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[Kotlin] Annotation

caution

This is not original content, but just a note from articles I read.

Annotation's architecture in JAVA​

img

From the image, we can know:

  • Each Annotation has only one RetentionPolicy.
  • Each Annotation can be associated with many ElementType.

RetentionPolicy​

class RetentionPolicy

The constants of this enumerated class describe the various policies for retaining annotations.

They are used in conjunction with the Retention meta-annotation interface to specify how long annotations are to be retained.

Enum values​

  • CLASS - Annotations are to be recorded in the class file by the compiler but need not be retained by the VM at run time.
  • RUNTIME - Annotations are to be recorded in the class file by the compiler and retained by the VM at run time, so they may be read reflectively.
  • SOURCE - Annotations are to be discarded by the compiler.

Kotlin (with AnnotationRetention enum)​

@Target(AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)
annotation class Prometheus(
    val name: String
)
info

In Java Language Specification section 9.6.4.2.:

If T does not have a (meta-)annotation m that corresponds to java.lang.annotation.Retention, then a Java compiler must treat T as if it does have such a meta-annotation m with an element whose value is java.lang.annotation.RetentionPolicy.CLASS.

ElementType​

class ElementType

The constants of this enumerated class provide a simple classification of the syntactic locations where annotations may appear in a Java program.

These constants are used in Target meta-annotations to specify where it is legal to write annotations of a given type.

read more detail about ElementType

read more detail about AnnotationTarget

Kotlin (with AnnotationTarget enum)​

@Target(AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)
annotation class Prometheus(
    val name: String
)

Reference​