Kotlin Academy articles

Check the series of articles "Mocking is not rocket science" at Kt. Academy describing MockK from the very basics of mocking up to description of all advanced features.

Basics
Expected behavior and behavior verification
MockK features
MockK advanced features

Spring support

springmockk introduced in official Spring Boot Kotlin tutorial

Quarkus support

quarkus-mockk adds support for mocking beans in Quarkus. Documentation can be found here

Kotlin version support

From version 1.13.0 MockK supports Kotlin 1.4 and higher

Known issues

PowerMock needs a workaround to run together with MockK #79. (not sure after workaround if it is generally usable or not, please somebody report it)
Inline functions cannot be mocked: see the discussion on this issue
Spies, mockkStatic may not work on JDK 16+; InaccessibleObjectException/IllegalAccessException: read more here

Table of contents:

auto-gen TOC: {:toc}

Examples, guides & articles

Testing Quarkus with Kotlin, JUnit and MockK
MockK의 흑마술을 파헤치자! (KO)
Unraveling MockK's black magic / MockKの「黒魔術」を解明する (JP, but readable through chrome translator)
Unraveling MockK's black magic(EN, translation)
Mockk Guidebook
“Kotlin Unit Testing with Mockk” by Marco Cattaneo
(Video) Use verify in MockK to validate function calls on mocked object
Testing With MockK paid course on raywenderlich.com
TDD for Android video tutorial part 1, part 2 by Ryan Kay
(Video)Android Developer Live Coding #13: Unit Testing with Mockk, Coroutines, Test Driven Development
MockK: intentions as of Nov 2018
KotlinConf 2018 - Best Practices for Unit Testing in Kotlin by Philipp Hauer
kotlin-fullstack-sample uses MockK project covered with tests
DZone article
Habrahabr article (RU)
Mocking in Kotlin with MockK - Yannick De Turck

Japanese guides and articles

Documentation translation to Japanese

Chinese guides and articles

用 Kotlin + Mockito 寫單元測試會碰到什麼問題？
MockK 功能介紹：mockk, every, Annotation, verify
MockK 功能介紹：Relaxed Mocks, 再談 Verify, Capture
如何測試 Static Method, Singleton

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:${mockkVersion}"
(Kotlin DSL)	testImplementation("io.mockk:mockk:${mockkVersion}")
	<dependency> <groupId>io.mockk</groupId> <artifactId>mockk-jvm</artifactId> <version>${mockkVersion}</version> <scope>test</scope> </dependency>
Unit	testImplementation "io.mockk:mockk-android:${mockkVersion}" testImplementation "io.mockk:mockk-agent:${mockkVersion}"
Instrumented	androidTestImplementation "io.mockk:mockk-android:${mockkVersion}" androidTestImplementation "io.mockk:mockk-agent:${mockkVersion}"

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) }

confirmVerified(car)

Annotations

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

JUnit4

JUnit 4 exposes a rule-based API to allow for some automation following the test lifecycle. MockK includes a rule which uses this to set up and tear down your mocks without needing to manually call MockKAnnotations.init(this). Example:

class CarTest {
  @get:Rule
  val mockkRule = MockKRule(this)

  @MockK
  lateinit var car1: Car

  @RelaxedMockK
  lateinit var car2: Car

  @Test
  fun something() {
     every { car1.drive() } just runs
     every { car2.changeGear(any()) } returns true
     // etc
  }
}

JUnit5

In JUnit5 you can use MockKExtension to initialize your mocks.

@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 the possibility to use @MockK and @RelaxedMockK on test function parameters:

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

Finally, this extension will call unmockkAll in a @AfterAll callback, ensuring your test environment is clean after each test class execution. You can disable this behavior by adding the @MockKExtension.KeepMocks annotation to your class or globally by setting the mockk.junit.extension.keepmocks=true property

Automatic verification confirmation

You can make sure that all stubbed methods are actually verified by also annotating your test class with @MockKExtension.ConfirmVerification.

This will internally call confirmVerified on all mocks after each test, to make sure there are no unnecessary stubbings.

Please note that this behavior may not work as expected when running tests in your IDE, as it is Gradle who takes care of handling the exception being thrown when these confirmVerified calls fail.

Automatic unnecessary stubbing check

You can make sure that all stubbed methods are useful - used at least once - by also annotating your test class with @MockKExtension.CheckUnnecessaryStub.

