Mock objects in JUnit - Test Execution Trace

Test Overview

This test uses a mock object to simulate a dependency and verifies that the service method calls the dependency correctly and returns the expected result.

Test Code - JUnit with Mockito

JUnit

import static org.mockito.Mockito.*;
import static org.junit.jupiter.api.Assertions.*;
import org.junit.jupiter.api.Test;

class UserServiceTest {

    interface UserRepository {
        String getUserEmail(int userId);
    }

    static class UserService {
        private final UserRepository userRepository;

        UserService(UserRepository userRepository) {
            this.userRepository = userRepository;
        }

        String getUserEmail(int userId) {
            return userRepository.getUserEmail(userId);
        }
    }

    @Test
    void testGetUserEmail() {
        // Create mock
        UserRepository mockRepo = mock(UserRepository.class);

        // Define behavior
        when(mockRepo.getUserEmail(1)).thenReturn("[e-mail]");

        // Inject mock into service
        UserService userService = new UserService(mockRepo);

        // Call method
        String email = userService.getUserEmail(1);

        // Verify result
        assertEquals("[e-mail]", email);

        // Verify interaction
        verify(mockRepo).getUserEmail(1);
    }
}

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initialized	-	PASS
2	Creates mock object for UserRepository using Mockito.mock()	Mock UserRepository instance created	-	PASS
3	Defines mock behavior: when getUserEmail(1) is called, return '[e-mail]'	Mock configured to return fixed email for userId 1	-	PASS
4	Creates UserService instance with mock UserRepository injected	UserService ready with mock dependency	-	PASS
5	Calls userService.getUserEmail(1)	Method call routed to mock, returns '[e-mail]'	Returned email equals '[e-mail]'	PASS
6	Verifies that mockRepo.getUserEmail(1) was called exactly once	Mockito verifies interaction	Mock method getUserEmail(1) was called once	PASS
7	Test ends successfully	All assertions passed	-	PASS

Failure Scenario

Failing Condition: Mock method getUserEmail(1) not called or returns unexpected value

Execution Trace Quiz - 3 Questions

What does the mock object simulate in this test?

A The UserService class

B The JUnit test runner

C The UserRepository dependency

D The assertEquals method

Key Result

Using mock objects helps isolate the unit under test by simulating dependencies, allowing focused and reliable testing of behavior without relying on real implementations.