Use XMLConstants.W3C_XML_SCHEMA_NS_URI
:
SchemaFactory factory = SchemaFactory.newInstance(
XMLConstants.W3C_XML_SCHEMA_NS_URI
);
The URI "http://www.w3.org/2001/XMLSchema-instance" (value of the XMLConstants.W3C_XML_SCHEMA_INSTANCE_NS_URI
) is not an XML schema language by itself, but a property (or feature, or additional mechanism, if you like) of the XML Schema.
The proper name for the W3C XML Schema is "http://www.w3.org/2001/XMLSchema" (value of the XMLConstants.W3C_XML_SCHEMA_NS_URI
).