Line Number of a DOM Element

Question

Hi There,
I'm trying to find the originating xml file line number of a DOM element (for purposes of error reporting).
It seems that you can somehow use the DOMLocator class, however I can't seem to find any explanation of how to do this.
Please could anyone tell me how?
Thanks a lot,
Chris.

Answer 1

What you're looking to do is validation, and there are multiple ways to do this, already supported by the parsers that come with the JDK. The downside is that writing a grammar that correctly describes your expected XML is not usually trivial.
There are three common forms of validation: DTD, XSD, and (arguably not common) Relax-NG.
DTD is the oldest, and is part of the [XML Specification|http://www.xml.com/axml/testaxml.htm]. It allows you to specify the ordering of elements, and the legal attributes for those elements, but not much more.
XSD, correctly known as [XML Schema|http://www.w3.org/TR/xmlschema-0/], is the W3C-approved validation grammar. The link I gave is to the first of three documents in its spec, and XSD grammars tend to be huge (I know one person who wrote one that was 60 pages long). The benefits of XSD (over DTD) is that it lets you specify min/max counts for elements, union-style groups, and also lets you validate (at some level) the textual content of both elements and attributes.
[Relax NG|http://relaxng.org/] is meant as a simpler alternative to XSD that retains a similar level of functionality. I've never used it, and haven't had any call to learn it in a professional environment, so can't say much more.
If you can wait a few weeks, I'm going to put a long article on XML parsing on my website :-) however, assuming you can't wait, here's a chunk of code that shows how you'd do a validating parse with XSD:
public static void basicSchemaExample()
throws Exception
    SchemaFactory xsFact = SchemaFactory.newInstance(XMLConstants.W3C_XML_SCHEMA_NS_URI);
    Schema xsd = xsFact.newSchema(new StreamSource(new StringReader(xsd)));
    DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
    dbf.setSchema(xsd);
    DocumentBuilder db = dbf.newDocumentBuilder();
    Document dom = db.parse(new InputSource(new StringReader(xml)));
}