Question

My code is too slow, but I’m not sure how to improve it. Reading from disk into DOM for a 1k-file takes about 20 ms, that might be okay depending on the disk, but then I’ve got another 20 ms for working on a xpath statement, which is far too much. Here is some sample code with time comments. How can I improve the code?

This happens at construction time:

    DocumentBuilderFactory dbFactory = DocumentBuilderFactory.newInstance();
DocumentBuilder dBuilder = this.dbFactory.newDocumentBuilder(); 
XPathExpression[] ex = new XPathExpression[]{about 30 different expressions}
XPathExpression mainEx =xPath.compile("/rootElement/firstLevel/secondLevel");

Then the code:

Document doc = this.dBuilder.parse("somefile.xml");
//took 20 ms until here
NodeList nodes = (NodeList) mainEx .evaluate,doc, XPathConstants.NODESET);
 //took another 20 ms until here !!!
    for (int i = 0; i < nodes.getLength(); i++) {
    Node n = nodes.item(i);
    for (XPathExpression e:ex) {
         String v = (String) e.evaluate(n, XPathConstants.STRING);
        if (v != null) {
            System.out.println(v);
        }
    }
    }
    //this only takes 5 ms

Answer 1

You’re probably suffering from this problem that I documented here:

Java XPath (Apache JAXP implementation) performance

Essentially, you should add these JVM arguments to heavily speed up Xalan’s XPath implementation:

-Dorg.apache.xml.dtm.DTMManager=
  org.apache.xml.dtm.ref.DTMManagerDefault

or

-Dcom.sun.org.apache.xml.internal.dtm.DTMManager=
  com.sun.org.apache.xml.internal.dtm.ref.DTMManagerDefault