The ecosystem has (at long last) standardized on JSpecify (https://jspecify.dev) for nullability annotations. JSpecify allows you to annotate a package or module with `@NullMarked` and your IDE and build (via ErrorProne+NullAway, typically) will check for null safety.

If you develop a library in Java and use it from Kotlin, the built-in Kotlin null-safety will recognize the JSpecify annotations on the library.

Null-restricted types are on the roadmap. See: https://openjdk.org/jeps/8303099