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Annotations are means of attaching metadata to code. To declare an annotation, put the annotation modifier in front of a class:

annotation class Fancy

Additional attributes of the annotation can be specified by annotating the annotation class with meta-annotations:

  • @Target specifies the possible kinds of elements which can be annotated with the annotation (such as classes, functions, properties, and expressions);
  • @Retention specifies whether the annotation is stored in the compiled class files and whether it's visible through reflection at runtime (by default, both are true);
  • @Repeatable allows using the same annotation on a single element multiple times;
  • @MustBeDocumented specifies that the annotation is part of the public API and should be included in the class or method signature shown in the generated API documentation.
@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION,
        AnnotationTarget.TYPE_PARAMETER, AnnotationTarget.VALUE_PARAMETER, 
        AnnotationTarget.EXPRESSION)
@Retention(AnnotationRetention.SOURCE)
@MustBeDocumented
annotation class Fancy

Usage

@Fancy class Foo {
    @Fancy fun baz(@Fancy foo: Int): Int {
        return (@Fancy 1)
    }
}

If you need to annotate the primary constructor of a class, you need to add the constructor keyword to the constructor declaration, and add the annotations before it:

class Foo @Inject constructor(dependency: MyDependency) { ... }

You can also annotate property accessors:

class Foo {
    var x: MyDependency? = null
        @Inject set
}

Constructors

Annotations can have constructors that take parameters.

annotation class Special(val why: String)

@Special("example") class Foo {}

Allowed parameter types are:

  • Types that correspond to Java primitive types (Int, Long etc.)
  • Strings
  • Classes (Foo::class)
  • Enums
  • Other annotations
  • Arrays of the types listed above

Annotation parameters cannot have nullable types, because the JVM does not support storing null as a value of an annotation attribute.

If an annotation is used as a parameter of another annotation, its name is not prefixed with the @ character:

annotation class ReplaceWith(val expression: String)

annotation class Deprecated(
        val message: String,
        val replaceWith: ReplaceWith = ReplaceWith(""))

@Deprecated("This function is deprecated, use === instead", ReplaceWith("this === other"))

If you need to specify a class as an argument of an annotation, use a Kotlin class (KClass). The Kotlin compiler will automatically convert it to a Java class, so that the Java code can access the annotations and arguments normally.

import kotlin.reflect.KClass

annotation class Ann(val arg1: KClass<*>, val arg2: KClass<out Any>)

@Ann(String::class, Int::class) class MyClass

Instantiation

In Java, an annotation type is a form of an interface, so you can implement it and use an instance. As an alternative to this mechanism, Kotlin lets you call a constructor of an annotation class in arbitrary code and similarly use the resulting instance.

annotation class InfoMarker(val info: String)

fun processInfo(marker: InfoMarker): Unit = TODO()

fun main(args: Array<String>) {
    if (args.isNotEmpty())
        processInfo(getAnnotationReflective(args))
    else
        processInfo(InfoMarker("default"))
}

Learn more about instantiation of annotation classes in this KEEP.

Lambdas

Annotations can also be used on lambdas. They will be applied to the invoke() method into which the body of the lambda is generated. This is useful for frameworks like Quasar, which uses annotations for concurrency control.

annotation class Suspendable

val f = @Suspendable { Fiber.sleep(10) }

Annotation use-site targets

When you're annotating a property or a primary constructor parameter, there are multiple Java elements which are generated from the corresponding Kotlin element, and therefore multiple possible locations for the annotation in the generated Java bytecode. To specify how exactly the annotation should be generated, use the following syntax:

class Example(@field:Ann val foo,    // annotate Java field
              @get:Ann val bar,      // annotate Java getter
              @param:Ann val quux)   // annotate Java constructor parameter

The same syntax can be used to annotate the entire file. To do this, put an annotation with the target file at the top level of a file, before the package directive or before all imports if the file is in the default package:

@file:JvmName("Foo")

package org.jetbrains.demo

If you have multiple annotations with the same target, you can avoid repeating the target by adding brackets after the target and putting all the annotations inside the brackets:

class Example {
     @set:[Inject VisibleForTesting]
     var collaborator: Collaborator
}

