Java HTML/XML - 如何添加元素到存儲在org.w3c.dom.Document中的根元素

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.OutputStreamWriter;
import java.io.Writer;

import javax.xml.parsers.DocumentBuilder;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.transform.Transformer;
import javax.xml.transform.TransformerFactory;
import javax.xml.transform.dom.DOMSource;
import javax.xml.transform.stream.StreamResult;

import org.w3c.dom.Document;
import org.w3c.dom.Element;
import org.w3c.dom.Node;

public class Main {

  public static void main(String[] args) throws Exception {
    InputStream is = new FileInputStream("test.xml");
    DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
    DocumentBuilder builder = factory.newDocumentBuilder();

    Document oldDoc = builder.parse(is);
    Node oldRoot = oldDoc.getDocumentElement();
    Document newDoc = builder.newDocument();
    Element newRoot = newDoc.createElement("newroot");
    newDoc.appendChild(newRoot);
    newRoot.appendChild(newDoc.importNode(oldRoot, true));

    ByteArrayOutputStream out = new ByteArrayOutputStream();
    DOMSource domSource = new DOMSource(newDoc);
    Writer writer = new OutputStreamWriter(out);
    StreamResult result = new StreamResult(writer);
    TransformerFactory tf = TransformerFactory.newInstance();
    Transformer transformer = tf.newTransformer();
    transformer.transform(domSource, result);
    writer.flush();

    InputStream isNewXML = new ByteArrayInputStream(out.toByteArray());
  }
}