Spring Boot Testing

Spring Boot Testing

This skill provides expert guide for testing Spring Boot 4 applications with modern patterns and best practices.

Core Principles

1. Test Pyramid: Unit (fast) > Slice (focused) > Integration (complete) 2. Right Tool: Use the narrowest slice that gives you confidence 3. AssertJ Style: Fluent, readable assertions over verbose matchers 4. Modern APIs: Prefer MockMvcTester and RestTestClient over legacy alternatives

Which Test Slice?

Test Slices Reference

• references/test-slices-overview.md - Decision matrix and comparison
• references/webmvctest.md - Web layer with MockMvc
• references/datajpatest.md - Data layer with Testcontainers
• references/restclienttest.md - REST client testing

Testing Tools Reference

• references/mockmvc-tester.md - AssertJ-style MockMvc (3.2+)
• references/mockmvc-classic.md - Traditional MockMvc (pre-3.2)
• references/resttestclient.md - Spring Boot 4+ REST client
• references/mockitobean.md - Mocking dependencies

Assertion Libraries

• references/assertj-basics.md - Scalars, strings, booleans, dates
• references/assertj-collections.md - Lists, Sets, Maps, arrays

Testcontainers

• references/testcontainers-jdbc.md - PostgreSQL, MySQL, etc.

Test Data Generation

• references/instancio.md - Generate complex test objects (3+ properties)

Performance & Migration

• references/context-caching.md - Speed up test suites
• references/sb4-migration.md - Spring Boot 4.0 changes

Quick Decision Tree

Testing a controller endpoint?
  Yes → @WebMvcTest with MockMvcTester

Testing repository queries?
  Yes → @DataJpaTest with Testcontainers (real DB)

Testing business logic in service?
  Yes → Plain JUnit + Mockito (no Spring context)

Testing external API client?
  Yes → @RestClientTest with MockRestServiceServer

Testing JSON mapping?
  Yes → @JsonTest

Need full integration test?
  Yes → @SpringBootTest with minimal context config

Spring Boot 4 Highlights

• RestTestClient: Modern alternative to TestRestTemplate
• @MockitoBean: Replaces @MockBean (deprecated)
• MockMvcTester: AssertJ-style assertions for web tests
• Modular starters: Technology-specific test starters
• Context pausing: Automatic pausing of cached contexts (Spring Framework 7)

Testing Best Practices

Code Complexity Assessment

When a method or class is too complex to test effectively:

1. Analyze complexity - If you need more than 5-7 test cases to cover a single method, it's likely too complex 2. Recommend refactoring - Suggest breaking the code into smaller, focused functions 3. User decision - If the user agrees to refactor, help identify extraction points 4. Proceed if needed - If the user decides to continue with the complex code, implement tests despite the difficulty

Example of refactoring recommendation:

// Before: Complex method hard to test
public Order processOrder(OrderRequest request) {
  // Validation, discount calculation, payment, inventory, notification...
  // 50+ lines of mixed concerns
}

// After: Refactored into testable units
public Order processOrder(OrderRequest request) {
  validateOrder(request);
  var order = createOrder(request);
  applyDiscount(order);
  processPayment(order);
  updateInventory(order);
  sendNotification(order);
  return order;
}

Avoid Code Redundancy

Create helper methods for commonly used objects and mock setup to enhance readability and maintainability.

Test Organization with @DisplayName

Use descriptive display names to clarify test intent:

@Test
@DisplayName("Should calculate discount for VIP customer")
void shouldCalculateDiscountForVip() { }

@Test
@DisplayName("Should reject order when customer has insufficient credit")
void shouldRejectOrderForInsufficientCredit() { }

Test Coverage Order

Always structure tests in this order:

1. Main scenario - The happy path, most common use case 2. Other paths - Alternative valid scenarios, edge cases 3. Exceptions/Errors - Invalid inputs, error conditions, failure modes

Test Production Scenarios

Write tests with real production scenarios in mind. This makes tests more relatable and helps understand code behavior in actual production cases.

Test Coverage Goals

Aim for 80% code coverage as a practical balance between quality and effort. Higher coverage is beneficial but not the only goal.

