Transformación simple: XML + XSL = XML transformado
import javax.xml.transform.*;
import javax.xml.transform.stream.*;

public class SimpleTransform {
    public static void main(String[] args) throws Exception {
        TransformerFactory tFactory = TransformerFactory.newInstance();
        Transformer transformer = tFactory.newTransformer(new StreamSource("birds.xsl"));

        transformer.transform(new StreamSource("birds.xml"), new StreamResult(System.out));
    }
}
Selección en el fichero XML de la hoja de estilos XSL como un "Processing Instruction"
import javax.xml.transform.*;
import javax.xml.transform.stream.*;

public class UseStylesheetPI {
    public static void main(String[] args) throws Exception {
        String media = null , title = null, charset = null;

        TransformerFactory tFactory = TransformerFactory.newInstance();
        Source stylesheet = tFactory.getAssociatedStylesheet(new StreamSource("birds.xml"), media, title, charset);

        Transformer transformer = tFactory.newTransformer(stylesheet);
        transformer.transform(new StreamSource("birds.xml"), new StreamResult(System.out));
  }
}