Migrate to JSpecify annotations for nullability constraints

[sbrannen]

Overview

Once JSpecify releases a relatively stable (potentially beta) version, we should migrate to JSpecify annotations for nullability constraints.

The previous plan was to meta-annotate annotations in the org.springframework.lang package with JSpecify annotations alongside the JSR-305 annotations, but JSpecify won't provide such meta annotations for various reasons. So the new plan is to leverage directly JSpecify annotations and deprecate Spring Framework null-safety annotations.

Related Issues

• Replace @Nonnull(when = When.MAYBE) by @CheckForNull in @Nullable #27183

[sdeleuze]

An interesting related blog post by Meta.

An important improvement would be IMO to update the Spring Framework build to check null-safety at build-time, not just rely on IntelliJ IDEA warnings, and potentially to publish guidelines for Spring portfolio projects and Spring applications that want to do the same.

[sdeleuze]

In addition to the change of artifact and meta-annotations, the switch to jSpecify is expected to allow Spring to refine the following aspects of null-safety:

• Support null-safety at generics, varargs and array elements levels (to be checked depending on which jSpecify version we leverage).
• Better tooling integration, with a build-time check in Spring Framework Gradle build.
• Prevent the unknown enum constant When.MAYBE compile-time warning when Spring null-safety annotations are used without JSR 305 available in the classpath.