The full list of supported use-site targets is:

  • file
  • property (annotations with this target are not visible to Java)
  • field
  • get (property getter)
  • set (property setter)
  • receiver (receiver parameter of an extension function or property)
  • param (constructor parameter)
  • setparam (property setter parameter)
  • delegate (the field storing the delegate instance for a delegated property)

To annotate the receiver parameter of an extension function, use the following syntax:

fun @receiver:Fancy String.myExtension() { ... }

If you don't specify a use-site target, the target is chosen according to the @Target annotation of the annotation being used. If there are multiple applicable targets, the first applicable target from the following list is used:

  • param
  • property
  • field

Java annotations

Java annotations are 100% compatible with Kotlin:

import org.junit.Test
import org.junit.Assert.*
import org.junit.Rule
import org.junit.rules.*

class Tests {
    // apply @Rule annotation to property getter
    @get:Rule val tempFolder = TemporaryFolder()

    @Test fun simple() {
        val f = tempFolder.newFile()
        assertEquals(42, getTheAnswer())
    }
}

Since the order of parameters for an annotation written in Java is not defined, you can't use a regular function call syntax for passing the arguments. Instead, you need to use the named argument syntax:

// Java
public @interface Ann {
    int intValue();
    String stringValue();
}
// Kotlin
@Ann(intValue = 1, stringValue = "abc") class C

Just like in Java, a special case is the value parameter; its value can be specified without an explicit name:

// Java
public @interface AnnWithValue {
    String value();
}
// Kotlin
@AnnWithValue("abc") class C

Arrays as annotation parameters

If the value argument in Java has an array type, it becomes a vararg parameter in Kotlin:

// Java
public @interface AnnWithArrayValue {
    String[] value();
}
// Kotlin
@AnnWithArrayValue("abc", "foo", "bar") class C

For other arguments that have an array type, you need to use the array literal syntax or arrayOf(...):

// Java
public @interface AnnWithArrayMethod {
    String[] names();
}
@AnnWithArrayMethod(names = ["abc", "foo", "bar"]) 
class C

Accessing properties of an annotation instance

Values of an annotation instance are exposed as properties to Kotlin code:

// Java
public @interface Ann {
    int value();
}
// Kotlin
fun foo(ann: Ann) {
    val i = ann.value
}

Ability to not generate JVM 1.8+ annotation targets

If a Kotlin annotation has TYPE among its Kotlin targets, the annotation maps to java.lang.annotation.ElementType.TYPE_USE in its list of Java annotation targets. This is just like how the TYPE_PARAMETER Kotlin target maps to the java.lang.annotation.ElementType.TYPE_PARAMETER Java target. This is an issue for Android clients with API levels less than 26, which don't have these targets in the API.

To avoid generating the TYPE_USE and TYPE_PARAMETER annotation targets, use the new compiler argument -Xno-new-java-annotation-targets.

Repeatable annotations

Just like in Java, Kotlin has repeatable annotations, which can be applied to a single code element multiple times. To make your annotation repeatable, mark its declaration with the @kotlin.annotation.Repeatable meta-annotation. This will make it repeatable both in Kotlin and Java. Java repeatable annotations are also supported from the Kotlin side.

The main difference with the scheme used in Java is the absence of a containing annotation, which the Kotlin compiler generates automatically with a predefined name. For an annotation in the example below, it will generate the containing annotation @Tag.Container:

@Repeatable
annotation class Tag(val name: String)

// The compiler generates the @Tag.Container containing annotation

You can set a custom name for a containing annotation by applying the @kotlin.jvm.JvmRepeatable meta-annotation and passing an explicitly declared containing annotation class as an argument:

@JvmRepeatable(Tags::class)
annotation class Tag(val name: String)

annotation class Tags(val value: Array<Tag>)

To extract Kotlin or Java repeatable annotations via reflection, use the KAnnotatedElement.findAnnotations() function.

Learn more about Kotlin repeatable annotations in this KEEP.