It is critical that a tool on JDK 8 can use the jrt file system to target a JDK 9 image. Testing this scenario is awkward because it requires a JDK 8 in the test environment. We need to identity a test suite or some way that we can test this scenario. As things stand then jrtfs on JDK 8 breaks on a regular basis and it can be weeks/months before there is a bug report (typically external).