Reflection

Reflection is a set of language and library features that allows you to introspect the structure of your program at runtime. Functions and properties are first-class citizens in Kotlin, and the ability to introspect them (for example, learning the name or the type of a property or function at runtime) is essential when using a functional or reactive style.

JVM dependency

On the JVM platform, the Kotlin compiler distribution includes the runtime component required for using the reflection features as a separate artifact, kotlin-reflect.jar. This is done to reduce the required size of the runtime library for applications that do not use reflection features.

To use reflection in a Gradle or Maven project, add the dependency on kotlin-reflect:

In Gradle:
dependencies { implementation(kotlin("reflect")) }
dependencies { implementation "org.jetbrains.kotlin:kotlin-reflect:2.1.0" }
In Maven:
<dependencies> <dependency> <groupId>org.jetbrains.kotlin</groupId> <artifactId>kotlin-reflect</artifactId> </dependency> </dependencies>

If you don't use Gradle or Maven, make sure you have kotlin-reflect.jar in the classpath of your project. In other supported cases (IntelliJ IDEA projects that use the command-line compiler or Ant), it is added by default. In the command-line compiler and Ant, you can use the -no-reflect compiler option to exclude kotlin-reflect.jar from the classpath.

Class references

The most basic reflection feature is getting the runtime reference to a Kotlin class. To obtain the reference to a statically known Kotlin class, you can use the class literal syntax:

val c = MyClass::class

The reference is a KClass type value.

Bound class references

You can get the reference to the class of a specific object with the same ::class syntax by using the object as a receiver:

val widget: Widget = ...
assert(widget is GoodWidget) { "Bad widget: ${widget::class.qualifiedName}" }

You will obtain the reference to the exact class of an object, for example, GoodWidget or BadWidget, regardless of the type of the receiver expression (Widget).

Callable references

References to functions, properties, and constructors can also be called or used as instances of function types.

The common supertype for all callable references is KCallable<out R>, where R is the return value type. It is the property type for properties, and the constructed type for constructors.

Function references

When you have a named function declared as below, you can call it directly (isOdd(5)):

fun isOdd(x: Int) = x % 2 != 0

Alternatively, you can use the function as a function type value, that is, pass it to another function. To do so, use the :: operator:

fun isOdd(x: Int) = x % 2 != 0

fun main() {
//sampleStart
    val numbers = listOf(1, 2, 3)
    println(numbers.filter(::isOdd))
//sampleEnd
}

Here ::isOdd is a value of function type (Int) -> Boolean.

Function references belong to one of the KFunction<out R> subtypes, depending on the parameter count. For instance, KFunction3<T1, T2, T3, R>.

:: can be used with overloaded functions when the expected type is known from the context. For example:

fun main() {
//sampleStart
    fun isOdd(x: Int) = x % 2 != 0
    fun isOdd(s: String) = s == "brillig" || s == "slithy" || s == "tove"
    
    val numbers = listOf(1, 2, 3)
    println(numbers.filter(::isOdd)) // refers to isOdd(x: Int)
//sampleEnd
}

Alternatively, you can provide the necessary context by storing the method reference in a variable with an explicitly specified type:

val predicate: (String) -> Boolean = ::isOdd // refers to isOdd(x: String)

If you need to use a member of a class or an extension function, it needs to be qualified: String::toCharArray.

Even if you initialize a variable with a reference to an extension function, the inferred function type will have no receiver, but it will have an additional parameter accepting a receiver object. To have a function type with a receiver instead, specify the type explicitly:

val isEmptyStringList: List<String>.() -> Boolean = List<String>::isEmpty

Example: function composition

Consider the following function:

fun <A, B, C> compose(f: (B) -> C, g: (A) -> B): (A) -> C {
    return { x -> f(g(x)) }
}