Use Jacoco maven plugin for coverage reporting and tracking.

Coverage Rules:

• 80+% coverage minimum
• Focus on meaningful assertions, not just execution

What to Prioritize: 1. Business-critical paths (payment processing, order validation) 2. Complex algorithms (pricing, discount calculations) 3. Error handling (exceptions, edge cases) 4. Integration points (external APIs, databases)

Dependencies (Spring Boot 4)

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-test</artifactId>
  <scope>test</scope>
</dependency>

<!-- For WebMvc tests -->
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-webmvc-test</artifactId>
  <scope>test</scope>
</dependency>

<!-- For Testcontainers -->
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-testcontainers</artifactId>
  <scope>test</scope>
</dependency>

AssertJ Basics

Fluent assertions for readable, maintainable tests.

Basic Assertions

Object Equality

assertThat(order.getStatus()).isEqualTo("PENDING");
assertThat(order.getId()).isNotEqualTo(0);
assertThat(order).isEqualTo(expectedOrder);
assertThat(order).isNotNull();
assertThat(nullOrder).isNull();

String Assertions

assertThat(order.getDescription())
  .isEqualTo("Test Order")
  .startsWith("Test")
  .endsWith("Order")
  .contains("Test")
  .hasSize(10)
  .matches("[A-Za-z ]+");

Number Assertions

assertThat(order.getAmount())
  .isEqualTo(99.99)
  .isGreaterThan(50)
  .isLessThan(100)
  .isBetween(50, 100)
  .isPositive()
  .isNotZero();

Boolean Assertions

assertThat(order.isActive()).isTrue();
assertThat(order.isDeleted()).isFalse();

Date/Time Assertions

assertThat(order.getCreatedAt())
  .isEqualTo(LocalDateTime.of(2024, 1, 15, 10, 30))
  .isBefore(LocalDateTime.now())
  .isAfter(LocalDateTime.of(2024, 1, 1))
  .isCloseTo(LocalDateTime.now(), within(5, ChronoUnit.SECONDS));

Optional Assertions

Optional<Order> maybeOrder = orderService.findById(1L);

assertThat(maybeOrder)
  .isPresent()
  .hasValueSatisfying(order -> {
    assertThat(order.getId()).isEqualTo(1L);
  });

assertThat(orderService.findById(999L)).isEmpty();

Exception Assertions

JUnit 5 Exception Handling

@Test
void shouldThrowException() {
  OrderService service = new OrderService();
  
  assertThatThrownBy(() -> service.findById(999L))
    .isInstanceOf(OrderNotFoundException.class)
    .hasMessage("Order 999 not found")
    .hasMessageContaining("999");
}

AssertJ Exception Handling

@Test
void shouldThrowExceptionWithCause() {
  assertThatExceptionOfType(OrderProcessingException.class)
    .isThrownBy(() -> service.processOrder(invalidOrder))
    .withCauseInstanceOf(ValidationException.class);
}

Custom Assertions

Create domain-specific assertions for reusable test code:

public class OrderAssert extends AbstractAssert<OrderAssert, Order> {
  
  public static OrderAssert assertThat(Order actual) {
    return new OrderAssert(actual);
  }
  
  private OrderAssert(Order actual) {
    super(actual, OrderAssert.class);
  }
  
  public OrderAssert isPending() {
    isNotNull();
    if (!"PENDING".equals(actual.getStatus())) {
      failWithMessage("Expected order status to be PENDING but was %s", actual.getStatus());
    }
    return this;
  }
  
  public OrderAssert hasTotal(BigDecimal expected) {
    isNotNull();
    if (!expected.equals(actual.getTotal())) {
      failWithMessage("Expected total %s but was %s", expected, actual.getTotal());
    }
    return this;
  }
}

Usage:

OrderAssert.assertThat(order)
  .isPending()
  .hasTotal(new BigDecimal("99.99"));

Soft Assertions

Collect multiple failures before failing:

@Test
void shouldValidateOrder() {
  Order order = orderService.findById(1L);
  
