Matej Novotny, There are a lot of issues with building Weld on JDK9. That's the reason why I started by cutting this big task in small pieces.
There are issues with the Format helpers (bcel has visibility restriction in JDK 9) Arquillian, the Groovy compiler and perhaps others.
Some of them have to be resolved on our side, some on others (Groovy team, Arquillian ?), some may be linked to JDK 9 bug or non finished features.
This bug was the first and obviously a problem on our side since tools.jar will never be available in the final JDK 9 release. Others needs exploration and action on our side or ticket raising to other team (Arquillian, Groovy, open JDK).