Annotations

@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
}

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

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.

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) }

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 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

// Java
public @interface AnnWithArrayValue {
    String[] value();
}

// Kotlin
@AnnWithArrayValue("abc", "foo", "bar") class C

// Java
public @interface AnnWithArrayMethod {
    String[] names();
}

@AnnWithArrayMethod(names = ["abc", "foo", "bar"])
class C

// Java
public @interface Ann {
    int value();
}

// Kotlin
fun foo(ann: Ann) {
    val i = ann.value
}

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.