NOTES & REQUIREMENTS:
This applies to UniFi Controller Versions 5.4 and later.
The UniFi controller is currently supported on Java versions 7 and 8. Java 6 is no longer supported. Future versions of the UniFi controller will drop Java version 7 support.
As Java version 8 is the current LTS (Long-Term Support) release as designated by Oracle, this version is recommended for all controller deployments.
Currently, Java version 9 is incompatible with the controller. We are aware of this incompatibility and will resolve it in the future.