This version is still in development and is not considered stable yet. For the latest stable version, please use Spring Framework 6.0.25!

Null-safety

Although Java does not let you express null-safety with its type system, the Spring Framework provides the following annotations in the org.springframework.lang package to let you declare nullability of APIs and fields:

  • @Nullable: Annotation to indicate that a specific parameter, return value, or field can be null.

  • @NonNull: Annotation to indicate that a specific parameter, return value, or field cannot be null (not needed on parameters, return values, and fields where @NonNullApi and @NonNullFields apply, respectively).

  • @NonNullApi: Annotation at the package level that declares non-null as the default semantics for parameters and return values.

  • @NonNullFields: Annotation at the package level that declares non-null as the default semantics for fields.

The Spring Framework itself leverages these annotations, but they can also be used in any Spring-based Java project to declare null-safe APIs and optionally null-safe fields. Nullability declarations for generic type arguments, varargs, and array elements are not supported yet. Nullability declarations are expected to be fine-tuned between Spring Framework releases, including minor ones. Nullability of types used inside method bodies is outside the scope of this feature.

Other common libraries such as Reactor and Spring Data provide null-safe APIs that use a similar nullability arrangement, delivering a consistent overall experience for Spring application developers.

Use cases

In addition to providing an explicit declaration for Spring Framework API nullability, these annotations can be used by an IDE (such as IDEA or Eclipse) to provide useful warnings related to null-safety in order to avoid NullPointerException at runtime.

They are also used to make Spring APIs null-safe in Kotlin projects, since Kotlin natively supports null-safety. More details are available in the Kotlin support documentation.

JSR-305 meta-annotations

Spring annotations are meta-annotated with JSR 305 annotations (a dormant but widespread JSR). JSR-305 meta-annotations let tooling vendors like IDEA or Kotlin provide null-safety support in a generic way, without having to hard-code support for Spring annotations.

It is neither necessary nor recommended to add a JSR-305 dependency to the project classpath to take advantage of Spring’s null-safe APIs. Only projects such as Spring-based libraries that use null-safety annotations in their codebase should add com.google.code.findbugs:jsr305:3.0.2 with compileOnly Gradle configuration or Maven provided scope to avoid compiler warnings.