注释类型 Nullable

@Target({METHOD,PARAMETER,FIELD})
@Retention(RUNTIME)
@Documented
@Nonnull(when=MAYBE)
@TypeQualifierNickname
public @interface Nullable

A common Spring annotation to declare that annotated elements can be null under some circumstance.

Leverages JSR-305 meta-annotations to indicate nullability in Java to common tools with JSR-305 support and used by Kotlin to infer nullability of Spring API.

Should be used at parameter, return value, and field level. Methods override should repeat parent @Nullable annotations unless they behave differently.

Can be used in association with @NonNullApi or @NonNullFields to override the default non-nullable semantic to nullable.