Revisit nullability annotations towards GA [SPR-15756]

[spring-projects-issues]

Rob Winch opened SPR-15756 and commented

Currently @NonNullApi can be specified at a package level only. It would be nice if there were a way to do this at a type level as well. This is of interest to users migrating to using the annotated approach to handling null values who haven't converted their entire code base appropriately and want to avoid warnings.

For example, consider a user who wants to implement WebSessionManager. If they create an implementation, like this:

public class MyWebSessionManager implements WebSessionManager {
	@Override
	public Mono<WebSession> getSession(ServerWebExchange serverWebExchange) {
		...
	}
}

The user is given the warning:

Not annotated method overrides method annotated with @NonNullApi....

To properly resolve this warning, the user would currently need to place @NonNullApi at the package level. However, any other APIs within the package would also be impacted. It would be nice if the @NonNullApi (or similar annotation) could be placed on the type level.

Obviously in the long run, it would be good for the user to expand through the entire package. However, there is some effort involved in migrating so something supporting the type level would be valuable.

---

Affects: 5.0 RC2

Reference URL: https://github.com/Kotlin/KEEP/blob/jsr-305/proposals/jsr-305-custom-nullability-qualifiers.md

Issue Links:

• Support null-safety at generics, varargs and array elements level [SPR-15942] #20496 Support null-safety at generics, varargs and array elements level ("is depended on by")
• Fix overridden methods nullability [SPR-15869] #20424 Fix overridden methods nullability
• Introduce null-safety of Spring Framework API [SPR-15540] #20099 Introduce null-safety of Spring Framework API
• Extend null-safety to field level [SPR-15720] #20276 Extend null-safety to field level

[spring-projects-issues]

Juergen Hoeller commented

In the above case, wouldn't an @Nullable declaration on the overridden method solve the problem, along the lines of #20424?

In any case, let's use this opportunity to revisit @NonNullApi overall, double-checking whether we're happy with its semantics.

[spring-projects-issues]

Sébastien Deleuze commented

Notice than JSR 305 meta-annotations should be applied to generic type arguments as well with Kotlin 1.0.5, which was the last known issue I am aware of.

[spring-projects-issues]

Sébastien Deleuze commented

Rob Winch We plan to introduce a Spring @NonNull annotation, similar to @Nullable with the opposite semantic. These both annotations should allow to deal with #20496 as well as the progressive nullability use case that you described.

[spring-projects-issues]

Sébastien Deleuze commented

Resolved via this commit, see the commit message for more details.