import org.w3c.dom.bootstrap.DOMImplementationRegistry;

import org.w3c.dom.ls.*;

final DOMImplementationRegistry registry = DOMImplementationRegistry.newInstance();

final DOMImplementationLS impl = (DOMImplementationLS) registry.getDOMImplementation("LS");

final LSSerializer writer = impl.createLSSerializer();

writer.getDomConfig().setParameter("format-pretty-print", Boolean.TRUE);

writer.getDomConfig().setParameter("xml-declaration", true);

String s = writer.writeToString(doc);