@Mock annotation in JUnit - Test Execution Trace

Test Overview

This test uses the @Mock annotation to create a mock object for a dependency. It verifies that the method under test calls the mocked dependency correctly and returns the expected result.

Test Code - JUnit 5 with Mockito

JUnit

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

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Mock;
import org.mockito.junit.jupiter.MockitoExtension;

@ExtendWith(MockitoExtension.class)
public class UserServiceTest {

    @Mock
    UserRepository userRepository;

    @Test
    void testGetUserName() {
        // Arrange
        UserService userService = new UserService(userRepository);
        when(userRepository.findNameById(1)).thenReturn("Alice");

        // Act
        String name = userService.getUserName(1);

        // Assert
        assertEquals("Alice", name);
        verify(userRepository).findNameById(1);
    }
}

class UserService {
    private final UserRepository userRepository;

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

    public String getUserName(int id) {
        return userRepository.findNameById(id);
    }
}

interface UserRepository {
    String findNameById(int id);
}

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts and MockitoExtension initializes the test class	UserServiceTest instance created with userRepository mocked	-	PASS
2	UserService instance created with mocked userRepository	UserService ready to use mocked dependency	-	PASS
3	Mock userRepository is stubbed to return 'Alice' when findNameById(1) is called	Mock behavior set	-	PASS
4	Call getUserName(1) on UserService	UserService calls mocked userRepository.findNameById(1)	-	PASS
5	Verify returned name is 'Alice'	Result from getUserName is 'Alice'	assertEquals("Alice", name)	PASS
6	Verify userRepository.findNameById(1) was called at least once	Mock interaction recorded	verify(userRepository).findNameById(1)	PASS
7	Test ends successfully	All assertions passed	-	PASS

Failure Scenario

Failing Condition: The mock userRepository does not return the expected value or method is not called

Execution Trace Quiz - 3 Questions

What does the @Mock annotation do in this test?

A Creates a fake UserRepository object to control its behavior

B Runs the real UserRepository code

C Injects a real database connection

D Marks the test method to be ignored

Key Result

Using @Mock allows you to isolate the class under test by replacing dependencies with controllable mock objects, making tests faster and more reliable.