XMLConstants.W3C_XML_SCHEMA_NS_URI: http://www.w3.org/2001/XMLSchema
XMLConstants.RELAXNG_NS_URI: http://relaxng.org/ns/structure/1.0
XMLConstants.XML_DTD_NS_URI: http://www.w3.org/TR/REC-xml
Other languages can be added with libraries
Only W3C schema supported out-of-the-box in JDK 5/6