Using XPath to pick data out of XML

This week I wrote a WattDepot sensor for the TED 5000 home energy meter. The TED 5000 gateway (a small Internet-connected embedded computer) provides a URI that generates XML showing the current power data. First, I needed to figure out what the XML meant. Once that was done, I wanted a quick and simple way to pick out the 2 pieces of data from the XML that I care about using Java.

WattDepot uses JAXB extensively for XML processing, but that was kinda heavyweight for my needs here. I had heard about XPath, and it sounded like the right type of tool for just grabbing a little data from XML. Turns out that Java 1.5 and later have XPath built-in, so there’s no additional dependencies.

IBM has a good tutorial on using XPath from Java by Elliotte Rusty Harold. Unfortunately, I was confused initially because all the XPath examples in the tutorial are for finding all XML nodes in a document that meet certain criteria, whereas I knew exactly where in the XML tree my data was lurking. Luckily, it turns out that XPath is really a lot like a path in a filesystem (duh), so traversing the tree is easy.

Say you have the following XML from TED (some parts elided):


The XPath that would pull out the value from PowerNow is /LiveData/Power/Total/PowerNow/text(), and for PowerMTD it is /LiveData/Power/Total/PowerMTD/text(). Simple!

Here a code fragment that extracts those two values from an XML file (stealing liberally from the XPath tutorial linked above):

public class XPathTest {

  public static void main(String[] args) throws ParserConfigurationException, SAXException,
      IOException, XPathExpressionException {
    if (args.length != 1) {
      System.out.println("Need XML filename arg.");
    DocumentBuilderFactory domFactory = DocumentBuilderFactory.newInstance();
    DocumentBuilder builder = domFactory.newDocumentBuilder();
    Document doc = builder.parse(args[0]);

    XPathFactory factory = XPathFactory.newInstance();
    XPath powerXpath = factory.newXPath();
    XPath energyXpath = factory.newXPath();
    XPathExpression exprPower = powerXpath.compile("/LiveData/Power/Total/PowerNow/text()");
    XPathExpression exprEnergy = energyXpath.compile("/LiveData/Power/Total/PowerMTD/text()");
    Object powerResult = exprPower.evaluate(doc, XPathConstants.NUMBER);
    Object energyResult = exprEnergy.evaluate(doc, XPathConstants.NUMBER);

    Double power = (Double) powerResult;
    Double energy = (Double) energyResult;
    System.out.println("Power from TED 5K: " + power + "W");
    System.out.println("Energy from TED 5K month to date: " + energy + "Wh");

It’s nice to have a quick and easy way to make use of XML from Java in my toolbox.


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