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Testing Annotations in Spring & JUnit

When writing tests in a Spring Boot application, you will frequently use a combination of JUnit 5 and Spring Framework annotations.

JUnit 5 Core Annotations​

  • @Test: Marks a method as a test method.
  • @ParameterizedTest: Allows a test to run multiple times with different arguments. Used with @ValueSource, @CsvSource, @MethodSource, etc.
  • @BeforeEach / @AfterEach: Executed before/after each test method in the class. Useful for resetting mocks or preparing test data.
  • @BeforeAll / @AfterAll: Executed once before/after all tests in the class. Must be static unless lifecycle is per-class. Useful for heavy setup like starting a testcontainer.

Parameterized Test Example​

@ParameterizedTest
@ValueSource(strings = {"racecar", "radar", "able was I ere I saw elba"})
void checkPalindromes(String candidate) {
    assertTrue(StringUtils.isPalindrome(candidate));
}

Mockito Annotations​

  • @Mock: Creates a mock instance of a class or interface.
  • @Spy: Creates a spy on a real object. Unlike a mock, a spy calls real methods unless explicitly stubbed.
  • @InjectMocks: Instantiates the tested object and automatically injects @Mock or @Spy fields into it.
@ExtendWith(MockitoExtension.class)
class UserServiceTest {

    @Mock
    private UserRepository userRepository;

    @InjectMocks
    private UserService userService;

    @Test
    void testFindUser() {
        when(userRepository.findById(1L)).thenReturn(Optional.of(new User()));
        assertNotNull(userService.findUser(1L));
    }
}

Spring Boot Test Annotations​

  • @SpringBootTest: Loads the complete Spring application context. Used for integration testing. Can start the server on a defined or random port using webEnvironment.
  • @MockBean (Deprecated in Spring Boot 3.4+ / use @MockitoBean): Replaces an existing bean in the Spring ApplicationContext with a mock.
  • @SpyBean (Deprecated in Spring Boot 3.4+ / use @MockitoSpyBean): Wraps a Spring bean with a Mockito spy, allowing you to stub specific methods while keeping real behavior for others.

Note on Spring Boot 3.4+​

In recent Spring Boot versions, @MockitoBean and @MockitoSpyBean replace the older @MockBean and @SpyBean annotations to better align with the core framework and improve integration test performance by avoiding context caching issues.

Sliced Testing Annotations​

Sometimes loading the full context with @SpringBootTest is too heavy. Spring provides "sliced" annotations that load only specific layers:

  • @WebMvcTest(MyController.class): Loads only the web layer (Controllers, Filters, ControllerAdvice) without the full context. Dependencies must be mocked.
  • @DataJpaTest: Loads only JPA configuration, auto-configures an in-memory database, and wraps tests in a transaction.
  • @JsonTest: Focuses solely on testing JSON serialization and deserialization.