I have an Document object that is modeling a XML like this one:

<RootNode xmlns="http://a.com/a" xmlns:b="http://b.com/b">
    <Child />
</RootNode>

Using Java DOM, I need to get the <Child> node and serialize it to XML, but keeping the root node namespaces. This is what I currently have, but it does not serialize the namespaces:

public static void main(String[] args) throws Exception {
    String xml = "<RootNode xmlns='http://a.com/a' xmlns:b='http://b.com/b'><Child /></RootNode>";

    DocumentBuilder builder = DocumentBuilderFactory.newInstance().newDocumentBuilder();
    Document doc = builder.parse(new ByteArrayInputStream(xml.getBytes()));
    Node childNode = doc.getFirstChild().getFirstChild();

    // serialize to string
    StringWriter sw = new StringWriter();
    DOMSource domSource = new DOMSource(childNode);
    StreamResult streamResult = new StreamResult(sw);
    TransformerFactory tf = TransformerFactory.newInstance();
    Transformer serializer = tf.newTransformer();
    serializer.transform(domSource, streamResult);
    String serializedXML = sw.toString();

    System.out.println(serializedXML);
}

Current output:

<?xml version="1.0" encoding="UTF-8"?>
<Child/>

Expected output:

<?xml version="1.0" encoding="UTF-8"?>
<Child xmlns='http://a.com/a' xmlns:b='http://b.com/b' />