How to mock a Kotlin singleton object?

Question

Given a Kotlin singleton object and a fun that call it's method

object SomeObject {
   fun someFun() {}
}

fun callerFun() {
   SomeObject.someFun()
}

Is there a way to mock call to SomeObject.someFun()?

Answer 1

Just make you object implement an interface, than you can mock you object with any mocking library. Here example of Junit + Mockito + Mockito-Kotlin:

import com.nhaarman.mockito_kotlin.mock
import com.nhaarman.mockito_kotlin.whenever
import org.junit.Assert.assertEquals
import org.junit.Test

object SomeObject : SomeInterface {
    override fun someFun():String {
        return ""
    }
}

interface SomeInterface {
    fun someFun():String
}

class SampleTest {

    @Test
    fun test_with_mock() {
        val mock = mock<SomeInterface>()

        whenever(mock.someFun()).thenReturn("42")

        val answer = mock.someFun()

        assertEquals("42", answer)
    }
}

Or in case if you want mock SomeObject inside callerFun:

import com.nhaarman.mockito_kotlin.mock
import com.nhaarman.mockito_kotlin.whenever
import org.junit.Assert.assertEquals
import org.junit.Test

object SomeObject : SomeInterface {
    override fun someFun():String {
        return ""
    }
}

class Caller(val someInterface: SomeInterface) {
    fun callerFun():String {
        return "Test ${someInterface.someFun()}"
    }
}

// Example of use
val test = Caller(SomeObject).callerFun()

interface SomeInterface {
    fun someFun():String
}

class SampleTest {

    @Test
    fun test_with_mock() {
        val mock = mock<SomeInterface>()
        val caller = Caller(mock)

        whenever(mock.someFun()).thenReturn("42")

        val answer = caller.callerFun()

        assertEquals("Test 42", answer)
    }
}

Answer 2

There's a very nice mocking library for Kotlin - Mockk, which allows you to mock objects, the exact same way you're desiring.

As of it's documentation:

---

Objects can be transformed to mocks following way:

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

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

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

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

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

val newObjectMock = mockk<MockObj>()

Answer 3

You can mock Object without any extra library, by using class delegates.

Here is my proposal

val someObjectDelegate : SomeInterface? = null

object SomeObject: by someObjectDelegate ?: SomeObjectImpl

object SomeObjectImpl : SomeInterface {

    fun someFun() {
        println("SomeObjectImpl someFun called")
    }
}

interface SomeInterface {
    fun someFun()
}

In your tests you can set delegate object that will change behaviour, otherwise it will use it's real implementation.

@Beofre
fun setUp() {
  someObjectDelegate = object : SomeInterface {
      fun someFun() {
          println("Mocked function")
      }
  }
  // Will call method from your delegate
  SomeObject.someFun()
}

Of course names above are bad, but for the sake of an example it shows the purpose.

After SomeObject is initialised delegate will handle all the functions.
More you can find in official documentation

Answer 4

Besides using Mockk library, which is quite convenient, one could mock an object simply with Mockito and reflection. A Kotlin object is just a regular Java class with a private constructor and an INSTANCE static field, with reflection one can replace the value of INSTANCE with a mocked object. After the test the original should be restored so that the change won't affect other tests

Using Mockito Kotlin (one needs to add an extension configuration as described here to mock final classes):

testCompile "com.nhaarman:mockito-kotlin:1.5.0"

A first fun could replace the value of the static INSTANCE field in the object class and return the previous value

fun <T> replaceObjectInstance(clazz: Class<T>, newInstance: T): T {

    if (!clazz.declaredFields.any {
                it.name == "INSTANCE" && it.type == clazz && Modifier.isStatic(it.modifiers)
            }) {
        throw InstantiationException("clazz ${clazz.canonicalName} does not have a static  " +
                "INSTANCE field, is it really a Kotlin \"object\"?")
    }

    val instanceField = clazz.getDeclaredField("INSTANCE")
    val modifiersField = Field::class.java.getDeclaredField("modifiers")
    modifiersField.isAccessible = true
    modifiersField.setInt(instanceField, instanceField.modifiers and Modifier.FINAL.inv())

    instanceField.isAccessible = true
    val originalInstance = instanceField.get(null) as T
    instanceField.set(null, newInstance)
    return originalInstance
}

Then you could have a fun that will create a mock instance of the object and replace the original value with the mocked one, returning the original so that it can be reset later

fun <T> mockObject(clazz: Class<T>): T {
    val constructor = clazz.declaredConstructors.find { it.parameterCount == 0 }
            ?: throw InstantiationException("class ${clazz.canonicalName} has no empty constructor, " +
                    "is it really a Kotlin \"object\"?")

    constructor.isAccessible = true

    val mockedInstance = spy(constructor.newInstance() as T)

    return replaceObjectInstance(clazz, mockedInstance)
}

Add some Kotlin sugar

class MockedScope<T : Any>(private val clazz: Class<T>) {

    fun test(block: () -> Unit) {
        val originalInstance = mockObject(clazz)
        block.invoke()
        replaceObjectInstance(clazz, originalInstance)
    }
}

fun <T : Any> withMockObject(clazz: Class<T>) = MockedScope(clazz)

And finally, given an object

object Foo {
    fun bar(arg: String) = 0
}

You could test it this way

withMockObject(Foo.javaClass).test {
    doAnswer { 1 }.whenever(Foo).bar(any())

    Assert.assertEquals(1, Foo.bar(""))
}

Assert.assertEquals(0, Foo.bar(""))

Answer 5

Short of manipulating byte code the answer is no, unless you are willing and able to change the code. The most straightforward way (and the way I would recommend) to mock callerFun's call to SomeObject.someFun() is to provide some way to slip it a mock object.

e.g.

object SomeObject {
    fun someFun() {}
}

fun callerFun() {
    _callerFun { SomeObject.someFun() }
}

internal inline fun _callerFun(caller: () -> Unit) {
    caller()
}

The idea here is to change something you're willing to change. If you're certain you want a singleton and a top-level function that acts on that singleton then one way, as demonstrated above, to make the top-level function testable without changing its public signature is to move its implementation to an internal function that allows slipping a mock.