Document的setXmlStandalone()方法不生效

public void test() throws ParserConfigurationException {
  DocumentBuilderFactory docFactory = DocumentBuilderFactory.newInstance();
  DocumentBuilder docBuilder = docFactory.newDocumentBuilder();
  Document doc = docBuilder.newDocument();
  doc.setXmlStandalone(true);  //无效

  Element request = doc.createElement("request");
  Element sn = doc.createElement("sn");
  Element user = doc.createElement("user");
  Element passwd = doc.createElement("passwd");
  Element args = doc.createElement("args");
  Element arg1 = doc.createElement("arg");
  arg1.setAttribute("name", "arg1");
  Element arg2 = doc.createElement("arg");
  arg2.setAttribute("name", "arg2");
  Element arg3 = doc.createElement("arg");
  arg3.setAttribute("name", "arg3");

  sn.setTextContent("201601011234567890");
  user.setTextContent("XXXX");
  passwd.setTextContent("XXXX");
  arg1.setTextContent("Y");
  arg2.setTextContent("YY");
  arg3.setTextContent("YYY");

  doc.appendChild(request);
  request.appendChild(sn);
  request.appendChild(user);
  request.appendChild(passwd);
  request.appendChild(args);
  args.appendChild(arg1);
  args.appendChild(arg2);
  args.appendChild(arg3);

  System.out.println(XmlTools.docToStr(doc, true));
}

输出结果

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<request>
<sn>201601011234567890</sn>
<user>XXXX</user>
<passwd>XXXX</passwd>
<args>
<arg name="arg1">Y</arg>
<arg name="arg2">YY</arg>
<arg name="arg3">YYY</arg>
</args>
</request>

我是想要standalone="yes"的，但是用doc.setXmlStandalone(true)方法行不通