Nullability annotations

[gavinking]

I'm now very happy with how the use of jakarta.annotation.Nullable and jakarta.annotation.Nonnull has turned out in the persistence spec, and I think we should do something similar here. That is:

• annotate methods and method parameters of Jakarta Data APIs with @Nullable or @Nonnull as appropriate, and even
• encourage the developer to annotate repository methods and their parameters with these annotations, allowing the repository implementation to make use of the additional information (we already support this in Hibernate).

There's no real downside to any of this that I can see.

[gavinking]

encourage the developer to annotate repository methods and their parameters with these annotations, allowing the repository implementation to make use of the additional information

The big one here is the ability to determine whether a repository method that returns a single entity is allowed to return null when there is no matching entity. For an implementation based on Persistence 4.0, this lets @Nullable vs. Nonnull choose between find()/getSIngleResultOrNull() and get()/getSingleResult(). Very useful!