Testing Annotations in Spring & JUnit

Who this guide is for

• New learners — start at The Big Picture to understand how annotations fit together before diving into specifics.
• Senior engineers — jump to Annotation Decision Matrix, Spring Boot 3.4+ Changes, or Interview Questions.

---

The Big Picture

When writing tests in a Spring Boot project, you deal with three layers of annotations:

┌────────────────────────────────────────────────────────────┐
│  Layer 1: JUnit 5 (Test Runner)                            │
│  @Test, @ParameterizedTest, @BeforeEach, @DisplayName      │
│  → Controls WHAT runs, WHEN, and HOW                       │
├────────────────────────────────────────────────────────────┤
│  Layer 2: Mockito (Test Doubles)                           │
│  @Mock, @Spy, @InjectMocks, @Captor                        │
│  → Controls DEPENDENCIES — replace real objects with fakes │
├────────────────────────────────────────────────────────────┤
│  Layer 3: Spring Boot Test (Application Context)           │
│  @SpringBootTest, @WebMvcTest, @DataJpaTest, @MockitoBean  │
│  → Controls SPRING CONTEXT — how much of the app loads     │
└────────────────────────────────────────────────────────────┘

Rule of Thumb

• Unit tests → Layer 1 + Layer 2 only (no Spring context → fast ⚡)
• Integration tests → Layer 1 + Layer 3 (Spring context → slower 🐢, but tests real wiring)

---

Layer 1: JUnit 5 Core Annotations

Lifecycle Annotations

class OrderServiceTest {

    @BeforeAll   // Runs ONCE before all tests (must be static)
    static void setupOnce() {
        // Expensive one-time setup: start Testcontainers, load CSV, etc.
    }

    @BeforeEach  // Runs BEFORE each test method
    void setup() {
        // Reset mocks, prepare test data, clean state
    }

    @Test
    void shouldCalculateTotal() { /* test logic */ }

    @AfterEach   // Runs AFTER each test method
    void tearDown() {
        // Close resources, clear caches
    }

    @AfterAll    // Runs ONCE after all tests (must be static)
    static void cleanupOnce() {
        // Stop containers, delete temp files
    }
}

Execution order for 2 test methods:

  @BeforeAll ──────────────────────────────────────────
       │
       ├── @BeforeEach → @Test (test1) → @AfterEach
       │
       ├── @BeforeEach → @Test (test2) → @AfterEach
       │
  @AfterAll ───────────────────────────────────────────

Test Annotations

Annotation	Purpose	Example
@Test	Marks a single test method	@Test void shouldReturnUser()
@ParameterizedTest	Runs the same test with different inputs	See example below
@RepeatedTest(5)	Runs the test 5 times (flaky test detection)	@RepeatedTest(5) void stressTest()
@DisplayName	Human-readable test name in reports	@DisplayName("Should reject expired coupons")
@Disabled	Skips a test (with reason)	@Disabled("Bug #1234 — fix pending")
@Timeout(5)	Fails if test takes longer than 5 seconds	@Timeout(value = 5, unit = SECONDS)
@Tag("slow")	Categorize tests for selective execution	Filter in Maven: -Dgroups=slow
@Nested	Group related tests in inner classes	Organize by scenario

Parameterized Tests — Test Many Inputs

// ── @ValueSource — simple single-value inputs ────────────
@ParameterizedTest(name = "isPalindrome({0})")
@ValueSource(strings = {"racecar", "radar", "level", "madam"})
void shouldDetectPalindromes(String word) {
    assertTrue(StringUtils.isPalindrome(word));
}

// ── @CsvSource — multiple arguments per test ─────────────
@ParameterizedTest(name = "add({0}, {1}) = {2}")
@CsvSource({
    "1,    1,    2",
    "0,    0,    0",
    "-1,   1,    0",
    "100,  200,  300"
})
void shouldAddNumbers(int a, int b, int expected) {
    assertEquals(expected, calculator.add(a, b));
}

// ── @MethodSource — complex objects as test data ─────────
@ParameterizedTest
@MethodSource("provideInvalidEmails")
void shouldRejectInvalidEmails(String email, String reason) {
    ValidationResult result = validator.validate(email);
    assertFalse(result.isValid(), "Expected invalid: " + reason);
}

static Stream<Arguments> provideInvalidEmails() {
    return Stream.of(
        Arguments.of("",                "empty string"),
        Arguments.of("no-at-sign",      "missing @"),
        Arguments.of("@no-local.com",   "missing local part"),
        Arguments.of("spaces [email protected]", "contains spaces"),
        Arguments.of(null,              "null value")
    );
}

