The method signature of ByteBuffer#flip changed in Java 9, causing code compiled with a newer JDK to crash when used on older runtimes despite the bytecode compatibility level.

I'm going to close this issue and treat it as an unsupported configuration. There could be many more lurking issues caused by backwards-incompatible standard library changes, and trying to patch around this greatly reduces code quality.

If you want to compile with Java 9+, you must use a Java 9+ runtime.