mockk: A Kotlin repository from Kutashov

Open collective community support
Verification timeout v1.8.6 #114
Relaxed unit returning functions v1.8.4 #91

Please report any issues

We are actively looking for contributors and co-authors. There are two ambitious goals, that look unachievable at first sight, but in my opinion is essential to win Kotlin competition and require just hard and inventive work. They are - support for Kotlin/JS and Kotlin/Native platforms. If you think you can accept one of this two challenges and join us please contact me.

Besides, contributions may cover:

documentation improvements
KDoc samples
refactorings
additional test coverage
bug fixes
features similar to Mockito
new features with user-oriented goals in mind
integration with RxJava, SpringFu, Spring test, e.t.c.

Table of contents:

auto-gen TOC: {:toc}

Nice features

annotations
mocking final classes and functions (via inlining)
pure Kotlin mocking DSL
matchers partial specification
chained calls
matcher expressions
mocking coroutines
capturing lambdas
object mocks
constructor mocks
private function mocking
property backing field access
extension function mocking (static mocks)
Android instrumented tests
multiplatform support (JS support is highly experimental)

Examples & articles

kotlin-fullstack-sample project covered with tests
DZone article
Habrahabr article (RU)

Kotlin Academy

Installation

All you need to get started is just to add a dependency to MockK library.

Gradle/maven dependency

Approach	Instruction
	testImplementation "io.mockk:mockk:{version}"
(Kotlin DSL)	testImplementation("io.mockk:mockk:{version}")
	<dependency> <groupId>io.mockk</groupId> <artifactId>mockk</artifactId> <version>{version}</version> <scope>test</scope> </dependency>
Unit	testImplementation "io.mockk:mockk:{version}"
Instrumented	androidTestImplementation "io.mockk:mockk-android:{version}"
Common multiplatform	testImplementation "io.mockk:mockk-common:{version}"

DSL examples

Simplest example. By default mocks are strict, so you need to provide some behaviour.

val car = mockk<Car>()

every { car.drive(Direction.NORTH) } returns Outcome.OK

car.drive(Direction.NORTH) // returns OK

verify { car.drive(Direction.NORTH) }

Annotations

You can use annotations to simplify creation of mock objects:

class TrafficSystem {
  lateinit var car1: Car
  
  lateinit var car2: Car
  
  lateinit var car3: Car
}

class CarTest {
  @MockK
  lateinit var car1: Car

  @RelaxedMockK
  lateinit var car2: Car

  @MockK(relaxUnitFun = true)
  lateinit var car3: Car

  @SpyK
  var car4 = Car()
  
  @InjectMockKs
  var trafficSystem = TrafficSystem()
  
  @Before
  fun setUp() = MockKAnnotations.init(this, relaxUnitFun = true) // turn relaxUnitFun on for all mocks

  @Test
  fun calculateAddsValues1() {
      // ... use car1, car2, car3 and car4
  }
}

Injection first tries to match properties by name, then by class or superclass. Check lookupType parameter for customization.

Properties are injected even if private is applied. Constructors for injection are selected from the biggest number of arguments to lowest.

@InjectMockKs by default is injecting only lateinit vars or vars that are not assigned. To change this use overrideValues = true. This would assign value even if it is already somehow initialized. To inject vals use injectImmutable = true. For shorter notation use @OverrideMockKs which do the same as @InjectMockKs by default, but turns this two flags on.

JUnit5

In JUnit5 you can use MockKExtension to initialize mock.

@ExtendWith(MockKExtension::class)
class CarTest {
  @MockK
  lateinit var car1: Car

  @RelaxedMockK
  lateinit var car2: Car

  @MockK(relaxUnitFun = true)
  lateinit var car3: Car

  @SpyK
  var car4 = Car()

  @Test
  fun calculateAddsValues1() {
      // ... use car1, car2, car3 and car4
  }
}

Additionally it adds possibility to use@MockK and @RelaxedMockK on test function parameters:

@Test
fun calculateAddsValues1(@MockK car1: Car, @RelaxedMockK car2: Car) {
  // ... use car1 and car2
}

Spy

Spies allow to mix mocks and real objects.

val car = spyk(Car()) // or spyk<Car>() to call default constructor

car.drive(Direction.NORTH) // returns whatever real function of Car returns

verify { car.drive(Direction.NORTH) }

Note: the spy object is a copy of a passed object.