// ── @EnumSource — test all enum values ───────────────────
@ParameterizedTest
@EnumSource(OrderStatus.class)
void allStatusesShouldHaveDisplayName(OrderStatus status) {
    assertNotNull(status.getDisplayName());
    assertFalse(status.getDisplayName().isBlank());
}

Nested Tests — Organize by Scenario

@DisplayName("OrderService")
class OrderServiceTest {

    @Nested
    @DisplayName("when creating an order")
    class WhenCreatingOrder {

        @Test
        @DisplayName("should save to database")
        void shouldSave() { /* ... */ }

        @Test
        @DisplayName("should publish OrderCreated event")
        void shouldPublishEvent() { /* ... */ }

        @Nested
        @DisplayName("with invalid input")
        class WithInvalidInput {

            @Test
            @DisplayName("should throw on null product")
            void shouldThrowOnNull() { /* ... */ }

            @Test
            @DisplayName("should throw on negative quantity")
            void shouldThrowOnNegative() { /* ... */ }
        }
    }
}

Report output:

OrderService
  ├── when creating an order
  │   ├── ✅ should save to database
  │   ├── ✅ should publish OrderCreated event
  │   └── with invalid input
  │       ├── ✅ should throw on null product
  │       └── ✅ should throw on negative quantity

---

Layer 2: Mockito Annotations

Annotation	What It Creates	When To Use
@Mock	A mock (all methods return defaults)	Replace a dependency you don't want to call
@Spy	A spy wrapping a real object	Need real behavior + override one method
@InjectMocks	Real object with mocks injected	Auto-wire @Mock/@Spy into the class under test
@Captor	An ArgumentCaptor	Capture arguments passed to a mock for detailed assertions

Always use @ExtendWith(MockitoExtension.class)

Without this, @Mock, @Spy, and @InjectMocks do nothing — they are not initialized. This is the #1 mistake beginners make.

Complete Example with @Captor

@ExtendWith(MockitoExtension.class)
class NotificationServiceTest {

    @Mock private EmailGateway emailGateway;
    @Mock private UserRepository userRepo;
    @Captor private ArgumentCaptor<EmailMessage> emailCaptor;
    @InjectMocks private NotificationService notificationService;

    @Test
    void sendOrderConfirmation_composesCorrectEmail() {
        // Arrange
        User user = new User("alice", "[email protected]");
        Order order = new Order("ORD-001", new BigDecimal("99.99"));
        when(userRepo.findById("alice")).thenReturn(Optional.of(user));

        // Act
        notificationService.sendOrderConfirmation("alice", order);

        // Assert — capture the actual email that was sent
        verify(emailGateway).send(emailCaptor.capture());
        EmailMessage sentEmail = emailCaptor.getValue();

        assertEquals("[email protected]", sentEmail.getTo());
        assertThat(sentEmail.getSubject()).contains("ORD-001");
        assertThat(sentEmail.getBody()).contains("$99.99");
    }

    @Test
    void sendOrderConfirmation_userNotFound_doesNotSendEmail() {
        when(userRepo.findById("unknown")).thenReturn(Optional.empty());

        assertThrows(UserNotFoundException.class,
            () -> notificationService.sendOrderConfirmation("unknown", anyOrder()));

        // Verify email was NEVER sent
        verifyNoInteractions(emailGateway);
    }
}

Common Mockito Methods Reference

// ── Stubbing ──────────────────────────────────────────────
when(mock.method(arg)).thenReturn(value);          // return value
when(mock.method(arg)).thenThrow(new RuntimeException());  // throw
when(mock.method(arg)).thenAnswer(invocation -> {  // dynamic
    String arg0 = invocation.getArgument(0);
    return arg0.toUpperCase();
});

// ── Verification ──────────────────────────────────────────
verify(mock).method(arg);                     // called exactly once
verify(mock, times(3)).method(arg);           // called exactly 3 times
verify(mock, never()).method(arg);            // never called
verify(mock, atLeastOnce()).method(any());    // at least once
verifyNoMoreInteractions(mock);               // no other calls
verifyNoInteractions(mock);                   // zero calls total

// ── Argument Matchers ─────────────────────────────────────
when(repo.findById(any())).thenReturn(...);   // any argument
when(repo.findByName(eq("alice"))).thenReturn(...); // exact value
when(repo.findByAge(anyInt())).thenReturn(...);
when(repo.findByName(argThat(name -> name.startsWith("A")))).thenReturn(...);

---

Layer 3: Spring Boot Test Annotations

@SpringBootTest — Full Application Context

// Loads the ENTIRE Spring context — all beans, all config
// Use for end-to-end integration tests
@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
class ApplicationIntegrationTest {

    @Autowired
    private TestRestTemplate restTemplate;

