Adding linebreak in xml file before root node

domparser, java, line-breaks, xml

Solution

You basically want a text node containing a line break after the comment node.

Element docElem = doc.getDocumentElement();

doc.insertBefore(doc.createComment("DO NOT EDIT THIS FILE"), docElem);
doc.insertBefore(doc.createTextNode("\\n"), docElem);

EDIT: It seems that appending even whitespace-only text nodes is not allowed at the root node of an `org.w3c.dom.Document`. This is 100% formally correct, but also unhelpful.

The way comments are rendered in the output of the `Transformer` is determined by the serializer it uses (there are different serializers for HTML, XML and plain text outputs). In the built-in XML serializer the end of a comment is defined as `-->` - without a newline.

Since the internals of `javax.xml.transform.Transformer` are hard-wired, the serializers are not public API and the class is marked as `final`, overriding that behavior or setting a custom serializer is impossible.

In other words, you are out of luck adding your line break in a clean way.

You can, however, safely add it in a slightly unclean way:

DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
DocumentBuilder db = dbf.newDocumentBuilder();

FileInputStream inputXml = new FileInputStream(new File("input.xml"));
Document doc = db.parse(inputXml);

// add the comment node    
doc.insertBefore(doc.createComment("THIS IS A COMMENT"), doc.getDocumentElement());

StringWriter outputXmlStringWriter = new StringWriter();
Transformer transformer = transformerFactory.newTransformer();
// "xml" + "UTF-8" "include XML declaration" is the default anyway, but let's be explicit
transformer.setOutputProperty(OutputKeys.METHOD, "xml");
transformer.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "no");
transformer.setOutputProperty(OutputKeys.ENCODING, "UTF-8");
transformer.transform(new DOMSource(doc), new StreamResult(outputXmlStringWriter));

// now insert our newline into the string & write an UTF-8 file
String outputXmlString = outputXmlStringWriter.toString()
    .replaceFirst("<!--", "\n<!--").replaceFirst("-->", "-->\n");

FileOutputStream outputXml = new FileOutputStream(new File("output.xml"));            
outputXml.write(outputXmlString.getBytes("UTF-8"));

Doing search-and-replace operations on XML strings is highly discouraged in general, but in this case there is little that can go wrong.

Problem

Original source