Question

Can the Spring @Async annotation be used on interface methods (so all implementations of that interface will implicitly have @Async on that method) or can it only be used on concrete classes?

Answer 1

I quickly tested it and having @Async on an interface will make implementation asynchronous.

If you want to try it yourself: You can easily test this by checking that the values of Thread.currentThread().getId() are different (before calling the method and inside the method) or just adding a Thread.sleep(10000); in the method expected to be asynchronous.

Also take note the return type must be void or java.util.concurrent.Future.