@MockBean for Spring mocking in JUnit - Test Execution Trace

Test Overview

This test verifies that a Spring service method returns the expected value by mocking a dependency using @MockBean. It checks that the service uses the mocked repository correctly.

Test Code - JUnit

JUnit

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.mockito.Mockito.when;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.boot.test.mock.mockito.MockBean;

@SpringBootTest
class UserServiceTest {

    @MockBean
    private UserRepository userRepository;

    @Autowired
    private UserService userService;

    @Test
    void testGetUserNameById() {
        // Arrange
        when(userRepository.findNameById(1L)).thenReturn("Alice");

        // Act
        String name = userService.getUserNameById(1L);

        // Assert
        assertEquals("Alice", name);
    }
}

// Supporting classes
interface UserRepository {
    String findNameById(Long id);
}

class UserService {
    private final UserRepository userRepository;

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

    public String getUserNameById(Long id) {
        return userRepository.findNameById(id);
    }
}

Execution Trace - 7 Steps

Step	Action	System State	Assertion	Result
1	Test starts	JUnit test runner initializes Spring context with @MockBean replacing UserRepository bean	-	PASS
2	Spring injects mocked UserRepository into UserService	UserService bean uses mocked UserRepository	-	PASS
3	Test sets up mock behavior: when findNameById(1L) called, return 'Alice'	Mock configured to return 'Alice' for id 1	-	PASS
4	Test calls userService.getUserNameById(1L)	UserService calls mocked UserRepository.findNameById(1L)	-	PASS
5	Mock returns 'Alice' as configured	Method returns 'Alice'	Check that returned name equals 'Alice'	PASS
6	Test asserts returned name is 'Alice'	Assertion compares expected 'Alice' with actual 'Alice'	assertEquals('Alice', name)	PASS
7	Test finishes successfully	Test passes with no errors	-	PASS

Failure Scenario

Failing Condition: Mock behavior not set or returns null, causing assertion to fail

Execution Trace Quiz - 3 Questions

What does the @MockBean annotation do in this test?

A Injects UserService into UserRepository

B Replaces the UserRepository bean with a mock in the Spring context

C Creates a real instance of UserRepository

D Runs the test without Spring context

Key Result

Using @MockBean in Spring tests allows you to replace real beans with mocks easily, so you can control dependencies and test service logic in isolation.