String xmlStr = "......";
StringReader sr = newStringReader(xmlStr);
InputSource is = newInputSource(sr);
DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
DocumentBuilder builder=factory.newDocumentBuilder();
Document doc = builder.parse(is);

TransformerFactory tf = TransformerFactory.newInstance();
Transformer t = tf.newTransformer();
t.setOutputProperty( "encoding","GB23121"); //解决中文问题，试过用GBK不行
ByteArrayOutputStream bos = newByteArrayOutputStream();
t.transform( newDOMSource(doc),newStreamResult(bos));
String xmlStr = bos.toString();

String xmlStr = "......";
Document document = DocumentHelper.parseText(xmlStr);