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());
	}
}