Now that we are using JDK 18+ (currently JDK 24 -- see JavaConventions
in buildSrc for details), we can reinstate links to JUnit 5 Javadoc APIs.
Closes gh-27497
This commit revisit the build configuration to enforce the following:
* A single Java toolchain is used consistently with a recent Java
version (here, Java 23) and language level
* the main source is compiled with the Java 17 "-release" target
* Multi-Release classes are compiled with their respective "-release"
target. For now, only "spring-core" ships Java 21 variants.
Closes gh-34507
This commit updates the whole Spring Framework codebase to use JSpecify
annotations instead of Spring null-safety annotations with JSR 305
semantics.
JSpecify provides signficant enhancements such as properly defined
specifications, a canonical dependency with no split-package issue,
better tooling, better Kotlin integration and the capability to specify
generic type, array and varargs element null-safety. Generic type
null-safety is not defined by this commit yet and will be specified
later.
A key difference is that Spring null-safety annotations, following
JSR 305 semantics, apply to fields, parameters and return values,
while JSpecify annotations apply to type usages. That's why this
commit moves nullability annotations closer to the type for fields
and return values.
See gh-28797
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