  SoftAssertions.assertSoftly(softly -> {
    softly.assertThat(order.getId()).isEqualTo(1L);
    softly.assertThat(order.getStatus()).isEqualTo("PENDING");
    softly.assertThat(order.getItems()).isNotEmpty();
  });
}

Satisfies Pattern

assertThat(order)
  .satisfies(o -> {
    assertThat(o.getId()).isPositive();
    assertThat(o.getStatus()).isNotBlank();
    assertThat(o.getCreatedAt()).isNotNull();
  });

Using with Spring

import static org.assertj.core.api.Assertions.assertThat;

@SpringBootTest
class OrderServiceTest {
  
  @Autowired
  private OrderService orderService;
  
  @Test
  void shouldCreateOrder() {
    Order order = orderService.create(new OrderRequest("Product", 2));
    
    assertThat(order)
      .isNotNull()
      .extracting(Order::getId, Order::getStatus)
      .containsExactly(1L, "PENDING");
  }
}

Static Import

Always use static import for clean assertions:

import static org.assertj.core.api.Assertions.assertThat;
import static org.assertj.core.api.Assertions.assertThatThrownBy;
import static org.assertj.core.api.Assertions.catchThrowable;

Key Benefits

1. Readable: Sentence-like structure 2. Type-safe: IDE autocomplete works 3. Rich API: Many built-in assertions 4. Extensible: Custom assertions for your domain 5. Better Errors: Clear failure messages

AssertJ Collections

AssertJ assertions for collections: List, Set, Map, arrays, and streams.

When to Use This Reference

• The value under test is a List, Set, Map, array, or Stream
• You need to assert on multiple elements, their order, or specific fields within them
• You are using extracting(), filteredOn(), containsExactly(), or similar collection methods
• Asserting a single scalar or single object → use assertj-basics.md instead

Basic Collection Checks

List<Order> orders = orderService.findAll();

assertThat(orders).isNotEmpty();
assertThat(orders).isEmpty();
assertThat(orders).hasSize(3);
assertThat(orders).hasSizeGreaterThan(0);
assertThat(orders).hasSizeLessThanOrEqualTo(10);

Containment Assertions

// Contains (any order, allows extras)
assertThat(orders).contains(order1, order2);

// Contains exactly these elements in this order (no extras)
assertThat(statuses).containsExactly("NEW", "PENDING", "COMPLETED");

// Contains exactly these elements in any order (no extras)
assertThat(statuses).containsExactlyInAnyOrder("COMPLETED", "NEW", "PENDING");

// Contains any of these elements (at least one match required)
assertThat(statuses).containsAnyOf("NEW", "CANCELLED");

// Does not contain
assertThat(statuses).doesNotContain("DELETED");

Extracting Fields

Extract a single field from each element before asserting:

assertThat(orders)
  .extracting(Order::getStatus)
  .containsExactly("NEW", "PENDING", "COMPLETED");

Extract multiple fields as tuples:

assertThat(orders)
  .extracting(Order::getId, Order::getStatus)
  .containsExactly(
    tuple(1L, "NEW"),
    tuple(2L, "PENDING"),
    tuple(3L, "COMPLETED")
  );

Filtering Before Asserting

assertThat(orders)
  .filteredOn(order -> order.getStatus().equals("PENDING"))
  .hasSize(2)
  .extracting(Order::getId)
  .containsExactlyInAnyOrder(1L, 3L);

// Filter by field value
assertThat(orders)
  .filteredOn("status", "PENDING")
  .hasSize(2);

Predicate Checks

assertThat(orders).allMatch(o -> o.getTotal().compareTo(BigDecimal.ZERO) > 0);
assertThat(orders).anyMatch(o -> o.getStatus().equals("COMPLETED"));
assertThat(orders).noneMatch(o -> o.getStatus().equals("DELETED"));

// With description for failure messages
assertThat(orders)
  .allSatisfy(o -> assertThat(o.getId()).isPositive());

Per-Element Ordered Assertions

Assert each element in order with individual conditions:

assertThat(orders).satisfiesExactly(
  first  -> assertThat(first.getStatus()).isEqualTo("NEW"),
  second -> assertThat(second.getStatus()).isEqualTo("PENDING"),
  third  -> {
    assertThat(third.getStatus()).isEqualTo("COMPLETED");
    assertThat(third.getTotal()).isGreaterThan(BigDecimal.ZERO);
  }
);

