Mockito dependency setup in JUnit - Test Execution Trace

Test Overview

This test checks if Mockito is correctly set up in a JUnit 5 project by verifying that a mock object is created and behaves as expected.

Test Code - JUnit 5 with Mockito

JUnit

import static org.mockito.Mockito.*;
import org.junit.jupiter.api.Test;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;
import org.junit.jupiter.api.extension.ExtendWith;
import static org.junit.jupiter.api.Assertions.*;
import java.util.List;

@ExtendWith(MockitoExtension.class)
public class MockitoSetupTest {

    @Mock
    private List<String> mockedList;

    @Test
    public void testMockCreation() {
        // Arrange
        when(mockedList.size()).thenReturn(3);

        // Act
        int size = mockedList.size();

        // Assert
        assertEquals(3, size, "Mocked list size should be 3");
    }
}

Execution Trace - 5 Steps

Step	Action	System State	Assertion	Result
1	Test starts and JUnit initializes the test class with MockitoExtension	JUnit test runner is ready; MockitoExtension prepares mocks	—	PASS
2	Mockito creates a mock instance of List<String> and injects it into mockedList	mockedList is a Mockito mock object	—	PASS
3	Test sets up behavior: when mockedList.size() is called, return 3	mockedList configured to return 3 for size()	—	PASS
4	Test calls mockedList.size()	mockedList.size() returns 3 as configured	assertEquals(3, size)	PASS
5	Test finishes successfully with assertion passing	Test passed, mock works as expected	assertEquals passed	PASS

Failure Scenario

Failing Condition: MockitoExtension is not registered or Mockito dependency is missing, so mock is not created

Execution Trace Quiz - 3 Questions

What does the @ExtendWith(MockitoExtension.class) annotation do in this test?

A It disables Mockito for this test

B It runs the test in a separate thread

C It initializes Mockito mocks before each test

D It sets the test timeout

Key Result

Always use @ExtendWith(MockitoExtension.class) in JUnit 5 tests to enable Mockito annotations and ensure mocks are properly created before tests run.

Practice

1. What is the main purpose of setting up Mockito as a dependency in a JUnit project?

easy

A. To automatically generate user interfaces

B. To create fake objects for testing without using real implementations

C. To improve the performance of the application in production

D. To deploy the application to a server

Solution

1. Step 1: Understand Mockito's role

   Mockito is a tool used to create mock objects that simulate real objects in tests.

2. Step 2: Identify the purpose in testing

   Using mocks helps isolate the code under test by replacing dependencies with fake versions.

3. Final Answer:

   To create fake objects for testing without using real implementations -> Option B

4. Quick Check:

   Mockito = mock objects for testing [OK]

Hint: Mockito creates fake objects to test code easily [OK]

Common Mistakes:

• Thinking Mockito improves production performance
• Confusing Mockito with UI tools
• Assuming Mockito deploys applications

2. Which of the following is the correct Maven dependency snippet to add Mockito version 4.11.0 for test scope?

easy

A.
org.mockito
mockito-core
4.11.0
test

B.
org.mockito
mockito-core
4.11.0

C.
org.mockito
mockito-all
3.0.0
compile

D.
org.mockito
mockito-core
5.0.0
test

Solution

1. Step 1: Check groupId and artifactId

   The correct groupId is 'org.mockito' and artifactId is 'mockito-core' for Mockito library.

2. Step 2: Verify version and scope

   Version 4.11.0 is correct and scope should be 'test' to use Mockito only during testing.

3. Final Answer:

   Correct Maven dependency with version 4.11.0 and test scope -> Option A

4. Quick Check:

   Maven dependency with test scope = org.mockito mockito-core 4.11.0 test [OK]

Hint: Use 'mockito-core' with test scope and correct version [OK]

Common Mistakes:

• Omitting the test scope
• Using wrong artifactId like 'mockito-all'
• Using incorrect or too new version

3. You want to use Mockito in a JUnit 5 project with Gradle. Which of the following Gradle dependency setups correctly adds Mockito version 4.11.0 only for test implementation?

easy

A. testImplementation 'org.mockito:mockito-core:4.11.0'

B. implementation 'org.mockito:mockito-core:4.11.0'

C. compileOnly 'org.mockito:mockito-core:4.11.0'

D. runtimeOnly 'org.mockito:mockito-core:4.11.0'

Solution

1. Step 1: Understand Gradle scopes

   testImplementation means the dependency is available only during test compilation and runtime.

2. Step 2: Choose correct scope for Mockito

   Mockito is used only in tests, so testImplementation is the correct scope.

3. Final Answer:

   testImplementation 'org.mockito:mockito-core:4.11.0' -> Option A

4. Quick Check:

   Mockito test dependency = testImplementation [OK]

Hint: Use testImplementation for test-only dependencies in Gradle [OK]

Common Mistakes:

• Using implementation instead of testImplementation
• Using compileOnly which excludes runtime
• Using runtimeOnly which excludes compile time

4. Given the following JUnit test code snippet using Mockito, what will be the output of System.out.println(mockService.getData());?

import static org.mockito.Mockito.*;

public class ServiceTest {
  @Test
  void testGetData() {
    DataService mockService = mock(DataService.class);
    when(mockService.getData()).thenReturn("Mocked Data");
    System.out.println(mockService.getData());
  }
}

medium

A. Original data from DataService

B. null

C. Throws NullPointerException

D. Mocked Data

Solution

1. Step 1: Understand mock behavior

   The mock object returns what is specified in when().thenReturn(), here "Mocked Data".

2. Step 2: Check the printed output

   Calling getData() on mockService returns "Mocked Data", so it prints that string.

3. Final Answer:

   Mocked Data -> Option D

4. Quick Check:

   Mockito when().thenReturn() sets output = Mocked Data [OK]

Hint: when().thenReturn() sets mock method output [OK]

Common Mistakes:

• Expecting null because it's a mock
• Assuming real method runs
• Thinking it throws an exception

5. Identify the error in the following Mockito setup code snippet:

import static org.mockito.Mockito.*;

public class UserServiceTest {
  @Mock
  UserRepository repo;

  @Test
  void testUser() {
    when(repo.findUser()).thenReturn(new User("Alice"));
    UserService service = new UserService(repo);
    assertEquals("Alice", service.getUserName());
  }
}

medium

A. Incorrect method name findUser() should be getUser()

B. UserService constructor should not accept repo

C. Missing MockitoAnnotations.openMocks(this) to initialize @Mock fields

D. assertEquals parameters are reversed

Solution

1. Step 1: Check @Mock initialization

   @Mock fields need initialization, usually by calling MockitoAnnotations.openMocks(this) in setup.

2. Step 2: Verify test setup completeness

   Without initialization, repo remains null causing NullPointerException when used.

3. Final Answer:

   Missing MockitoAnnotations.openMocks(this) to initialize @Mock fields -> Option C

4. Quick Check:

   @Mock requires initialization = Missing MockitoAnnotations.openMocks(this) to initialize @Mock fields [OK]

Hint: Always initialize @Mock with MockitoAnnotations.openMocks(this) [OK]

Common Mistakes:

• Forgetting to initialize @Mock fields
• Assuming @Mock works without setup
• Mixing method names without checking