What's the best way to retrieve two pieces of data from an XML file?

Question

I've got an XML document that is in either a pre or post FO transformed state that I need to extract some information from. In the pre-case, I need to pull out two tags that represent the pageWidth and pageHeight and in the post case I need to extract the page-height and page-width parameters from a specific tag (I forget which one it is off the top of my head).

What I'm looking for is an efficient/easily maintainable way to grab these two elements. I'd like to only read the document a single time fetching the two things I need.

I initially started writing something that would use BufferedReader + FileReader, but then I'm doing string searching and it gets messy when the tags span multiple lines. I then looked at the DOMParser, which seems like it would be ideal, but I don't want to have to read the entire file into memory if I could help it as the files could potentially be large and the tags I'm looking for will nearly always be close to the top of the file. I then looked into SAXParser, but that seems like a big pile of complicated overkill for what I'm trying to accomplish.

Anybody have any advice? Or simple implementations that would accomplish my goal? Thanks.

Edit: I forgot to mention that due to various limitations I have, whatever I use has to be "builtin" to core Java, in which I can't use and/or download any 3rd party XML tools.

Answer 1

You can use XPath to search for your tags. Here is a tutorial on forming XPath expressions. And here is an article on using XPath with Java.

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An easy to use parser (dom, sax) is dom4j. It would be quite easier to use than the built-in SAXParser.

Answer 2

While XPath is very good for querying XML data, I am not aware of good and fast XPath implementation for Java (they all use DOM model at least).

I would recommend you to stick with StAX. It is extremely fast even for huge files, and it's cursor API is rather trivial:

XMLInputFactory f = XMLInputFactory.newInstance();
XMLStreamReader r = f.createXMLStreamReader("my.xml");
try {
  while (r.hasNext()) {
    r.next();
    . . .
  }
} finally {
  r.close()
}

Consult StAX tutorial and XMLStreamReader javadocs for more information.

Answer 3

try "XMLDog"

This uses sax to evaluate xpaths