This commit introduces null-safety checks for spring-core at build-time
in order to validate the consistency of Spring null-safety annotations
and generate errors when inconsistencies are detected during a build
(similar to what is done with Checkstyle).
In order to make that possible, this commit also introduces a new
org.springframework.lang.Contract annotation inspired from
org.jetbrains.annotations.Contract, which allows to specify semantics
of methods like Assert#notNull in order to prevent artificial
additional null checks in Spring Framework code base.
This commit only checks org.springframework.core package, follow-up
commits will also extend the analysis to other modules, after related
null-safety refinements.
See gh-32475
Historically, we have rarely intentionally thrown a
NullPointerException in the Spring Framework. Instead, we prefer to
throw either an IllegalArgumentException or IllegalStateException
instead of a NullPointerException.
However, changes to the code in recent times have introduced the use of
Objects.requireNonNull(Object) which throws a NullPointerException
without an explicit error message.
The latter ends up providing less context than a NullPointerException
thrown by the JVM (since Java 14) due to actually de-referencing a
null-pointer. See https://openjdk.org/jeps/358.
In light of that, this commit revises our current use of
Objects.requireNonNull(Object) by removing it or replacing it with
Assert.notNull().
However, we still use Objects.requireNonNull(T, String) in a few places
where we are required to throw a NullPointerException in order to
comply with a third-party contract such as Reactive Streams.
Closes gh-32430
Rework AnnotatedElementAdapter to avoid cloning the underlying
Annotation array if it is empty when getAnnotations() is called.
Additionally, make the class static and add a factory method that
returns a singleton instance for null or empty Annotation arrays.
Closes gh-32405
KClass instantiation in CoroutinesUtils is suboptimal, and should be
replaced by KTypes#isSubtypeOf checks using pre-instantiated types for
Flow, Mono and Publisher.
This commit impact on performances is significant since a throughput
increase between 2x and 3x has been measured on basic endpoints.
Closes gh-32390
