If a Selector is closed at around the same thing that another thread attempts to set the interest set of a SelectionKey representing a registration of a channel with that selector then it's possible for interestOps(int) to throw ClosedSelectorException.

interestOps(int), interestOpsAnd(int) and interestOpsOr(int) should not throw ClosedSelectorException, they throw CancelledKeyException when the key is cancelled (closing a selector will cancelled the keys).

This is small regression that crept in when the Selector implementations were re-implemented in JDK 11. More specifically, each of the Selector implementations defines a setEventOps method that queues the key for the registration. The setEventOps should not call ensureOpen (the ensureOpen can be removed in most cases).