This will internally call checkUnnecessaryStub on all mocks after each test, to make sure there are no unnecessary stubbings.

Spy

Spies allow you to mix mocks and real objects.

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

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

verify { car.drive(Direction.NORTH) }

confirmVerified(car)

Note 1: the spy object is a copy of the passed object. Note 2: there is a known issue if using a spy with a suspending function: mockk#554

Relaxed mock

A relaxed mock is the mock that returns some simple value for all functions. This allows you to skip specifying behavior for each case, while still stubbing 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) }

confirmVerified(car)

Note: relaxed mocking is working badly with generic return types. A class cast exception is usually thrown in this case. Opt for stubbing manually in the case of a 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 the relaxed mock would throw a class cast exception on the next line
every { func() } returns Car() // or you can return mockk() for example 

func()

Partial mocking

Sometimes, you need to stub some functions, but still call the real method on others, or on specific arguments. This is possible by passing callOriginal() to answers, which works for both relaxed and non-relaxed mocks.

class Adder {
 fun addOne(num: Int) = num + 1
}

val adder = mockk<Adder>()

every { adder.addOne(any()) } returns -1
every { adder.addOne(3) } answers { callOriginal() }

assertEquals(-1, adder.addOne(2))
assertEquals(4, adder.addOne(3)) // original function is called

Mock relaxed for functions returning Unit

If you want Unit-returning functions to be relaxed, you can use relaxUnitFun = true as an argument to the mockk function, @MockKannotation or MockKAnnotations.init function.

Function:

mockk<ClassBeingMocked>(relaxUnitFun = true)

Annotation:

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

MockKAnnotations.init:

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

Object mocks

Objects can be turned into mocks in the following way:

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

mockkObject(ObjBeingMocked) // applies mocking to an Object

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

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

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

To revert back, use unmockkObject or unmockkAll (more destructive: cancels object, static and constructor mocks)

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

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

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

Despite the Kotlin language restrictions, you can create new instances of objects if required by testing logic:

val newObjectMock = mockk<ObjBeingMocked>()

Class mock

Sometimes you need a mock of an arbitrary class. Use mockkClass in those cases.

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 don't own, you need to mock newly created objects. For this purpose, the 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) }

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

In case a class has more than one constructor, each can be mocked separately:

class MockCls(private val a: Int = 0) {
  constructor(x: String) : this(x.toInt())  
  fun add(b: Int) = a + b
}

mockkConstructor(MockCls::class)

every { constructedWith<MockCls>().add(1) } returns 2
every { 
    constructedWith<MockCls>(OfTypeMatcher<String>(String::class)).add(2) // Mocks the constructor which takes a String
} returns 3
every {
    constructedWith<MockCls>(EqMatcher(4)).add(any()) // Mocks the constructor which takes an Int
} returns 4

assertEquals(2, MockCls().add(1))
assertEquals(3, MockCls("2").add(2))
assertEquals(4, MockCls(4).add(7))

verify { 
    constructedWith<MockCls>().add(1)
    constructedWith<MockCls>("2").add(2)
    constructedWith<MockCls>(EqMatcher(4)).add(7)
}

Note that in this case, a prototype mock is created for every set of argument matchers passed to constructedWith.

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

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

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

confirmVerified(car)

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() }

confirmVerified(car)

Note: if the function's return type is generic then the information about the actual type is gone.
To make chained calls work, additional information is required.
Most of the time the framework will catch the cast exception and do autohinting.
In the case it is explicitly required, use hint before making the next call.

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

Hierarchical mocking

From version 1.9.1 mocks may be chained into hierarchies:

interface AddressBook {
    val contacts: List<Contact>
}

interface Contact {
    val name: String
    val telephone: String
    val address: Address
}

interface Address {
    val city: String
    val zip: String
}

val addressBook = mockk<AddressBook> {
    every { contacts } returns listOf(
        mockk {
            every { name } returns "John"
            every { telephone } returns "123-456-789"
            every { address.city } returns "New-York"
            every { address.zip } returns "123-45"
        },
        mockk {
            every { name } returns "Alex"
            every { telephone } returns "789-456-123"
            every { address } returns mockk {
                every { city } returns "Wroclaw"
                every { zip } returns "543-21"
            }
        }
    )
}

Capturing

You can capture an argument to a CapturingSlot or MutableList:

val car = mockk<Car>()

