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);
  }
}

Verify Method Invocations

@Test
void shouldSendEmailOnUserCreation() {
  User newUser = new User(1L, "Alice", "alice@example.com");
  when(userRepository.save(any(User.class))).thenReturn(newUser);

  enrichmentService.registerNewUser("Alice", "alice@example.com");

  verify(userRepository).save(any(User.class));
  verify(emailService).sendWelcomeEmail("alice@example.com");
}

For additional patterns (multiple dependencies, argument captors, async services, InOrder verification), see references/examples.md.

Best Practices

• Use @ExtendWith(MockitoExtension.class) for JUnit 5 integration
• Mock only direct dependencies of the service under test
• Verify interactions to ensure correct collaboration
• Test one behavior per test method - keep tests focused
• Use descriptive variable names: expectedUser, actualUser, captor
• Create real instances for value objects and DTOs (don't mock them)

Constraints and Warnings

• Do not mock value objects or DTOs; create real instances with test data.
• Avoid mocking too many dependencies; consider refactoring if a service has too many collaborators.
• Tests must be independent; do not rely on execution order.
• Be cautious with @Spy; partial mocking is harder to understand and maintain.
• Do not test private methods directly; test them through public method behavior.
• Argument matchers (any(), eq()) cannot be mixed with actual values in the same stub.
• Avoid over-verifying; verify only interactions important to the test scenario.

References

• Mockito Documentation
• JUnit 5 User Guide
• AssertJ Assertions