Relaxed mock

Relaxed mock is the mock that returns some simple value for all functions. This allows to skip specifying behavior for each case, while still allow to stub things you need. For reference types chained mocks are returned.

val car = mockk<Car>(relaxed = true)

car.drive(Direction.NORTH) // returns null

verify { car.drive(Direction.NORTH) }

Note: relaxed mocking is working badly with generic return type. Usually in this case class cast exception is thrown. You need to specify stubbing manually for case of generic return type.

Workaround:

val func = mockk<() -> Car>(relaxed = true) // in this case invoke function has generic return type

// this line is workaround, without it relaxed mock would throw class cast exception on the next line
every { func() } returns Car() // or you can return mockk() for example 

func()

Unit returning function relaxed mock

In case you would like Unit returning functions to be relaxed. You can use relaxUnitFun = true as an argument to mockk function, @MockKannotation or MockKAnntations.init function.

Function:

mockk<MockCls>(relaxUnitFun = true)

Annotation:

@MockK(relaxUnitFun = true)
lateinit var mock1: RurfMockCls
init {
    MockKAnnotations.init(this)
}

MockKAnnotations.init:

@MockK
lateinit var mock2: RurfMockCls
init {
    MockKAnnotations.init(this, relaxUnitFun = true)
}

Object mocks

Objects can be transformed to mocks following way:

object MockObj {
  fun add(a: Int, b: Int) = a + b
}

mockkObject(MockObj) // aplies mocking to an Object

assertEquals(3, MockObj.add(1, 2))

every { MockObj.add(1, 2) } returns 55

assertEquals(55, MockObj.add(1, 2))

To revert back use unmockkAll or unmockkObject:

@Before
fun beforeTests() {
    mockkObject(MockObj)
    every { MockObj.add(1,2) } returns 55
}

@Test
fun willUseMockBehaviour() {
    assertEquals(55, MockObj.add(1,2))
}

@After
fun afterTests() {
    unmockkAll()
    // or unmockkObject(MockObj)
}

Despite Kotlin language limits you can create new instances of objects if testing logic needs that:

val newObjectMock = mockk<MockObj>()

Class mock

Sometimes you need mock of arbitary class. Use mockkClass in this case.

val car = mockkClass(Car::class)

every { car.drive(Direction.NORTH) } returns Outcome.OK

car.drive(Direction.NORTH) // returns OK

verify { car.drive(Direction.NORTH) }

Enumeration mocks

Enums can be mocked using mockkObject:

enum class Enumeration(val goodInt: Int) {
    CONSTANT(35),
    OTHER_CONSTANT(45);
}

mockkObject(Enumeration.CONSTANT)
every { Enumeration.CONSTANT.goodInt } returns 42
assertEquals(42, Enumeration.CONSTANT.goodInt)

Constructor mocks

Sometimes, especially in code you are not owning, you need to mock newly created objects. For this purpose following constructs are provided:

class MockCls {
  fun add(a: Int, b: Int) = a + b
}

mockkConstructor(MockCls::class)

every { anyConstructed<MockCls>().add(1, 2) } returns 4

assertEquals(4, MockCls().add(1, 2)) // note new object is created

verify { anyConstructed<MockCls>().add(1, 2) }

Basic idea is that just after constructor of mocked class is executed(any of them), objects become constructed mock. Mocking behavior of such mock is connected to special prototype mock denoted by anyConstructed<MockCls>(). There is one instance per class of such prototype mock. Call recording also happens to prototype mock. If no behavior for function is specified original function is executed.