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

every {
  car.recordTelemetry(
    speed = capture(slot), // makes mock match calls with any value for `speed` and record it in a slot
    direction = Direction.NORTH // makes mock and capturing only match calls with specific `direction`. Use `any()` to match calls with any `direction`
  )
} answers {
  println(slot.captured)

  Outcome.RECORDED
}


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

  Outcome.RECORDED
}

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

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

confirmVerified(car)

Verification atLeast, atMost or exactly times

You can check the call count with the 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

confirmVerified(car)

Verification order

verifyAll verifies that all calls happened without checking their order.
verifySequence verifies that the calls happened in a specified sequence.
verifyOrder verifies that calls happened in a specific order.
wasNot Called verifies that the mock (or the 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
}

confirmVerified(obj)

Verification confirmation

To double-check that all calls were verified by verify... constructs, you can use confirmVerified:

confirmVerified(mock1, mock2)

It doesn't make much sense to use it for verifySequence and verifyAll, as these verification methods already exhaustively cover all calls with verification.

It will throw an exception if there are some calls left without verification.

Some calls can be excluded from this confirmation, check the next section for more details.

val car = mockk<Car>()

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

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

verify {
    car.drive(Direction.SOUTH)
    car.drive(Direction.NORTH)
}

confirmVerified(car) // makes sure all calls were covered with verification

Unnecessary stubbing

Because clean & maintainable test code requires zero unnecessary code, you can ensure that there is no unnecessary stubs.

checkUnnecessaryStub(mock1, mock2)

It will throw an exception if there are some declared calls on the mocks that are not used by the tested code. This can happen if you have declared some really unnecessary stubs or if the tested code doesn't call an expected one.

Recording exclusions

To exclude unimportant calls from being recorded, you can use excludeRecords:

excludeRecords { mock.operation(any(), 5) }

All matching calls will be excluded from recording. This may be useful if you are using exhaustive verification: verifyAll, verifySequence or confirmVerified.

val car = mockk<Car>()

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

excludeRecords { car.drive(Direction.SOUTH) }

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

verify {
    car.drive(Direction.NORTH)
}

confirmVerified(car) // car.drive(Direction.SOUTH) was excluded, so confirmation is fine with only car.drive(Direction.NORTH)

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 wait until one of two following states: either verification is passed or the timeout is reached.

Returning Unit

If a function returns Unit, you can use the justRun construct:

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

val obj = mockk<MockedClass>()

justRun { obj.sum(any(), 3) }

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)
}

Other ways to write justRun { obj.sum(any(), 3) }:

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

Coroutines

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

Gradle
testImplementation "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) }

And to simulate a never returning suspend function, you can use coJustAwait:

runTest {
    val car = mockk<Car>()

    coJustAwait { car.drive(any()) } // car.drive(...) will never return

    val job = launch(UnconfinedTestDispatcher()) {
        car.drive(Direction.NORTH)
    }

    coVerify { car.drive(Direction.NORTH) }

    job.cancelAndJoin() // Don't forget to cancel the job
}

Note: there is a known issue if using a spy with a suspending function: mockk#554

Top Level functions

Kotlin lets you declare functions that don’t belong to any class or object, called top-level functions. These calls are translated to static methods in jvm environments, and a special Java class is generated to hold the functions. These top-level functions can be mocked using mockkStatic. You just need to import the function and pass a reference as the argument:

import com.cars.buildCar

val testCar = Car()
mockkStatic(::buildCar)
every { buildCar() } returns testCar

assertEquals(testCar, buildCar())

verify { buildCar() }

Mocking a function doesn't affect other functions declared in the same file.

Extension functions

There are three types of extension function in Kotlin:

class-wide
object-wide
module-wide

For an object or a class, you can mock extension functions just by creating a 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 functions you need to build mockkStatic(...) with the module's class name as an argument. For example "pkg.FileKt" for module File.kt in the 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()
}

In jvm environments you can replace the class name with a function reference:

mockkStatic(Obj::extensionFunc)

Note that this will mock the whole pkg.FileKt class, and not just extensionFunc.

This syntax also applies for extension properties:

val Obj.squareValue get() = value * value

mockkStatic(Obj::squareValue)

If @JvmName is used, specify it as a class name.