    @Test
    void healthCheck_returns200() {
        ResponseEntity<String> response =
            restTemplate.getForEntity("/actuator/health", String.class);
        assertEquals(HttpStatus.OK, response.getStatusCode());
    }
}

webEnvironment Option	Behavior
MOCK (default)	Simulated servlet environment — use MockMvc
RANDOM_PORT	Starts real HTTP server on random port — use TestRestTemplate
DEFINED_PORT	Starts server on server.port
NONE	No web environment at all — service-layer tests

Sliced Test Annotations — Load Only What You Need

Full context (@SpringBootTest):
  ┌──────────────────────────────────────────────────────┐
  │ Controllers + Services + Repos + Config + Security + │
  │ Kafka + Redis + Scheduling + ...                     │
  └──────────────────────────────────────────────────────┘
  ⏱️ Startup time: 5-30 seconds

@WebMvcTest slice:
  ┌───────────────────────────────────┐
  │ Controllers + Filters + Advice   │
  └───────────────────────────────────┘
  ⏱️ Startup time: 1-3 seconds

@DataJpaTest slice:
  ┌───────────────────────────────────┐
  │ Repositories + JPA + In-mem DB   │
  └───────────────────────────────────┘
  ⏱️ Startup time: 2-5 seconds

@WebMvcTest — Controller Layer Only

@WebMvcTest(OrderController.class)   // only loads OrderController + web layer
class OrderControllerTest {

    @Autowired private MockMvc mockMvc;

    @MockitoBean    // Spring Boot 3.4+
    private OrderService orderService;

    @Test
    void getOrder_found_returns200() throws Exception {
        when(orderService.findById("ORD-1"))
            .thenReturn(Optional.of(new OrderDto("ORD-1", "SHIPPED")));

        mockMvc.perform(get("/api/orders/ORD-1"))
            .andExpect(status().isOk())
            .andExpect(jsonPath("$.orderId").value("ORD-1"))
            .andExpect(jsonPath("$.status").value("SHIPPED"));
    }

    @Test
    void getOrder_notFound_returns404() throws Exception {
        when(orderService.findById("UNKNOWN"))
            .thenReturn(Optional.empty());

        mockMvc.perform(get("/api/orders/UNKNOWN"))
            .andExpect(status().isNotFound());
    }
}

Common @WebMvcTest Mistake

@WebMvcTest does NOT load @Service, @Repository, or @Component beans. If your controller depends on OrderService, you must provide a @MockitoBean for it. Forgetting this causes NoSuchBeanDefinitionException.

@DataJpaTest — Repository Layer Only

@DataJpaTest     // auto-configures: JPA, in-memory H2, @Entity scanning
@AutoConfigureTestDatabase(replace = Replace.NONE)  // use Testcontainers instead
class OrderRepositoryTest {

    @Autowired private OrderRepository orderRepo;
    @Autowired private TestEntityManager em;

    @Test
    void findByStatus_returnsMatchingOrders() {
        em.persist(new Order("PROD-1", 2, "PENDING"));
        em.persist(new Order("PROD-2", 1, "SHIPPED"));
        em.persist(new Order("PROD-3", 3, "PENDING"));
        em.flush();

        List<Order> pending = orderRepo.findByStatus("PENDING");

        assertEquals(2, pending.size());
        assertTrue(pending.stream().allMatch(o -> o.getStatus().equals("PENDING")));
    }

    @Test
    void customQuery_calculatesRevenueByCategory() {
        // Test your @Query methods against real SQL
        em.persist(new Order("laptop", 1, "COMPLETED", new BigDecimal("1299")));
        em.persist(new Order("laptop", 1, "COMPLETED", new BigDecimal("999")));
        em.flush();

        BigDecimal revenue = orderRepo.calculateRevenueByCategory("laptop");
        assertEquals(new BigDecimal("2298"), revenue);
    }
}

@JsonTest — Serialization/Deserialization Only

@JsonTest
class OrderDtoJsonTest {

    @Autowired private JacksonTester<OrderDto> json;

    @Test
    void serialize_producesExpectedJson() throws Exception {
        OrderDto dto = new OrderDto("ORD-1", "SHIPPED", LocalDate.of(2024, 1, 15));

        assertThat(json.write(dto))
            .extractingJsonPathStringValue("$.orderId").isEqualTo("ORD-1")
            .extractingJsonPathStringValue("$.status").isEqualTo("SHIPPED")
            .extractingJsonPathStringValue("$.date").isEqualTo("2024-01-15");
    }

    @Test
    void deserialize_parsesJsonCorrectly() throws Exception {
        String content = """
            {"orderId": "ORD-1", "status": "SHIPPED", "date": "2024-01-15"}
            """;

        assertThat(json.parse(content))
            .isEqualTo(new OrderDto("ORD-1", "SHIPPED", LocalDate.of(2024, 1, 15)));
    }
}