Partial argument matching

You can mix both regular arguments and matchers:

val car = mockk<Car>()

every { 
  car.recordTelemetry(
    speed = more(50),
    direction = Direction.NORTH, // here eq() is used
    lat = any(),
    long = any()
  )
} returns Outcome.RECORDED

obj.recordTelemetry(60, Direction.NORTH, 51.1377382, 17.0257142)

verify { obj.recordTelemetry(60, Direction.NORTH, 51.1377382, 17.0257142) }

Chained calls

You can stub chains of calls:

val car = mockk<Car>()

every { car.door(DoorType.FRONT_LEFT).windowState() } returns WindowState.UP

car.door(DoorType.FRONT_LEFT) // returns chained mock for Door
car.door(DoorType.FRONT_LEFT).windowState() // returns WindowState.UP

verify { car.door(DoorType.FRONT_LEFT).windowState() }

Note: in case function return type is generic the information about actual type is erased. To make chained calls work additional information is required. Most of the times framework will catch the cast exception and do autohinting. But in the case it is explicitly needed just place hint before calls.

every { obj.op2(1, 2).hint(Int::class).op1(3, 4) } returns 5

Capturing

You can capture an argument to a CapturingSlot or MutableList:

val car = mockk<Car>()

val slot = slot<Double>()
val list = mutableListOf<Double>()

every {
  obj.recordTelemetry(
    speed = capture(slot),
    direction = Direction.NORTH
  )
} answers {
  println(slot.captured)

  Outcome.RECORDED
}


every {
  obj.recordTelemetry(
    speed = capture(list),
    direction = Direction.SOUTH
  )
} answers {
  println(list.captured())

  Outcome.RECORDED
}

obj.recordTelemetry(speed = 15, direction = Direction.NORTH) // prints 15
obj.recordTelemetry(speed = 16, direction = Direction.SOUTH) // prints 16

verify(exactly = 2) { obj.recordTelemetry(speed = or(15, 16), direction = any()) }

Verification atLeast, atMost or exactly times

You can check call count with atLeast, atMost or exactly parameters:

val car = mockk<Car>(relaxed = true)

car.accelerate(fromSpeed = 10, toSpeed = 20)
car.accelerate(fromSpeed = 10, toSpeed = 30)
car.accelerate(fromSpeed = 20, toSpeed = 30)

// all pass
verify(atLeast = 3) { car.accelerate(allAny()) }
verify(atMost  = 2) { car.accelerate(fromSpeed = 10, toSpeed = or(20, 30)) }
verify(exactly = 1) { car.accelerate(fromSpeed = 10, toSpeed = 20) }
verify(exactly = 0) { car.accelerate(fromSpeed = 30, toSpeed = 10) } // means no calls were performed

Verification order

verifyAll verifies that all calls happened without checking it's order. verifySequence verifies that the exact sequence happened and verifyOrder that calls happened in order. wasNot Called verifies that mock or list of mocks was not called at all.

class MockedClass {
    fun sum(a: Int, b: Int) = a + b
}

val obj = mockk<MockedClass>()
val slot = slot<Int>()

every {
    obj.sum(any(), capture(slot))
} answers {
    1 + firstArg<Int>() + slot.captured
}

obj.sum(1, 2) // returns 4
obj.sum(1, 3) // returns 5
obj.sum(2, 2) // returns 5

verifyAll {
    obj.sum(1, 3)
    obj.sum(1, 2)
    obj.sum(2, 2)
}

verifySequence {
    obj.sum(1, 2)
    obj.sum(1, 3)
    obj.sum(2, 2)
}

verifyOrder {
    obj.sum(1, 2)
    obj.sum(2, 2)
}

val obj2 = mockk<MockedClass>()
val obj3 = mockk<MockedClass>()
verify {
    listOf(obj2, obj3) wasNot Called
}

Verification timeout

To verify concurrent operations you can use timeout = xxx:

mockk<MockCls> {
    every { sum(1, 2) } returns 4

    Thread {
        Thread.sleep(2000)
        sum(1, 2)
    }.start()

    verify(timeout = 3000) { sum(1, 2) }
}

This will will wait one of two states: either verification is passed or timeout is reached.