KHttp.kt:

@file:JvmName("KHttp")

package khttp
// ... KHttp code

Testing code:

mockkStatic("khttp.KHttp")

Sometimes you need to know a little bit more to mock an extension function. For example the extension function File.endsWith() has a 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. Use Tools -> Kotlin -> Show Kotlin Bytecode or check .class files in JAR archive to detect such names.

Varargs

From version 1.9.1, more extended vararg handling is possible:

interface ClsWithManyMany {
    fun manyMany(vararg x: Any): Int
}

val obj = mockk<ClsWithManyMany>()

every { obj.manyMany(5, 6, *varargAll { it == 7 }) } returns 3

println(obj.manyMany(5, 6, 7)) // 3
println(obj.manyMany(5, 6, 7, 7)) // 3
println(obj.manyMany(5, 6, 7, 7, 7)) // 3

every { obj.manyMany(5, 6, *anyVararg(), 7) } returns 4

println(obj.manyMany(5, 6, 1, 7)) // 4
println(obj.manyMany(5, 6, 2, 3, 7)) // 4
println(obj.manyMany(5, 6, 4, 5, 6, 7)) // 4

every { obj.manyMany(5, 6, *varargAny { nArgs > 5 }, 7) } returns 5

println(obj.manyMany(5, 6, 4, 5, 6, 7)) // 5
println(obj.manyMany(5, 6, 4, 5, 6, 7, 7)) // 5

every {
    obj.manyMany(5, 6, *varargAny {
        if (position < 3) it == 3 else it == 4
    }, 7)
} returns 6

println(obj.manyMany(5, 6, 3, 4, 7)) // 6
println(obj.manyMany(5, 6, 3, 4, 4, 7)) // 6

Private functions mocking / dynamic calls

IF you need to mock private functions, you can do it via a dynamic call.

class Car {
    fun drive() = accelerate()

    private fun accelerate() = "going faster"
}

val mock = spyk<Car>(recordPrivateCalls = true)

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

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

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

If you want to verify private calls, you should create a spyk with recordPrivateCalls = true

Additionally, a more verbose syntax allows you to get and set properties, combined with the same dynamic calls:

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

every { mock getProperty "speed" } returns 33
every { mock setProperty "acceleration" value less(5) } just runs
justRun { mock invokeNoArgs "privateMethod" }
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 the backing fields via fieldValue and use value for the value being set.

Note: in the examples below, we use propertyType to specify the type of the fieldValue. This is needed because it is possible to capture the type automatically for the getter. Use nullablePropertyType to specify a 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
}

Multiple interfaces

Adding additional behaviours via interfaces and stubbing them:

val spy = spyk(System.out, moreInterfaces = arrayOf(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()

Mocking Nothing

Nothing special here. If you have a function returning Nothing:

fun quit(status: Int): Nothing {
    exitProcess(status)
}

Then you can for example throw an exception as behaviour:

every { quit(1) } throws Exception("this is a test")

Clearing vs Unmocking

clear - deletes the internal state of objects associated with a mock, resulting in an empty object
unmock - re-assigns transformation of classes back to original state prior to mock

Matcher extensibility

A very simple way to create new matchers is by attaching a function to MockKMatcherScope or MockKVerificationScope and using the match function:

fun MockKMatcherScope.seqEq(seq: Sequence<String>) = match<Sequence<String>> {
    it.toList() == seq.toList()
}

It's also possible to create more advanced matchers by implementing the Matcher interface.

Custom matchers

Example of a custom matcher that compares list without order:

@Test
fun test() {
    class MockCls {
        fun op(a: List<Int>) = a.reversed()
    }

    val mock = mockk<MockCls>()

    every { mock.op(any()) } returns listOf(5, 6, 9)

    println(mock.op(listOf(1, 2, 3)))

    verify { mock.op(matchListWithoutOrder(3, 2, 1)) }

}

data class ListWithoutOrderMatcher<T>(
    val expectedList: List<T>,
    val refEq: Boolean
) : Matcher<List<T>> {
    val map = buildCountsMap(expectedList, refEq)

    override fun match(arg: List<T>?): Boolean {
        if (arg == null) return false
        return buildCountsMap(arg, refEq) == map
    }

    private fun buildCountsMap(list: List<T>, ref: Boolean): Map<Any?, Int> {
        val map = mutableMapOf<Any?, Int>()

        for (item in list) {
            val key = when {
                item == null -> nullKey
                refEq -> InternalPlatform.ref(item)
                else -> item
            }
            map.compute(key, { _, value -> (value ?: 0) + 1 })
        }

        return map
    }

    override fun toString() = "matchListWithoutOrder($expectedList)"

