DOCTYPE宣言がないXMLを外部DTDを使用してparseする方法

import java.io.*;
import org.apache.xerces.parsers.*;
import org.apache.xml.serialize.*;
import org.w3c.dom.*;
import org.xml.sax.InputSource;
import javax.xml.parsers.*;
import javax.xml.transform.*;
import javax.xml.transform.dom.*;
import javax.xml.transform.stream.*;

public class CombineTest {

	public static void main(String[] args) throws Exception {
		// args[0] is XML filename
		// args[1] is DTD filename
		OutputStream out = processXerces(args);
//		OutputStream out = processJAXP(args);
		System.out.println(out.toString());
		
		DOMParser parser = new DOMParser();
		parser.setFeature("http://xml.org/sax/features/validation", true);
		parser.parse(
			new InputSource(
				new ByteArrayInputStream(((ByteArrayOutputStream) out).toByteArray())
			)
		);
	}

	/** Xerces version */
	private static OutputStream processXerces(String[] args) throws Exception {
		DOMParser parser = new DOMParser();
		parser.parse(args[0]);
		Document srcXML = parser.getDocument();

		DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
		DocumentBuilder db = dbf.newDocumentBuilder();
		DOMImplementation di = db.getDOMImplementation();
		DocumentType dt
			= di.createDocumentType(
				srcXML.getDocumentElement().getNodeName(),
				null,
				args[1]
			);
		
		Document combined = XMLDocumentFactory.combine(srcXML, dt);
		OutputFormat of = new OutputFormat(combined);
		of.setEncoding("UTF-8");
		of.setStandalone(false);
		of.setIndenting(true);
		
		OutputStream out = new ByteArrayOutputStream();
		XMLSerializer serializer = new XMLSerializer(out, of);
		serializer.asDOMSerializer();
		serializer.serialize(combined.getDocumentElement());
		
		return out;
	}

	/** JAXP version */
	private static OutputStream processJAXP(String[] args) throws Exception {
		DOMResult domresult = new DOMResult();
		TransformerFactory tf = TransformerFactory.newInstance();
		Transformer transformer = tf.newTransformer();
		transformer.transform(new StreamSource(new FileInputStream(args[0])), domresult);
		Document srcXML = (Document) domresult.getNode();

		DocumentBuilder db = DocumentBuilderFactory.newInstance().newDocumentBuilder();
		DOMImplementation di = db.getDOMImplementation();
		DocumentType dt
			= di.createDocumentType(
				srcXML.getDocumentElement().getNodeName(),
				null,
				args[1]
			);
		
		Document combined = XMLDocumentFactory.combine(srcXML, dt);

		transformer.setOutputProperty("encoding", "UTF-8");
		transformer.setOutputProperty("standalone", "no");
		transformer.setOutputProperty("indent", "yes");

		OutputStream out = new ByteArrayOutputStream();
		transformer.transform(new DOMSource(combined), new StreamResult(out));
		
		return out;
	}

	/** XML and DTD are combined */
	public static Document combine(Document doc, DocumentType docType)
			throws ParserConfigurationException {
		DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
		DocumentBuilder db = dbf.newDocumentBuilder();
		DOMImplementation di = db.getDOMImplementation();
		Document combined
			= di.createDocument(
				"",
				doc.getDocumentElement().getNodeName(),
				docType
			);
		
		NodeList nodes = doc.getDocumentElement().getChildNodes();
		for (int i = 0; i < nodes.getLength(); ++i) {
			Node node = combined.importNode(nodes.item(i), true);
			combined.getDocumentElement().appendChild(node);
		}
		
		return combined;
	}
}