Jakarta Commons Cookbook

12.4. Applying XPath Queries to Complex Object Graphs

Problem

You need to use XPath expressions to select objects from a complex object graph referencing the contents of a Map and using expressions with variable references.

Solution

Use Jakarta Commons JXPath to select objects from a collection using XPath queries. The following example uses Commons Digester to parse an XML file into an object graph, and selects Planet objects with a radius greater than 5000:

import org.apache.commons.digester.Digester;
import org.apache.commons.digester.xmlrules.DigesterLoader;
import org.apache.commons.jxpath.JXPathContext;

List planets = new ArrayList( );

// Parse Planet XML into a List of Planet objects
InputStream input = getClass( ).getResourceAsStream("./planets.xml");
URL rules = getClass( ).getResource("./planet-digester-rules.xml");
Digester digester = DigesterLoader.createDigester(rules);
digester.push(planets);
digester.parse( input );

// Select all planets with a radius greater than 5000
System.out.println( "Planet Name where radius > 5000");
JXPathContext context = JXPathContext.newContext( planets );
Iterator iterator = context.iterate(".[@radius > 5000]/name");
while( iterator.hasNext( ) ) {
    Object o  = (Object) iterator.next( );
    System.out.println( "Object: " + o);
}

The Planet objects are filtered and the names of planets with sufficient radii are printed to the console:

Planet Name where radius > 5000
Object: Venus
Object: Saturn

This object graph was created from an XML document ...