    @Suppress("UNCHECKED_CAST")
    override fun substitute(map: Map<Any, Any>): Matcher<List<T>> {
        return copy(expectedList = expectedList.map { map.getOrDefault(it as Any?, it) } as List<T>)
    }

    companion object {
        val nullKey = Any()
    }
}

inline fun <reified T : List<E>, E : Any> MockKMatcherScope.matchListWithoutOrder(
    vararg items: E,
    refEq: Boolean = true
): T = match(ListWithoutOrderMatcher(listOf(*items), refEq))

Settings file

To adjust parameters globally, there are a few settings you can specify in a resource file.

How to use:

Create a io/mockk/settings.properties file in src/main/resources.
Put any of the following options:

relaxed=true|false
relaxUnitFun=true|false
recordPrivateCalls=true|false
stackTracesOnVerify=true|false
stackTracesAlignment=left|center

stackTracesAlignment determines whether to align the stack traces to the center (default), or to the left (more consistent with usual JVM stackTraces).

DSL tables

Here are a few tables to help you master the DSL.

Top level functions

Function	Description
`mockk<T>(...)`	builds a regular mock
`spyk<T>()`	builds a spy using the default constructor
`spyk(obj)`	builds a spy by copying from `obj`
`slot`	creates a capturing slot
`every`	starts a stubbing block
`coEvery`	starts a stubbing block for coroutines
`verify`	starts a verification block
`coVerify`	starts a verification block for coroutines
`verifyAll`	starts a verification block that should include all calls
`coVerifyAll`	starts a verification block that should include all calls for coroutines
`verifyOrder`	starts a verification block that checks the order
`coVerifyOrder`	starts a verification block that checks the order for coroutines
`verifySequence`	starts a verification block that checks whether all calls were made in a specified sequence
`coVerifySequence`	starts a verification block that checks whether all calls were made in a specified sequence for coroutines
`excludeRecords`	exclude some calls from being recorded
`confirmVerified`	confirms that all recorded calls were verified
`checkUnnecessaryStub`	confirms that all recorded calls are used at least once
`clearMocks`	clears specified mocks
`registerInstanceFactory`	allows you to redefine the way of instantiation for certain object
`mockkClass`	builds a regular mock by passing the class as parameter
`mockkObject`	turns an object into an object mock, or clears it if was already transformed
`unmockkObject`	turns an object mock back into a regular object
`mockkStatic`	makes a static mock out of a class, or clears it if it was already transformed
`unmockkStatic`	turns a static mock back into a regular class
`clearStaticMockk`	clears a static mock
`mockkConstructor`	makes a constructor mock out of a class, or clears it if it was already transformed
`unmockkConstructor`	turns a constructor mock back into a regular class
`clearConstructorMockk`	clears the constructor mock
`unmockkAll`	unmocks object, static and constructor mocks
`clearAllMocks`	clears regular, 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 the value is null
`isNull(inverse=true)`	checks if the value is not null
`ofType(type)`	checks if the value belongs to the type
`match { it.startsWith("string") }`	matches via the passed predicate
`coMatch { it.startsWith("string") }`	matches via the passed coroutine predicate
`matchNullable { it?.startsWith("string") }`	matches nullable value via the passed predicate
`coMatchNullable { it?.startsWith("string") }`	matches nullable value via the passed coroutine predicate
`eq(value)`	matches if the value is equal to the provided value via the `deepEquals` function
`eq(value, inverse=true)`	matches if the value is not equal to the provided value via the `deepEquals` function
`neq(value)`	matches if the value is not equal to the provided value via the `deepEquals` function
`refEq(value)`	matches if the value is equal to the provided value via reference comparison
`refEq(value, inverse=true)`	matches if the value is not equal to the provided value via reference comparison
`nrefEq(value)`	matches if the value is not equal to the provided value via reference comparison
`cmpEq(value)`	matches if the value is equal to the provided value via the `compareTo` function
`less(value)`	matches if the value is less than the provided value via the `compareTo` function
`more(value)`	matches if the value is more than the provided value via the `compareTo` function
`less(value, andEquals=true)`	matches if the value is less than or equal to the provided value via the `compareTo` function
`more(value, andEquals=true)`	matches if the value is more than or equal to the provided value via the `compareTo` function
`range(from, to, fromInclusive=true, toInclusive=true)`	matches if the value is in range via the `compareTo` function
`and(left, right)`	combines two matchers via a logical and
`or(left, right)`	combines two matchers via a 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 a lambda
`captureCoroutine()`	captures a coroutine
`invoke(...)`	calls a matched argument
`coInvoke(...)`	calls a matched argument for a coroutine
`hint(cls)`	hints the next return type in case it's gotten erased
`anyVararg()`	matches any elements in a vararg
`varargAny(matcher)`	matches if any element matches the matcher
`varargAll(matcher)`	matches if all elements match the matcher
`any...Vararg()`	matches any elements in vararg (specific to primitive type)
`varargAny...(matcher)`	matches if any element matches the matcher (specific to the primitive type)
`varargAll...(matcher)`	matches if all elements match the matcher (specific to the primitive type)

