怎么样把xml Document对象转换成字符串

public String doc2String(Document doc) {
String str = null;

try{
TransformerFactory factory = TransformerFactory.newInstance();
Transformer trans = factory.newTransformer();
trans.setOutputProperty(OutputKeys.ENCODING, "gb2312"); // 处理汉字编码

ByteArrayOutputStream byteOut = new ByteArrayOutputStream();
StreamResult strOut = new StreamResult(byteOut);

Source xmlSource = new DOMSource(doc);

trans.transform(xmlSource, strOut);

str = strOut.toString();
}catch(Exception e){
e.printStackTrace();
}

return str;
}