Nested / Flat Collections

// flatExtracting: flatten one level of nested collections
assertThat(orders)
  .flatExtracting(Order::getItems)
  .extracting(OrderItem::getProduct)
  .contains("Laptop", "Mouse");

Recursive Field Comparison

Compare elements by fields instead of object identity:

assertThat(orders)
  .usingRecursiveFieldByFieldElementComparator()
  .containsExactlyInAnyOrder(expectedOrder1, expectedOrder2);

// Ignore specific fields (e.g. generated IDs or timestamps)
assertThat(orders)
  .usingRecursiveFieldByFieldElementComparatorIgnoringFields("id", "createdAt")
  .containsExactly(expectedOrder1, expectedOrder2);

Map Assertions

Map<String, Integer> stockByProduct = inventoryService.getStock();

assertThat(stockByProduct)
  .isNotEmpty()
  .hasSize(3)
  .containsKey("Laptop")
  .doesNotContainKey("Fax Machine")
  .containsEntry("Laptop", 10)
  .containsEntries(entry("Laptop", 10), entry("Mouse", 50));

assertThat(stockByProduct)
  .hasEntrySatisfying("Laptop", qty -> assertThat(qty).isGreaterThan(0));

Array Assertions

String[] roles = user.getRoles();

assertThat(roles).hasSize(2);
assertThat(roles).contains("ADMIN");
assertThat(roles).containsExactlyInAnyOrder("USER", "ADMIN");

Set Assertions

Set<String> tags = product.getTags();

assertThat(tags).contains("electronics", "sale");
assertThat(tags).doesNotContain("expired");
assertThat(tags).hasSizeGreaterThanOrEqualTo(1);

Static Import

import static org.assertj.core.api.Assertions.assertThat;
import static org.assertj.core.api.Assertions.tuple;
import static org.assertj.core.api.Assertions.entry;

Key Points

1. `containsExactly` vs `containsExactlyInAnyOrder` — use the former when order matters 2. `extracting()` before containment checks — avoids implementing equals() on domain objects 3. `filteredOn()` + `extracting()` — compose to assert a subset of a collection precisely 4. `satisfiesExactly()` — use when each element needs different assertions 5. `usingRecursiveFieldByFieldElementComparator()` — preferred over equals() for DTOs and records

Context Caching

Optimize Spring Boot test suite performance through context caching.

How Context Caching Works

Spring's TestContext Framework caches application contexts based on their configuration "key". Tests with identical configurations reuse the same context.

What Affects the Cache Key

• @ContextConfiguration
• @TestPropertySource
• @ActiveProfiles
• @WebAppConfiguration
• @MockitoBean definitions
• @TestConfiguration imports

Cache Key Examples

Same Key (Context Reused)

@WebMvcTest(OrderController.class)
class OrderControllerTest1 {
  @MockitoBean private OrderService orderService;
}

@WebMvcTest(OrderController.class)
class OrderControllerTest2 {
  @MockitoBean private OrderService orderService;
}
// Same context reused

Different Key (New Context)

@WebMvcTest(OrderController.class)
@ActiveProfiles("test")
class OrderControllerTest1 { }

@WebMvcTest(OrderController.class)
@ActiveProfiles("integration")
class OrderControllerTest2 { }
// Different contexts loaded

Viewing Cache Statistics

Spring Boot Actuator

management:
  endpoints:
    web:
      exposure:
        include: metrics

Access: GET /actuator/metrics/spring.test.context.cache

Debug Logging

logging.level.org.springframework.test.context.cache=DEBUG

Optimizing Cache Hit Rate

Group Tests by Configuration

 tests/
   unit/           # No context
   web/            # @WebMvcTest
   repository/     # @DataJpaTest  
   integration/    # @SpringBootTest

Minimize @TestPropertySource Variations

Bad (multiple contexts):

