標準のAPIを使い続けたいと仮定して...
DOMImplementationLSを使用できます:
Document document = node.getOwnerDocument();
DOMImplementationLS domImplLS = (DOMImplementationLS) document
.getImplementation();
LSSerializer serializer = domImplLS.createLSSerializer();
String str = serializer.writeToString(node);
<?xml version = "1.0" encoding = "UTF-16"?>宣言が気になる場合は、代わりにトランスフォーマーを使用できます。
TransformerFactory transFactory = TransformerFactory.newInstance();
Transformer transformer = transFactory.newTransformer();
StringWriter buffer = new StringWriter();
transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "yes");
transformer.transform(new DOMSource(node),
new StreamResult(buffer));
String str = buffer.toString();