Curious about what exactly our situation running on Java 9 would be, I took a stab at compiling/running ABCL under:
"1.5.0-dev" "Java_HotSpot(TM)_64-Bit_Server_VM-Oracle_Corporation-9-ea+145" "x86_64-Mac_OS_X-10.12.1"
The good news is that setting the javac source/target to "1.6" seems to be fine, so the assertion that the compiler will continue to work, albeit inefficiently as noted by Alessio, seems to be born out.
The bad news is that due to the changes for "Project Jigsaw" aka [modular-runtime-images][], ABCL can no longer load ABCL-CONTRIB and therefore prevents ASDF from working. The root problem stems from Java 9 no longer using jar archives for system classes, which means that the boot classloader is no longer a descendant of URLClassloader which our current code relies on.
Ticket [424][] tracks this issue, containing a patch that at least gets ABCL compiled/running the base ANSI system under Java 9.
[423]: http://abcl.org/trac/ticket/423 [modular-runtime-images] https://bugs.openjdk.java.net/browse/JDK-8061971