@TestPropertySource(properties = "app.feature-x=true")
class FeatureXTest { }

@TestPropertySource(properties = "app.feature-y=true")
class FeatureYTest { }

Better (grouped):

@TestPropertySource(properties = {"app.feature-x=true", "app.feature-y=true"})
class FeaturesTest { }

Use @DirtiesContext Sparingly

Only when context state truly changes:

@Test
@DirtiesContext // Forces context rebuild after test
void testThatModifiesBeanDefinitions() { }

Best Practices

1. Group by configuration - Keep tests with same config together 2. Limit property variations - Use profiles over individual properties 3. Avoid @DirtiesContext - Prefer test data cleanup 4. Use narrow slices - @WebMvcTest vs @SpringBootTest 5. Monitor cache hits - Enable debug logging occasionally

@DataJpaTest

Testing JPA repositories with isolated data layer slice.

Basic Structure

@DataJpaTest
@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
@Testcontainers
class OrderRepositoryTest {
  
  @Container
  @ServiceConnection
  static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18");
  
  @Autowired
  private OrderRepository orderRepository;
  
  @Autowired
  private TestEntityManager entityManager;
}

What Gets Loaded

• Repository beans
• EntityManager / TestEntityManager
• DataSource
• Transaction manager
• No web layer, no services, no controllers

Testing Custom Queries

@Test
void shouldFindOrdersByStatus() {
  // Given - Using var for cleaner code
  var pending = new Order("PENDING");
  var completed = new Order("COMPLETED");
  entityManager.persist(pending);
  entityManager.persist(completed);
  entityManager.flush();
  
  // When
  var pendingOrders = orderRepository.findByStatus("PENDING");
  
  // Then - Using sequenced collection methods
  assertThat(pendingOrders).hasSize(1);
  assertThat(pendingOrders.getFirst().getStatus()).isEqualTo("PENDING");
}

Testing Native Queries

@Test
void shouldExecuteNativeQuery() {
  entityManager.persist(new Order("PENDING", BigDecimal.valueOf(100)));
  entityManager.persist(new Order("PENDING", BigDecimal.valueOf(200)));
  entityManager.flush();
  
  var total = orderRepository.calculatePendingTotal();
  
  assertThat(total).isEqualTo(new BigDecimal("300.00"));
}

Testing Pagination

@Test
void shouldReturnPagedResults() {
  // Insert 20 orders using IntStream
  IntStream.range(0, 20).forEach(i -> {
    entityManager.persist(new Order("PENDING"));
  });
  entityManager.flush();
  
  var page = orderRepository.findByStatus("PENDING", PageRequest.of(0, 10));
  
  assertThat(page.getContent()).hasSize(10);
  assertThat(page.getTotalElements()).isEqualTo(20);
  assertThat(page.getContent().getFirst().getStatus()).isEqualTo("PENDING");
}

Testing Lazy Loading

@Test
void shouldLazyLoadOrderItems() {
  var order = new Order("PENDING");
  order.addItem(new OrderItem("Product", 2));
  entityManager.persist(order);
  entityManager.flush();
  entityManager.clear(); // Detach from persistence context
  
  var found = orderRepository.findById(order.getId());
  
  assertThat(found).isPresent();
  // This will trigger lazy loading
  assertThat(found.get().getItems()).hasSize(1);
  assertThat(found.get().getItems().getFirst().getProduct()).isEqualTo("Product");
}

Testing Cascading

@Test
void shouldCascadeDelete() {
  var order = new Order("PENDING");
  order.addItem(new OrderItem("Product", 2));
  entityManager.persist(order);
  entityManager.flush();
  
  orderRepository.delete(order);
  entityManager.flush();
  
  assertThat(entityManager.find(OrderItem.class, order.getItems().getFirst().getId()))
    .isNull();
}

Testing @Query Methods

@Query("SELECT o FROM Order o WHERE o.createdAt > :date AND o.status = :status")
List<Order> findRecentByStatus(@Param("date") LocalDateTime date, 
                               @Param("status") String status);

