Print empty XML element as opening tag, closing tag

Question

Is there any way to print empty elements like <tag></tag> rather than <tag /> using org.w3c.dom? I'm modifying XML files that need to be diff'ed against old versions of themselves for review.

If it helps, the code that writes the XML to the file:

TransformerFactory t = TransformerFactory.newInstance();
Transformer transformer = t.newTransformer();

DOMSource source = new DOMSource(doc);
StringWriter xml = new StringWriter();
StreamResult result = new StreamResult(xml);
transformer.transform(source, result);

File f = new File("output.xml");
FileWriter writer = new FileWriter(f);
BufferedWriter out = new BufferedWriter(writer);
out.write(xml.toString());
out.close();

Thanks.

Answer 1

I'm assuming the empty elements are actually ELEMENT_NODEs with no children within the document. Try adding an empty text node to them instead. That may trick the writer into believing there is a text node there, so it will write it out as if there was one. But the text node won't output anything because it is an empty string.

Calling this method with the document as both parameters should do the trick:

private static void fillEmptyElementsWithEmptyTextNodes(
    final Document doc, final Node root)
{
    final NodeList children = root.getChildNodes();
    if (root.getType() == Node.ELEMENT_NODE &&
        children.getLength() == 0)
    {
        root.appendChild(doc.createTextNode(""));
    }

    // Recurse to children.
    for(int i = 0; i < children.getLength(); ++i)
    {
        final Node child = children.item(i);
        fillEmptyElementsWithEmptyTextNodes(doc, child);
    }
}

Answer 2

You may want to consider converting both the old and the new XML file to Canonical XML - http://en.wikipedia.org/wiki/Canonical_XML - before comparing them with e.g. diff.

James Clark has a small Java program to do so on http://www.jclark.com/xml/