Returning Unit

If the function is returning Unit you can use just Runs construct:

class MockedClass {
    fun sum(a: Int, b: Int): Unit {
        println(a + b)
    }
}

val obj = mockk<MockedClass>()

every { obj.sum(any(), 3) } just Runs

obj.sum(1, 1)
obj.sum(1, 2)
obj.sum(1, 3)

verify {
    obj.sum(1, 1)
    obj.sum(1, 2)
    obj.sum(1, 3)
}

Coroutines

To mock coroutines you need to add dependency to the support library.

Gradle
testCompile "org.jetbrains.kotlinx:kotlinx-coroutines-core:x.x"

Maven
<dependency> <groupId>org.jetbrains.kotlinx</groupId> <artifactId>kotlinx-coroutines-core</artifactId> <version>x.x</version> <scope>test</scope> </dependency>

Then you can use coEvery, coVerify, coMatch, coAssert, coRun, coAnswers or coInvoke to mock suspend functions

val car = mockk<Car>()

coEvery { car.drive(Direction.NORTH) } returns Outcome.OK

car.drive(Direction.NORTH) // returns OK

coVerify { car.drive(Direction.NORTH) }

Extension functions

There a 3 cases of extension function:

class wide
object wide
module wide

In case of object and class you can mock extension function just by creating regular mockk:

data class Obj(val value: Int)

class Ext {
    fun Obj.extensionFunc() = value + 5
}

with(mockk<Ext>()) {
    every {
        Obj(5).extensionFunc()
    } returns 11

    assertEquals(11, Obj(5).extensionFunc())

    verify {
        Obj(5).extensionFunc()
    }
}

To mock module wide extension function you need to build mockkStatic(...) with argument specifying module class name. For example "pkg.FileKt" for module "File.kt" in "pkg" package

data class Obj(val value: Int)

// declared in File.kt ("pkg" package)
fun Obj.extensionFunc() = value + 5

mockkStatic("pkg.FileKt")

every {
    Obj(5).extensionFunc()
} returns 11

assertEquals(11, Obj(5).extensionFunc())

verify {
    Obj(5).extensionFunc()
}

Sometimes you need to know a little bit more to mock extension function. For example File.endsWith() extension function has totally unpredictable classname:

   mockkStatic("kotlin.io.FilesKt__UtilsKt")
   every { File("abc").endsWith(any<String>()) } returns true
   println(File("abc").endsWith("abc"))

This is standard Kotlin behaviour that may be unpredictable for user of mocking library. Use Tools -> Kotlin -> Show Kotlin Bytecode or check .class files in JAR archive to detect such names.

Private functions mocking / dynamic calls

In case you have a need to mock private function, you can do it via dynamic call.

class Car {
    fun drive() = accelerate()

    private fun accelerate() = "going faster"
}

val mock = spyk<Car>()

every { mock["accelerate"]() } returns "going not so fast"

assertEquals("going not so fast", mock.drive())

verifySequence {
    mock.drive()
    mock["accelerate"]()
}

In case you want private calls to be verified, you should create spyk with recordPrivateCalls = true

Additionally more verbose syntax allows to get and set properties, do same dynamic calls:

val mock = spyk(Team(), recordPrivateCalls = true)

every { mock getProperty "speed" } returns 33
every { mock setProperty "acceleration" value less(5) } just Runs
every { mock invoke "openDoor" withArguments listOf("left", "rear") } returns "OK"

verify { mock getProperty "speed" }
verify { mock setProperty "acceleration" value less(5) }
verify { mock invoke "openDoor" withArguments listOf("left", "rear") }

Property backing fields

You can access fields backing properties via fieldValue and use value for value being set.

Note in examples below usage of propertyType to specify type of fieldValue. This is needed because it is possible to capture type automatically only for getter. Use nullablePropertyType to specify nullable type.

val mock = spyk(MockCls(), recordPrivateCalls = true)

every { mock.property } answers { fieldValue + 6 }
every { mock.property = any() } propertyType Int::class answers { fieldValue += value }
every { mock getProperty "property" } propertyType Int::class answers { fieldValue + 6 }
every { mock setProperty "property" value any<Int>() } propertyType Int::class answers  { fieldValue += value }
every {
    mock.property = any()
} propertyType Int::class answers {
    fieldValue = value + 1
} andThen {
    fieldValue = value - 1
}

More interfaces

Adding additional behaviours via interfaces and stubbing them:

val spy = spyk(System.out, moreInterfaces = Runnable::class)

spy.println(555)

every {
    (spy as Runnable).run()
} answers {
    (self as PrintStream).println("Run! Run! Run!")
}

val thread = Thread(spy as Runnable)
thread.start()
thread.join()

DSL tables

Here are few tables helping to master the DSL.

