ARTEMIS-3296 enabled the build to run on Java 16 and got the PR build passing there. It disabled running the tests in the compatibility-tests module to achieve this though.
In its current state, the tests look to fail primarily due to an older groovy 2.5.x version, however upgrading this to the 3.0.8 release wasnt enough to get things working, with the tests progressing but seeming to get into an infinite-reconnect scenario where connection was failing and reporting a client-server version mismatch, and eventually OOM errors etc as well.
Work is needed to see whether other changes, such as adding specific module exports, can get the compatibility tests running on Java 16+. Its always possible some of the previous client/server versions they use wont actually run on 16, so perhaps even only a subset can.