Taken from David's blog.

/*
 * AntRunner.java
 */

import java.io.File;
import org.apache.tools.ant.DefaultLogger;
import org.apache.tools.ant.Project;
import org.apache.tools.ant.ProjectHelper;
import org.apache.tools.ant.taskdefs.Ant;


public class AntRunner {

    public AntRunner() {
    }

    public static void main(String args[]) throws Exception {
        File buildFile = new File("/path/to/build.xml");
        Project p = new Project();
        p.setUserProperty("ant.file", buildFile.getAbsolutePath());
        p.init();
        ProjectHelper helper = ProjectHelper.getProjectHelper();
        p.addReference("ant.projectHelper", helper);
        helper.parse(p, buildFile);
        DefaultLogger consoleLogger = new DefaultLogger();
        consoleLogger.setErrorPrintStream(System.err);
        consoleLogger.setOutputPrintStream(System.out);
        consoleLogger.setMessageOutputLevel(Project.MSG_INFO);
        p.addBuildListener(consoleLogger);
        p.executeTarget(p.getDefaultTarget());
    }
}