It returns a composition of two functions passed to it: compose(f, g) = f(g(*)). You can apply this function to callable references:

fun <A, B, C> compose(f: (B) -> C, g: (A) -> B): (A) -> C {
    return { x -> f(g(x)) }
}

fun isOdd(x: Int) = x % 2 != 0

fun main() {
//sampleStart
    fun length(s: String) = s.length
    
    val oddLength = compose(::isOdd, ::length)
    val strings = listOf("a", "ab", "abc")
    
    println(strings.filter(oddLength))
//sampleEnd
}

Property references

To access properties as first-class objects in Kotlin, use the :: operator:

val x = 1

fun main() {
    println(::x.get())
    println(::x.name) 
}

The expression ::x evaluates to a KProperty0<Int> type property object. You can read its value using get() or retrieve the property name using the name property. For more information, see the docs on the KProperty class.

For a mutable property such as var y = 1, ::y returns a value with the KMutableProperty0<Int> type which has a set() method:

var y = 1

fun main() {
    ::y.set(2)
    println(y)
}

A property reference can be used where a function with a single generic parameter is expected:

fun main() {
//sampleStart
    val strs = listOf("a", "bc", "def")
    println(strs.map(String::length))
//sampleEnd
}

To access a property that is a member of a class, qualify it as follows:

fun main() {
//sampleStart
    class A(val p: Int)
    val prop = A::p
    println(prop.get(A(1)))
//sampleEnd
}

For an extension property:

val String.lastChar: Char
    get() = this[length - 1]

fun main() {
    println(String::lastChar.get("abc"))
}

Interoperability with Java reflection

On the JVM platform, the standard library contains extensions for reflection classes that provide a mapping to and from Java reflection objects (see package kotlin.reflect.jvm). For example, to find a backing field or a Java method that serves as a getter for a Kotlin property, you can write something like this:

import kotlin.reflect.jvm.*
 
class A(val p: Int)
 
fun main() {
    println(A::p.javaGetter) // prints "public final int A.getP()"
    println(A::p.javaField)  // prints "private final int A.p"
}

To get the Kotlin class that corresponds to a Java class, use the .kotlin extension property:

fun getKClass(o: Any): KClass<Any> = o.javaClass.kotlin

Constructor references

Constructors can be referenced just like methods and properties. You can use them wherever the program expects a function type object that takes the same parameters as the constructor and returns an object of the appropriate type. Constructors are referenced by using the :: operator and adding the class name. Consider the following function that expects a function parameter with no parameters and return type Foo:

class Foo

fun function(factory: () -> Foo) {
    val x: Foo = factory()
}

Using ::Foo, the zero-argument constructor of the class Foo, you can call it like this:

function(::Foo)

Callable references to constructors are typed as one of the KFunction<out R> subtypes depending on the parameter count.

Bound function and property references

You can refer to an instance method of a particular object:

fun main() {
//sampleStart
    val numberRegex = "\\d+".toRegex()
    println(numberRegex.matches("29"))
     
    val isNumber = numberRegex::matches
    println(isNumber("29"))
//sampleEnd
}

Instead of calling the method matches directly, the example uses a reference to it. Such a reference is bound to its receiver. It can be called directly (like in the example above) or used whenever a function type expression is expected:

fun main() {
//sampleStart
    val numberRegex = "\\d+".toRegex()
    val strings = listOf("abc", "124", "a70")
    println(strings.filter(numberRegex::matches))
//sampleEnd
}

Compare the types of the bound and the unbound references. The bound callable reference has its receiver "attached" to it, so the type of the receiver is no longer a parameter:

val isNumber: (CharSequence) -> Boolean = numberRegex::matches

val matches: (Regex, CharSequence) -> Boolean = Regex::matches

A property reference can be bound as well:

fun main() {
//sampleStart
    val prop = "abc"::length
    println(prop.get())
//sampleEnd
}

You don't need to specify this as the receiver: this::foo and ::foo are equivalent.

Bound constructor references

A bound callable reference to a constructor of an inner class can be obtained by providing an instance of the outer class:

class Outer {
    inner class Inner
}

val o = Outer()
val boundInnerCtor = o::Inner

Last modified: 25 September 2024