I would say it depends, its quite easy to unknowingly depend on non-standard behavior that can break like HashMap iteration order. What often causes issues is

I would say it depends, its quite easy to unknowingly depend on non-standard behavior that can break like HashMap iteration order. What often causes issues is that every release has a new byte code version and the ecosystem relies heavily on byte code libraries so these will have to be updated across the board (eg. you use middleware that uses a framework that uses a byte code library). Things on the top of my head that can break with Java 9: - new gc logging options and format - rt.jar and tools.jar are gone - new JDK layout - Corba, XML and Annotation classes no longer visible by default - application class loader is no longer and URL class loader - a different hack is required to munmap files It's not too bad in general.