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December 31, 2024

Unit Testing in Kotlin with MockK

mockk

springboot

test

Contents

Mock Any Instance

mockkConstructor(StudentPackage::class)
every { anyConstructed<StudentPackage>().getNumOfClasses() } returns 42
every { anyConstructed<StudentPackage>().getCourseId() } returns 123

A more practical example is to mock the Joda DateTime object:

val fixedTime = 1640995200000L
mockkConstructor(DateTime::class)
every { anyConstructed<DateTime>().millis } returns fixedTime

Now we can unit-test any "current time" related logic, any instance of

DateTime().millis

will return fixedTime.

Partially Mock an Object

Sometimes we wish to keep the real implementation of an object with only some methods / attributes being faked. Then:

val studentPackage = spyk<StudentPackage>(StudentPackage(
    startDate = 0L.toDouble(),  // Use dummy values for constructor
    paidAt = 0L.toDouble(),
    officialEndDate = 0L.toDouble(),
    expiryDate = 0L.toDouble(),
    min = 0,
    courseId = 0,
    createdAt = 0L.toDouble(),
    createdAtHk = "",
    numOfClasses = 0,
    defaultClassroom = Classroom.CAUSEWAY_BAY,
    uuid = UUID.randomUUID(),
    id = 0
))

Now we can override studentPackage partially by:

every { spyPackage.getNumOfClasses() } returns 42

When can we use every {}?

Note that every {} can only be applied on objects created through MockK methods:

  1. mockk() - fully mocked object
  2. spyk() - real object with mocking capabilities
  3. mockkConstructor() - mocks all instances of a class
  4. mockkObject() - for mocking Kotlin object (singleton)
  5. mockkStatic() - for mocking static methods

Mock an object with all default dummy implementation

Sometimes we are simply interested in how some attributes of an object affect the result:

// relaxed = true => every method/field have default dummy value
val mockClass: Class = mockk(relaxed = true)
every { mockClass.id } returns 1
every { mockClass.hourUnixTimestamp } returns (fixedTime - 3600000L).toDouble()

Complete Test Example

@Test
fun `no class in the past can be added`() {
    val fixedTime = 1640995200000L // 2022-01-01 00:00:00
    mockkConstructor(DateTime::class)
    every { anyConstructed<DateTime>().millis } returns fixedTime

    val studentPackage = StudentPackage(
        startDate = 0L.toDouble(),
        paidAt = 0L.toDouble(),
        officialEndDate = 0L.toDouble(),
        expiryDate = 0L.toDouble(),
        min = 0,
        courseId = 0,
        createdAt = 0L.toDouble(),
        createdAtHk = "",
        numOfClasses = 0,
        defaultClassroom = Classroom.CAUSEWAY_BAY,
        uuid = UUID.randomUUID(),
        id = 0
    )

    val mockClass: Class = mockk(relaxed = true)
    every { mockClass.id } returns 1
    every { mockClass.hourUnixTimestamp } returns (fixedTime - 3600000L).toDouble()

    val exception = assertThrows<TimetableException> {
        studentPackage.addClasses(listOf(mockClass))
    }
    assertEquals("Only classes in the future can be created.", exception.message)
}