With JDK 9 development underway in the OpenJDK Community, and based on our experiences with open source projects testing against JDK 9 Early Access builds in the Quality Outreach effort, it seems like a good idea to compile the accumulated wisdom to make it easier for new projects to start testing against JDK 9.
JDK 9 has not been released yet. That means the tips and suggestions accumulated here should not be taken as the final word on how to adjust your code for JDK 9 as some things may (and probably will) change as the set of JEPs targeted for JDK 9 changes. If in doubt, ask - preferably on the adoption-discuss mailing list.