---

📋 Annotation Decision Matrix

Use this table to pick the right annotation for your test scenario:

I want to test...	Annotation	Mocking	Context	Speed
Pure business logic	@ExtendWith(MockitoExtension.class)	@Mock / @InjectMocks	None	⚡ ~ms
Controller endpoints	@WebMvcTest	@MockitoBean	Web slice	🐢 ~2s
JPA repository queries	@DataJpaTest	—	JPA slice	🐢 ~3s
JSON serialization	@JsonTest	—	JSON slice	⚡ ~1s
Full app wiring	@SpringBootTest	@MockitoBean (optional)	Full	🐌 ~10s
External HTTP APIs	@SpringBootTest + WireMock	—	Full + mock server	🐌 ~10s
Kafka consumer/producer	@SpringBootTest + @EmbeddedKafka	—	Full + embedded	🐌 ~15s

---

🔄 Spring Boot 3.4+ Migration

Spring Boot 3.4 introduced new annotations to replace the older @MockBean and @SpyBean:

Old (Deprecated)	New (Spring Boot 3.4+)	Change
@MockBean	@MockitoBean	Better context caching, clearer naming
@SpyBean	@MockitoSpyBean	Same improvements

// ── Before (Spring Boot < 3.4) ────────────────────────────
@SpringBootTest
class OldStyleTest {
    @MockBean private PaymentClient paymentClient;   // deprecated
    @SpyBean private OrderService orderService;      // deprecated
}

// ── After (Spring Boot 3.4+) ──────────────────────────────
@SpringBootTest
class NewStyleTest {
    @MockitoBean private PaymentClient paymentClient;     // ✅
    @MockitoSpyBean private OrderService orderService;    // ✅
}

Why The Change?

The old @MockBean sometimes caused context caching issues — Spring would create separate application contexts for tests that use different @MockBean combinations, dramatically slowing down test suites. The new annotations are designed to integrate more cleanly with Spring's test context framework.

---

Interview Questions

For New Learners

Q: What is the difference between @Mock and @MockitoBean?

@Mock (Mockito) creates a mock in plain unit tests — no Spring context is involved. @MockitoBean (Spring Boot) replaces an actual bean in the Spring ApplicationContext with a mock — used in integration tests where Spring manages the wiring.

Q: Why use @WebMvcTest instead of @SpringBootTest for controller tests?

@WebMvcTest loads only the web layer (controllers, filters, advice) — starting in ~2 seconds. @SpringBootTest loads the entire application context (services, repos, configs, Kafka, Redis...) — starting in 10+ seconds. Use @WebMvcTest for speed when you only need to test HTTP request/response behavior.

Q: What does @BeforeEach do?

It marks a method that runs before every single @Test method in the class. Use it to reset mocks, prepare test data, or set up clean state so tests don't affect each other.

For Senior Engineers

Q: How do you choose between @Mock, @MockitoBean, and @MockitoSpyBean?

Use @Mock in pure unit tests (no Spring context) — fastest. Use @MockitoBean in integration tests when you need to replace a Spring-managed bean entirely (e.g., mock an external payment gateway). Use @MockitoSpyBean when you need the real bean behavior but want to verify or override one specific method (e.g., spy on a service to verify event publishing while keeping real business logic).

Q: Why can excessive @SpringBootTest usage slow teams down?

Each unique combination of @MockitoBean declarations creates a separate Spring context. If 50 test classes each mock different beans, Spring may create 50 separate contexts — each taking 10+ seconds to start. Fix: standardize mock combinations using a shared base class, or prefer @WebMvcTest/@DataJpaTest slices.

Q: What signals indicate your test annotation strategy is wrong?

(1) Test suite takes 15+ minutes to run locally. (2) Tests fail when reordered. (3) Every test class uses @SpringBootTest even for pure logic. (4) You see ApplicationContext was cached X times warnings with high X values. (5) Developers skip running tests locally because they're too slow.

Q: How should you handle deprecated @MockBean during a Spring Boot upgrade?

Migrate incrementally: start with new test classes using @MockitoBean, then update existing tests component-by-component. Both annotations work simultaneously during the migration window. Add a checkstyle/lint rule to prevent new usages of @MockBean and track migration progress.

Q: How do you test configuration properties safely?

Use @SpringBootTest(classes = {MyConfig.class}) with minimal config scope instead of loading the full app. Test binding, defaults, validation (@Validated), and profile-specific overrides (@ActiveProfiles("test")). Use @ConfigurationPropertiesTest for focused property binding tests.