/kotlin-tooling-java-to-kotlin

Mockito to MockK Conversion Guide

When This Applies

Detected when imports match org.mockito.*.

While Mockito works in Kotlin, MockK is the idiomatic Kotlin mocking library. Convert to MockK when the project already uses MockK or is doing a full Kotlin migration. If the project wants to keep Mockito, convert only the Java syntax to Kotlin syntax using the mockito-kotlin helper library.

Key Rules

1. MockK conversion table

2. Coroutine support in MockK

For suspending functions, use coEvery and coVerify instead of every and verify:

coEvery { foo.suspendBar() } returns x
coVerify { foo.suspendBar() }

3. Keeping Mockito (syntax-only conversion)

If keeping Mockito, use the mockito-kotlin library (org.mockito.kotlin) for Kotlin-friendly wrappers:

• mock<Foo>() instead of Mockito.mock(Foo::class.java) — uses reified generics.
• whenever(foo.bar()) instead of Mockito.`when`(foo.bar()) — avoids backtick- escaping when (it is a Kotlin keyword).
• argumentCaptor<T>() — type-safe captor via reified generics.
• any() — properly handles Kotlin's non-null types.

4. Relaxed mocks

MockK supports relaxed mocks that return default values without explicit stubbing: mockk<Foo>(relaxed = true). This has no direct Mockito equivalent (Mockito's RETURNS_DEFAULTS is the closest).

Example 1: Converting to MockK

Java Input

package com.acme.service;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;
import static org.mockito.ArgumentMatchers.anyLong;

import org.junit.Before;
import org.junit.Test;
import org.mockito.ArgumentCaptor;

/**
 * Tests for OrderService using Mockito mocks.
 */
public class OrderServiceTest {

    private UserRepository userRepository;
    private OrderRepository orderRepository;
    private OrderService orderService;

    @Before
    public void setUp() {
        userRepository = mock(UserRepository.class);
        orderRepository = mock(OrderRepository.class);
        orderService = new OrderService(userRepository, orderRepository);
    }

    @Test
    public void testCreateOrderForUser() {
        User user = new User(1L, "Alice");
        when(userRepository.findById(1L)).thenReturn(user);

        orderService.createOrder(1L, "ITEM-100");

        ArgumentCaptor<Order> captor = ArgumentCaptor.forClass(Order.class);
        verify(orderRepository).save(captor.capture());
        assertEquals("ITEM-100", captor.getValue().getItemCode());
        assertEquals(1L, captor.getValue().getUserId());
    }

    @Test
    public void testGetOrderCount() {
        when(orderRepository.countByUserId(anyLong())).thenReturn(5);

        int count = orderService.getOrderCount(1L);

        assertEquals(5, count);
        verify(orderRepository).countByUserId(1L);
    }
}

Kotlin Output (MockK)

package com.acme.service

import io.mockk.every
import io.mockk.mockk
import io.mockk.slot
import io.mockk.verify
import org.junit.jupiter.api.BeforeEach
import org.junit.jupiter.api.Test
import kotlin.test.assertEquals

/**
 * Tests for OrderService using MockK mocks.
 */
class OrderServiceTest {

    private val userRepository = mockk<UserRepository>()
    private val orderRepository = mockk<OrderRepository>()
    private val orderService = OrderService(userRepository, orderRepository)

    @Test
    fun `should create order for user`() {
        val user = User(1L, "Alice")
        every { userRepository.findById(1L) } returns user
        every { orderRepository.save(any()) } returns Unit

        orderService.createOrder(1L, "ITEM-100")

        val orderSlot = slot<Order>()
        verify { orderRepository.save(capture(orderSlot)) }
        assertEquals("ITEM-100", orderSlot.captured.itemCode)
        assertEquals(1L, orderSlot.captured.userId)
    }

    @Test
    fun `should return order count`() {
        every { orderRepository.countByUserId(any()) } returns 5

        val count = orderService.getOrderCount(1L)

        assertEquals(5, count)
        verify { orderRepository.countByUserId(1L) }
    }
}

Key points:

• mock(Foo.class) → mockk<Foo>() using reified generics.
• @Before setUp is eliminated — mocks are initialized inline with property declarations. This works because MockK mocks do not require a runner.
• when(...).thenReturn(...) → every { ... } returns ....
• ArgumentCaptor → slot<T>() with capture(slot), accessed via slot.captured.
• anyLong() → any() (MockK's any() handles all types).
• verify(foo).bar() → verify { foo.bar() }.

Example 2: Keeping Mockito (mockito-kotlin syntax)

Java Input

package com.acme.service;

import static org.junit.Assert.assertEquals;
import static org.mockito.Mockito.mock;
import static org.mockito.Mockito.verify;
import static org.mockito.Mockito.when;

import org.junit.Before;
import org.junit.Test;

/**
 * Tests for PricingService using Mockito.
 */
public class PricingServiceTest {

    private PriceRepository priceRepository;
    private PricingService pricingService;

    @Before
    public void setUp() {
        priceRepository = mock(PriceRepository.class);
        pricingService = new PricingService(priceRepository);
    }

    @Test
    public void testGetPrice() {
        when(priceRepository.findPriceByItemCode("ITEM-1")).thenReturn(9.99);
        double price = pricingService.getPrice("ITEM-1");
        assertEquals(9.99, price, 0.001);
        verify(priceRepository).findPriceByItemCode("ITEM-1");
    }
}

Kotlin Output (mockito-kotlin)

package com.acme.service

import org.junit.jupiter.api.BeforeEach
import org.junit.jupiter.api.Test
import org.mockito.kotlin.mock
import org.mockito.kotlin.verify
import org.mockito.kotlin.whenever
import kotlin.test.assertEquals

/**
 * Tests for PricingService using Mockito.
 */
class PricingServiceTest {

    private val priceRepository = mock<PriceRepository>()
    private val pricingService = PricingService(priceRepository)

    @Test
    fun `should return price for item`() {
        whenever(priceRepository.findPriceByItemCode("ITEM-1")).thenReturn(9.99)

        val price = pricingService.getPrice("ITEM-1")

        assertEquals(9.99, price, 0.001)
        verify(priceRepository).findPriceByItemCode("ITEM-1")
    }
}

Key points:

• mock(Foo.class) → mock<Foo>() from org.mockito.kotlin (reified generics).
• when(...) → whenever(...) to avoid backtick-escaping the when keyword.
• verify stays the same — org.mockito.kotlin.verify wraps Mockito's verify.
• The setUp method is eliminated — mocks are initialized inline.
• assertEquals with a delta parameter works the same way from kotlin.test.