As Java 9 has been released Openfire should support it at some point. Currently if i try to run Openfire 4.1.5 after deleting its internal jre folder (x64 version) and have Java 9 (64-bit) installed it still says it can't find suitable java. This might be the case of just wrong path variable, though usually Openfire was able to find system Java (when using 32-bit one). Java 9 comes only in 64-bit version.
I have filed for MINA update
We're now compiling and running integration tests with JDK11, though there may still be edge cases, I think this is probably OK for 4.4.0
Gonna claim victory on this issue and can open new ones for specific problems found.