@Test
void shouldFindRecentOrders() {
  var old = new Order("PENDING");
  old.setCreatedAt(LocalDateTime.now().minusDays(10));
  var recent = new Order("PENDING");
  recent.setCreatedAt(LocalDateTime.now().minusHours(1));
  
  entityManager.persist(old);
  entityManager.persist(recent);
  entityManager.flush();
  
  var recentOrders = orderRepository.findRecentByStatus(
    LocalDateTime.now().minusDays(1), "PENDING");
  
  assertThat(recentOrders).hasSize(1);
  assertThat(recentOrders.getFirst().getId()).isEqualTo(recent.getId());
}

Using H2 vs Real Database

H2 (Default - Not Recommended for Production Parity)

@DataJpaTest // Uses embedded H2 by default
class OrderRepositoryH2Test {
  // Fast but may miss DB-specific issues
}

Testcontainers (Recommended)

@DataJpaTest
@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
@Testcontainers
class OrderRepositoryPostgresTest {
  @Container
  @ServiceConnection
  static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18");
}

Transaction Behavior

Tests are @Transactional by default and roll back after each test.

@Test
@Rollback(false) // Don't roll back (rarely needed)
void shouldPersistData() {
  orderRepository.save(new Order("PENDING"));
  // Data will remain in database after test
}

Key Points

1. Use TestEntityManager for setup data 2. Always flush() after persist() to trigger SQL 3. Clear() the entity manager to test lazy loading 4. Use real database (Testcontainers) for accurate results 5. Test both success and failure cases 6. Leverage Java 25 var keyword for cleaner variable declarations 7. Use sequenced collection methods (getFirst(), getLast(), reversed())

Instancio

Generate complex test objects automatically. Use when entities/DTOs have 3+ properties.

When to Use

• Objects with 3 or more properties
• Setting up test data for repositories
• Creating DTOs for controller tests
• Avoiding repetitive builder/setter calls

Dependency

<dependency>
  <groupId>org.instancio</groupId>
  <artifactId>instancio-junit</artifactId>
  <version>5.5.1</version>
  <scope>test</scope>
</dependency>

Basic Usage

Simple Object

final var order = Instancio.create(Order.class);
// All fields populated with random data

List of Objects

final var orders = Instancio.ofList(Order.class).size(5).create();
// 5 orders with random data

Customizing Values

Set Specific Fields

final var order = Instancio.of(Order.class)
  .set(field(Order::getStatus), "PENDING")
  .set(field(Order::getTotal), new BigDecimal("99.99"))
  .create();

Supply Generated Values

final var order = Instancio.of(Order.class)
  .supply(field(Order::getEmail), () -> "user" + UUID.randomUUID() + "@test.com")
  .create();

Ignore Fields

final var order = Instancio.of(Order.class)
  .ignore(field(Order::getId)) // Let DB generate
  .create();

Complex Objects

Nested Objects

final var order = Instancio.of(Order.class)
  .set(field(Order::getCustomer), Instancio.create(Customer.class))
  .set(field(Order::getItems), Instancio.ofList(OrderItem.class).size(3).create())
  .create();

All Fields Random

// When you need fully random but valid data
final var randomOrder = Instancio.create(Order.class);
// Customer, items, addresses - all populated

Spring Boot Integration

Repository Test Setup

@DataJpaTest
@AutoConfigureTestDatabase
@Testcontainers
class OrderRepositoryTest {
  
  @Container
  @ServiceConnection
  static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18");
  
  @Autowired
  private OrderRepository orderRepository;
  
  @Test
  void shouldFindOrdersByStatus() {
    // Given: Create 10 random orders with PENDING status
    final var orders = Instancio.ofList(Order.class)
      .size(10)
      .set(field(Order::getStatus), "PENDING")
      .create();
    
    orderRepository.saveAll(orders);
    
    // When
    final var found = orderRepository.findByStatus("PENDING");
    
    // Then
    assertThat(found).hasSize(10);
  }
}

Controller Test Setup

@WebMvcTest(OrderController.class)
class OrderControllerTest {
  
  @Autowired
  private MockMvcTester mvc;
  
  @MockitoBean
  private OrderService orderService;
  