Top level functions

Function	Description
`mockk<T>(...)`	builds a regular mock
`spyk<T>()`	builds a spy using default constructor
`spyk(obj)`	builds a spy by copying from `obj`
`slot`	creates capturing slot
`every`	starts stubbing block
`coEvery`	starts stubbing block for coroutines
`verify`	starts verification block
`coVerify`	starts verification block for coroutines
`verifyAll`	starts verification block that should include all calls
`verifyOrder`	starts verification block that checks order
`verifySequence`	starts verification block that checks all calls goes in sepecified sequence
`clearMocks`	clears specified mocks
`registerInstanceFactory`	allow to redefine way of instantiation for certain object
`mockkClass`	builds a regular mock, just class is passed as a parameter
`mockkObject`	makes any object an object mock or clears it if already transformed
`unmockkObject`	makes an object mock regular object
`mockkStatic`	makes static mock out of a class or clears it if already transformed
`unmockkStatic`	makes static mock back a regular class
`clearStaticMockk`	clears static mock
`mockkConstructor`	makes constructor mock out of a class or clears it if already transformed
`unmockkConstructor`	makes constructor mock back a regular class
`clearConstructorMockk`	clears constructor mock
`unmockkAll`	unmock object, static and constructor mocks

Matchers

By default simple arguments are matched using eq()

Matcher	Description
`any()`	matches any argument
`allAny()`	special matcher that uses any() instead of eq() for matchers that are provided as simple arguments
`isNull()`	checks if values is null
`isNull(inverse=true)`	checks if values is not null
`ofType(type)`	checks if values belongs to the type
`match { it.startsWith("string") }`	matches via passed predicate
`coMatch { it.startsWith("string") }`	matches via passed coroutine predicate
`matchNullable { it?.startsWith("string") }`	matches nullable value via passe predicate
`coMatchNullable { it?.startsWith("string") }`	matches nullable value via passed coroutine predicate
`eq(value)`	matches if value is equal to the provided via deepEquals function
`eq(value, inverse=true)`	matches if value is not equal to the provided via deepEquals function
`neq(value)`	matches if value is not equal to the provided via deepEquals function
`refEq(value)`	matches if value is equal to the provided via reference comparation
`refEq(value, inverse=true)`	matches if value is not equal to the provided via reference comparation
`nrefEq(value)`	matches if value is not equal to the provided via reference comparation
`cmpEq(value)`	matches if value is equal to the provided via compareTo function
`less(value)`	matches if value is less to the provided via compareTo function
`more(value)`	matches if value is more to the provided via compareTo function
`less(value, andEquals=false)`	matches if value is less or equals to the provided via compareTo function
`more(value, andEquals=false)`	matches if value is more or equals to the provided via compareTo function
`range(from, to, fromInclusive=true, toInclusive=true)`	matches if value is in range via compareTo function
`and(left, right)`	combines two matchers via logical and
`or(left, right)`	combines two matchers via logical or
`not(matcher)`	negates the matcher
`capture(slot)`	captures a value to a `CapturingSlot`
`capture(mutableList)`	captures a value to a list
`captureNullable(mutableList)`	captures a value to a list together with null values
`captureLambda()`	captures lambda
`captureCoroutine()`	captures coroutine
`invoke(...)`	calls matched argument
`coInvoke(...)`	calls matched argument for coroutine
`hint(cls)`	hints next return type in case it's got erased

Few special matchers available in verification mode only:

