output = feature.createLSOutput();
        output.setCharacterStream(new PrintWriter(newfnm));
        ser.write(doc, output);


    }
    public static void main(String[] args) throws Exception {
        createFile("C:\\bef.xml");
        displayFile("C:\\bef.xml");
        changeFile("C:\\bef.xml", 177, "C:\\aft.xml", 299);
        displayFile("C:\\aft.xml");
    }
}

OK ?