Annotations

Basic Notes

@JvaName
@JvaField
@JvnStatic
@JvmOverloads
@Throws

Meta annotation

Four Kotlin meta-annotations
@Target
Annotation classes corresponding to target annotations are kotlin.annotation.Target Specifies the applicable target of a new annotation
The allowedTargets attribute is used to set the applicable target, this property has 15 constants

@Retention
Retention period annotation, specifying the valid range of a new annotation
value attribute, which is used to set the retention period, there are three enumeration constants

@Repeatable
Repeatable annotation, which allows repeated annotations in the same program element, repeatable annotations must be annotated with this @Repeatable

@MustBeDocumented
Document annotations, which can decorate code elements (classes, interfaces, functions, properties, etc.), and document generation tools can extract these annotation information

Custom annotations

The simplest custom annotation code:
annotation calss annotation name declares annotation
annotation calss annotation name (val value: String') declares an annotation with attributes
annotation calss annotation name (valvalue: String = "annotation information") declares an annotation with a default value
The use of these custom annotations needs to be constrained with meta-annotations

Annotation target declaration

When multiple elements may be modified, you can use the annotation target declaration to specify the specific element modified by the annotation
Annotation target list
file file
property attribute, which is invisible in java using this target
field field
get getter accessor
set setter accessor
receiver extension function or function parameter
param constructor parameter
setparam setter accessor parameter
delegate save obsceneAttribute fields

Get annotation information through reflection

Get reflection instantiation
val clz = user::class
val ann = clz.findAnnotation get all annotation information