I wanted to start moving all my projects to Java 9, but quickly discovered that a lot of Maven plugins I depend on aren't ready for Java 9 yet.
japicmp doesn't support Java 9 because javassist doesn't support Java 9 yet.
maven-bundle-plugin doesn't support Java 9 because BND doesn't support Java 9 yet.
Update (2017-10-03): John Poth has offered a workaround
maven-dependency-plugin doesn't support Java 9. See this ticket.