SseEventSourceImpl calls shutdownNow() and awaitTermination() on the executor it is given.

This is wrong if it is using the executor passed when the Client was created, since there is nothing in the ClientBuilder JavaDoc that says the client owns the executor. Therefore the executor may be used for other things besides the Client.

It is also wrong if the executor is a ManagedExecutorService. JSR-236 section 3.1.6.1 explicitly bans applications from using lifecycle methods, and WildFly enforces this ban.

Therefore, SseEventSourceImpl should not call lifecycle methods on the executor.