Michael Hintenaus, in your reproducer there's a META-INF/validation.xml file which we need to parse. JAXB is used for that which requires the addition of the java.xml.bind module on Java 9 (it isn't enabled by default). If you remove the XML file, JAXB won't be used and the module won't be needed.

So it's essentially a case of "Works as designed", I'll add a sentence to the docs about that requirement, though (and reword that issue accordingly).