Why is JSpecify considered a breaking change for some Kotlin Spring projects upgrading to Spring Framework 7?

Kotlin's compiler enforces null-safety at compile time, and it does so for Spring's own APIs by reading whichever nullability annotations Spring's method signatures carry - previously Spring's own org.springframework.lang.Nullable/NonNull annotations, now JSpecify's @Nullable/@NonNull in Spring Framework 7.

// if a return type's nullability annotation changes,
// Kotlin's inferred type for it changes too
val user: User = userLookup.findByEmail(email) // was non-null, may now require User?

Two things compound the risk on upgrade. First, the annotation set itself changed, so even a semantically identical @Nullable usage is now read via a different type Kotlin's compiler resolves differently in edge cases. Second, and more consequential, Spring 7 tightened nullability checking to only honor annotations declared directly on a method, no longer inheriting them from an overridden superclass or interface method - so a method that relied on inherited annotations for its nullability contract may now appear unannotated (and therefore platform-typed or defaulted) to Kotlin's compiler. The practical effect is that an upgrade can silently change which Spring API calls Kotlin considers nullable, surfacing as new compiler warnings, new smart-cast failures, or previously safe !! non-null assertions becoming genuinely risky - it is rarely a hard compile error across the board, but it is not a safe, unreviewed drop-in upgrade for a Kotlin codebase either.

Invest now in Acorns!!! 🚀
Join Acorns and get your $5 bonus!

Acorns is a micro-investing app that automatically invests your "spare change" from daily purchases into diversified, expert-built portfolios of ETFs. It is designed for beginners, allowing you to start investing with as little as $5. The service automates saving and investing. Disclosure: I may receive a referral bonus.

Invest now!!! Get Free equity stock (US, UK only)!

Use Robinhood app to invest in stocks. It is safe and secure. Use the Referral link to claim your free stock when you sign up!.

The Robinhood app makes it easy to trade stocks, crypto and more.

Webull! Receive free stock by signing up using the link: Webull signup.