How to create an empty DOCTYPE using W3C DOM in Java?
doctype, dom, java, xml
Solution
Although this question is two years old, it is a top search result in some web search engine, so maybe it is a useful shortcut. See the question Set HTML5 doctype with XSLT referring to http://www.w3.org/html/wg/drafts/html/master/syntax.html#doctype-legacy-string, which says:
For the purposes of HTML generators that cannot output HTML markup with the short DOCTYPE "`<!DOCTYPE html>`", a DOCTYPE legacy string may be inserted into the DOCTYPE [...]
In other words, `<!DOCTYPE html SYSTEM "about:legacy-compat">` or `<!DOCTYPE html SYSTEM 'about:legacy-compat'>`, case-insensitively except for the part in single or double quotes.
Leading to a line of Java code like this:
trans.setOutputProperty(OutputKeys.DOCTYPE_SYSTEM, "about:legacy-compat");
Problem
I am trying to read an XML document and output it into a new XML document using the W3C DOM API in Java. To handle DOCTYPEs, I am using the following code (from an input Document `doc` to a target File `target`): ``` TransformerFactory transfac = TransformerFactory.newInstance(); Transformer trans = transfac.newTransformer(); trans.setOutputProperty(OutputKeys.OMIT_XML_DECLARATION, "no"); // omit '<?xml version="1.0"?>' trans.setOutputProperty(OutputKeys.INDENT, "yes"); // if a doctype was set, it needs to persist if (doc.getDoctype() != null) { DocumentType doctype = doc.getDoctype(); trans.setOutputProperty(OutputKeys.DOCTYPE_SYSTEM, doctype.getSystemId()); trans.setOutputProperty(OutputKeys.DOCTYPE_PUBLIC, doctype.getPublicId()); } FileWriter sw = new FileWriter(target); StreamResult result = new StreamResult(sw); DOMSource source = new DOMSource(doc); trans.transform(source, result); ``` This works fine for both XML documents with and without DOCTYPEs. However, I am now coming across a `NullPointerException` when trying to transform the following input XML document: ``` <?xml version='1.0' encoding='UTF-8'?> <!DOCTYPE permissions > <permissions> // ... </permissions> ``` HTML 5 uses a similar syntax for its DOCTYPEs, and it is valid. But I have no idea how to handle this using the W3C DOM API - trying to set the `DOCTYPE_SYSTEM` to `null` throws an exception. Can I still use the W3C DOM API to output an empty doctype?