You can also see Spring Data's implementation: https://github.com/spring-projects/spring-data-jpa/blob/master/src/main/java/org/springframework/data/jpa/provider/PersistenceProvider.java#L400

I'm not sure async would mean reading from the query from multiple threads.
Think of an async REST resource or an async servlet. They would load the data on a single thread, just that on a different thread than the one handling the request.