Is it possible parameterize a compiled Java XPath expression ala PreparedStatement style semantics?

Question

Is it possible either with the stock JAX-P xpath expression engine or another compliant engine to compile an xpath expression that would allow for parametrization?

I'm looking to see if is if there is an API that would allow a developer to set placeholders within a compiled xpath and replace those values at run time.

Any insight on this as to whether or not it is possible and if there are caveats, pitfalls, or just plain "don't do that" type advice would be greatly appreciated.

(Note I corrected a "crossing of the streams" ... was having a conversation with a coworker with regard to xpath & regular expressions ... just happened to get mentally tongue tied ... sorry for the confusion)

Answer 1

Here's a very simple implementation of javax.xml.xpath.XPathVariableResolver

import java.util.HashMap;
import java.util.Map;
import javax.xml.namespace.QName;
import javax.xml.xpath.XPathVariableResolver;

public class SimpleVariableResolver implements XPathVariableResolver {

    private static final Map<QName, Object> vars = 
        new HashMap<QName, Object>();

    public void addVariable(QName name, Object value) {
        vars.put(name, value);
    }

    public Object resolveVariable(QName name) {
        return vars.get(name);
    }

}

Use it like this:

public static void main(String[] args) throws ParserConfigurationException,
        SAXException, IOException, XPathExpressionException {

    DocumentBuilderFactory domFactory = DocumentBuilderFactory.newInstance();
    DocumentBuilder builder = domFactory.newDocumentBuilder();
    Document doc = builder.parse("workbook.xml");

    XPath xpath = XPathFactory.newInstance().newXPath();
    SimpleVariableResolver resolver = new SimpleVariableResolver();
    resolver.addVariable(new QName(null, "id"), 2);
    xpath.setXPathVariableResolver(resolver);
    XPathExpression expr = xpath.compile("/root/element[@id=$id]");
    Object result = expr.evaluate(doc, XPathConstants.NODESET);

    NodeList nodes = (NodeList) result;
    for (int i = 0; i < nodes.getLength(); i++) {
        System.out.println(nodes.item(i).getTextContent());
    }
}

On this document:

<root>
    <element id="1">one</element>
    <element id="2">two</element>
    <element id="3">three</element>
</root>

Output:

two