unit-test-service-layer

Unit Testing Service Layer with Mockito

Overview

Provides patterns for unit testing @Service classes using Mockito. Mocks repository calls, verifies method invocations, tests exception scenarios, and stubs external API responses. Enables fast, isolated tests without Spring container or database.

When to Use

• Testing business logic in @Service classes
• Mocking repository and external client dependencies
• Verifying service interactions with mocked collaborators
• Testing error handling and edge cases in services
• Writing fast, isolated unit tests (no database, no API calls)

Instructions

Follow this workflow to test service layer with Mockito, including validation checkpoints:

1. Setup Test Class

Use @ExtendWith(MockitoExtension.class) to enable Mockito annotations.

2. Declare Mocks with @Mock and @InjectMocks

Use @Mock for dependencies (repositories, clients) and @InjectMocks for the service under test.

3. Arrange-Act-Assert with Validation

Arrange: Create test data and configure mock return values using when().thenReturn().

Act: Execute the service method being tested.

Assert:

• Verify returned values with AssertJ assertions
• Verify mock interactions with verify()
• Validation checkpoint: Run test and confirm green bar

4. Test Exception Scenarios

Configure mocks to throw exceptions with when().thenThrow().

Validation checkpoint: Verify exception type and message

5. Verify Complete Coverage

• Run full test suite: mvn test or gradle test
• Check coverage report: mvn test jacoco:report
• Validation checkpoint: Confirm all service methods have corresponding tests

Examples

Basic Service Test Pattern

@ExtendWith(MockitoExtension.class)
class UserServiceTest {

  @Mock
  private UserRepository userRepository;

  @InjectMocks
  private UserService userService;

  @Test
  void shouldReturnUserWhenFound() {
    // Arrange
    User expected = new User(1L, "Alice");
    when(userRepository.findById(1L)).thenReturn(Optional.of(expected));

    // Act
    User result = userService.getUser(1L);

    // Assert
    assertThat(result.getName()).isEqualTo("Alice");
    verify(userRepository).findById(1L);
  }

  @Test
  void shouldThrowWhenUserNotFound() {
    // Arrange
    when(userRepository.findById(999L)).thenReturn(Optional.empty());

    // Act & Assert
    assertThatThrownBy(() -> userService.getUser(999L))
      .isInstanceOf(UserNotFoundException.class);
  }
}