Matcher	Description
`run { code }`	matches any value and allows to execute some code
`runNullable { code }`	matches any nullable value and allows to execute some code
`coRun { code }`	matches any value and allows to execute some coroutine code
`coRunNullable { code }`	matches any nullable value and allows to execute some coroutine code
`assert(msg) { predicate }`	matches any value and checks the assertion
`assertNullable(msg) { predicate }`	matches any nullable value and checks the assertion
`coAssert(msg) { predicate }`	matches any value and checks the coroutine assertion
`coAssertNullable(msg) { predicate }`	matches any nullable value and checks the coroutine assertion

Validators

Validator	Description
`verify { mock.call() }`	Do unordered verification that call were performed
`verify(inverse=true) { mock.call() }`	Do unordered verification that call were not performed
`verify(atLeast=n) { mock.call() }`	Do unordered verification that call were performed at least `n` times
`verify(atMost=n) { mock.call() }`	Do unordered verification that call were performed at most `n` times
`verify(exactly=n) { mock.call() }`	Do unordered verification that call were performed at exactly `n` times
`verifyAll { mock.call1(); mock.call2() }`	Do unordered verification that only the specified calls were executed for mentioned mocks
`verifyOrder { mock.call1(); mock.call2() }`	Do verification that sequence of calls went one after another
`verifySequence { mock.call1(); mock.call2() }`	Do verification that only the specified sequence of calls were executed for mentioned mocks
`verify { mock wasNot Called }`	Do verification that mock was not called
`verify { listOf(mock1, mock2) wasNot Called }`	Do verification that list of mocks were not called

Answers

Answer can be followed by one or more additional answers.

Answer	Description
`returns value`	specify that matched call returns one specified value
`returnsMany list`	specify that matched call returns value from the list, returning each time next element
`throws ex`	specify that matched call throws an exception
`answers { code }`	specify that matched call answers with code block scoped with `answer scope`
`coAnswers { code }`	specify that matched call answers with coroutine code block with `answer scope`
`answers answerObj`	specify that matched call answers with Answer object
`answers { nothing }`	specify that matched call answers null
`just Runs`	specify that matched call is returning Unit (returns null)
`propertyType Class`	specify type of backing field accessor
`nullablePropertyType Class`	specify type of backing field accessor as nullable type

Additional answer

Next answer is returned on each consequent call and last value is persisted. So this has similiar to returnsMany semantics.

Addititonal answer	Description
`andThen value`	specify that matched call returns one specified value
`andThenMany list`	specify that matched call returns value from the list, returning each time next element
`andThenThrows ex`	specify that matched call throws an exception
`andThen { code }`	specify that matched call answers with code block scoped with `answer scope`
`coAndThen { code }`	specify that matched call answers with coroutine code block with `answer scope`
`andThenAnswer answerObj`	specify that matched call answers with Answer object
`andThen { nothing }`	specify that matched call answers null

Answer scope

Parameter	Description
`call`	a call object that consists of invocation and matcher
`invocation`	contains information regarding actual function invoked
`matcher`	contains information regarding matcher used to match invocation
`self`	reference the object invocation made
`method`	reference to the function invocation made
`args`	reference to arguments of invocation
`nArgs`	number of invocation argument
`arg(n)`	n-th argument
`firstArg()`	first argument
`secondArg()`	second argument
`thirdArg()`	third argument
`lastArg()`	last argument
`captured()`	the last element in the list for convenience when capturing to the list
`lambda<...>().invoke()`	call captured lambda
`coroutine<...>().coInvoke()`	call captured coroutine
`nothing`	null value for returning nothing as an answer
`fieldValue`	accessor to property backing field
`fieldValueAny`	accessor to property backing field with `Any?` type
`value`	value being set casted to same type as property backing field
`valueAny`	value being set with `Any?` type

Funding

Getting Help

To ask questions, please use stackoverflow or gitter.

Chat/Gitter: https://gitter.im/mockk-io/Lobby
Stack Overflow: http://stackoverflow.com/questions/tagged/mockk

To report bugs, please use the GitHub project.

Project Page: https://github.com/mockk/mockk
Reporting Bugs: https://github.com/mockk/mockk/issues

Kutashov/mockk