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core-null-safety.adoc

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Null-safety

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

  • {api-spring-framework}/lang/NonNull.html[@NonNull] annotation where specific parameter, return value or field cannot be null (not needed on parameter and return value where @NonNullApi and @NonNullFields apply) .

  • {api-spring-framework}/lang/Nullable.html[@Nullable] annotation where specific parameter, return value or field can be null.

  • {api-spring-framework}/lang/NonNullApi.html[@NonNullApi] annotation at package level declares non-null as the default behavior for parameters and return values.

  • {api-spring-framework}/lang/NonNullFields.html[@NonNullFields] annotation at package level declares non-null as the default behavior for fields.

Spring Framework leverages itself these annotations, but they can also be used in any Spring based Java project to declare null-safe APIs and optionally null-safe fields. Generic type arguments, varargs and array elements nullability are not supported yet, but should be in an upcoming release, see SPR-15942 for up-to-date information. Nullability declaration are expected to be fine-tuned between Spring Framework release, including minor ones. Nullability of types used inside method bodies is outside of the scope of this feature.

Note

Libraries like Reactor or Spring Data provide null-safe APIs leveraging this feature.

Use cases

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

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

JSR 305 meta-annotations

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

It is not necessary nor recommended to add JSR 305 dependency in project classpath to take advantage of Spring null-safe API. Only projects like Spring-based libraries using 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 compile warnings.