A few special matchers available in verification mode only:

Matcher	Description
`withArg { code }`	matches any value and allows to execute some code
`withNullableArg { code }`	matches any nullable value and allows to execute some code
`coWithArg { code }`	matches any value and allows to execute some coroutine code
`coWithNullableArg { code }`	matches any nullable value and allows to execute some coroutine code

Validators

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

Answers

An Answer can be followed up by one or more additional answers.

Answer	Description
`returns value`	specify that the matched call returns a specified value
`returnsMany list`	specify that the matched call returns a value from the list, with subsequent calls returning the next element
`returnsArgument(n)`	specify that the matched call returns the nth argument of that call
`throws ex`	specify that the matched call throws an exception
`throwsMany ex`	specify that the matched call throws an exception from the list, with subsequent calls throwing the next exception
`answers { code }`	specify that the matched call answers with a code block scoped with `answer scope`
`coAnswers { code }`	specify that the matched call answers with a coroutine code block with `answer scope`
`answers answerObj`	specify that the matched call answers with an Answer object
`answers { nothing }`	specify that the matched call answers null
`just Runs`	specify that the matched call is returning Unit (returns null)
`just Awaits`	specify that the matched call never returns (available since v1.13.3)
`propertyType Class`	specify the type of the backing field accessor
`nullablePropertyType Class`	specify the type of the backing field accessor as a nullable type

Additional answer(s)

A next answer is returned on each consequent call and the last value is persisted. So this is similar to the returnsMany semantics.

Additional answer	Description
`andThen value`	specify that the matched call returns one specified value
`andThenMany list`	specify that the matched call returns a value from the list, with subsequent calls returning the next element
`andThenThrows ex`	specify that the matched call throws an exception
`andThenThrowsMany ex`	specify that the matched call throws an exception from the list, with subsequent calls throwing the next exception
`andThen { code }`	specify that the matched call answers with a code block scoped with `answer scope`
`coAndThen { code }`	specify that the matched call answers with a coroutine code block with `answer scope`
`andThenAnswer answerObj`	specify that the matched call answers with an Answer object
`andThen { nothing }`	specify that the matched call answers null
`andThenJust Runs`	specify that the matched call is returning Unit (available since v1.12.2)
`andThenJust Awaits`	specify that the matched call is never returning (available since v1.13.3)

Answer scope

Parameter	Description
`call`	a call object that consists of an invocation and a matcher
`invocation`	contains information regarding the actual function invoked
`matcher`	contains information regarding the matcher used to match the invocation
`self`	reference to the object invocation made
`method`	reference to the function invocation made
`args`	reference to the invocation arguments
`nArgs`	number of invocation arguments
`arg(n)`	nth 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 a list
`lambda<...>().invoke()`	call the captured lambda
`coroutine<...>().coInvoke()`	call the captured coroutine
`nothing`	null value for returning `nothing` as an answer
`fieldValue`	accessor to the property backing field
`fieldValueAny`	accessor to the property backing field with `Any?` type
`value`	value being set, cast to the same type as the property backing field
`valueAny`	value being set, with `Any?` type
`callOriginal`	calls the original function

Vararg scope

Parameter	Description
`position`	the position of an argument in a vararg array
`nArgs`	overall count of arguments in a vararg array

Funding

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Backers

Thank you to all our backers! 🙏

Contributors

This project exists thanks to all the people who contribute.

Getting Help

To ask questions, please use Stack Overflow 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

Files

README.md

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