Java evaluate XPath against a org.w3c.dom.Node

Question

Dipping my toe in a little Java at the minute and have a question about XPath.

I have a large Xml and I want to use XPath to be able to grab a specific node and then fire further XPath calls against this small chunk of Xml.

Here s rough outline of my Xml:

<Page>
  <ComponentPresentations>
    <ComponentPresentation>
      <Component>
        <Title>
      <ComponentTemplate> 
    <ComponentPresentation>
      <Component>
        <Title>
      <ComponentTemplate>

My first XPath selects the <Component> node based upon the value of a <ComponenTemplate> Id value:

String componentExpFormat = "/Page/ComponentPresentations/ComponentPresentation/ComponentTemplate/Id[text()='%1$s']/ancestor::ComponentPresentation";
String componentExp = String.format(componentExpFormat, template);
XPathExpression expComponent = xPath.compile(componentExp);
Node componentXml = (Node) expComponent.evaluate(xmldoc, XPathConstants.NODE);

This gives me the <Component> I want but I can;t seem to be able to then XPath against the Node:

String componentExpTitle = "/Component/Fields/item/value/Field/Name[text()='title']/parent::node()/Values/string";                                  
XPathExpression expTitle = xPath.compile(componentExpTitle);
String eventName = expTitle.evaluate(componentXml, XPathConstants.STRING).toString();

Without this I'll have to include the full XPath each time:

/Page/ComponentPresentations/ComponentPresentation/ComponentTemplate/Id[text()='%1$s']/ancestor::ComponentPresentation/Component/Fields/item/value/Field/Name[text()='title']/parent::node()/Values/string

Is that the only way?

Cheers

Your componentExpTitle is an absolute path, remove the leading slash and try it. — parsifal
– parsifal, Commented May 13, 2013 at 16:42

Ian Roberts · Accepted Answer · 2013-05-13 16:52:47Z

3

An XPath expression with a leading slash

/Component/Fields/item

is absolute, and when you evaluate it with a particular context node it will start looking from the root of the document that the context node belongs to. If you remove the leading slash

Component/Fields/item

it will look for Component children of the context node.

As an aside, you can simplify those XPaths quite a bit, you don't need all the up and down the tree stuff with ancestor::, and you also don't need to use text():

componentExpFormat = "/Page/ComponentPresentations/ComponentPresentation[ComponentTemplate/Id='%1$s']";
componentExpTitle = "Component/Fields/item/value/Field[Name='title']/Values/string";

answered May 13, 2013 at 16:52

Ian Roberts

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1 Comment

Neil Over a year ago

Ian, if SO would let me vote more than once I would because you my friend have made the next couple of hours waaaay easier for me - and I've improved my XPath along the way! Cheers!

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