  @Test
  void shouldReturnOrder() {
    // Given: Random order with specific ID
    Order order = Instancio.of(Order.class)
      .set(field(Order::getId), 1L)
      .create();
    
    given(orderService.findById(1L)).willReturn(order);
    
    // When/Then
    assertThat(mvc.get().uri("/orders/1"))
      .hasStatus(HttpStatus.OK)
      .bodyJson()
      .convertTo(OrderResponse.class)
      .satisfies(response -> {
        assertThat(response.getId()).isEqualTo(1L);
      });
  }
}

Patterns

Builder Pattern Alternative

// Instead of:
Order order = Order.builder()
  .id(1L)
  .status("PENDING")
  .customer(Customer.builder().name("John").build())
  .items(List.of(
    OrderItem.builder().product("A").price(10).build(),
    OrderItem.builder().product("B").price(20).build()
  ))
  .build();

// Use:
Order order = Instancio.of(Order.class)
  .set(field(Order::getId), 1L)
  .set(field(Order::getStatus), "PENDING")
  .create();
// Customer and items auto-generated

Seeded Data

// Consistent "random" data for reproducible tests
Order order = Instancio.of(Order.class)
  .withSeed(12345L)
  .create();
// Same data every test run with seed 12345

Common Patterns

Email Generation

String email = Instancio.gen().net().email();

Date Generation

LocalDateTime createdAt = Instancio.gen().temporal()
  .localDateTime()
  .past()
  .create();

String Patterns

String phone = Instancio.gen().text().pattern("+1-###-###-####");

Comparison

Best Practices

1. Use for 3+ property objects - Not worth it for simple objects 2. Set only what's relevant - Let Instancio fill the rest 3. Use with Testcontainers - Great for database seeding 4. Set IDs explicitly - When testing specific scenarios 5. Ignore auto-generated fields - Like createdAt, updatedAt

Links

• Instancio Documentation
• JUnit 5 Extension

@MockitoBean

Mocking dependencies in Spring Boot tests (replaces deprecated @MockBean in Spring Boot 4+).

Overview

@MockitoBean replaces the deprecated @MockBean annotation in Spring Boot 4.0+. It creates a Mockito mock and registers it in the Spring context, replacing any existing bean of the same type.

Basic Usage

@WebMvcTest(OrderController.class)
class OrderControllerTest {
  
  @MockitoBean
  private OrderService orderService;
  
  @MockitoBean
  private UserService userService;
}

Supported Test Slices

• @WebMvcTest - Mock service/repository dependencies
• @WebFluxTest - Mock reactive service dependencies
• @SpringBootTest - Replace real beans with mocks

Stubbing Methods

Basic Stub

@Test
void shouldReturnOrder() {
  Order order = new Order(1L, "PENDING");
  given(orderService.findById(1L)).willReturn(order);
  
  // Test code
}

Multiple Returns

given(orderService.findById(anyLong()))
  .willReturn(new Order(1L, "PENDING"))
  .willReturn(new Order(2L, "COMPLETED"));

Throwing Exceptions

given(orderService.findById(999L))
  .willThrow(new OrderNotFoundException(999L));

Argument Matching

given(orderService.create(argThat(req -> req.getQuantity() > 0)))
  .willReturn(1L);

given(orderService.findByStatus(eq("PENDING")))
  .willReturn(List.of(new Order()));

Verifying Interactions

Verify Method Called

verify(orderService).findById(1L);

Verify Never Called

verify(orderService, never()).delete(any());

Verify Count

verify(orderService, times(2)).findById(anyLong());
verify(orderService, atLeastOnce()).findByStatus(anyString());

Verify Order

InOrder inOrder = inOrder(orderService, userService);
inOrder.verify(orderService).findById(1L);
inOrder.verify(userService).getUser(any());

Resetting Mocks

Mocks are reset between tests automatically. To reset mid-test:

Mockito.reset(orderService);

@MockitoSpyBean for Partial Mocking

Use @MockitoSpyBean to wrap a real bean with Mockito.

@SpringBootTest
class OrderServiceIntegrationTest {
  
  @MockitoSpyBean
  private PaymentGatewayClient paymentClient;
  
  @Test
  void shouldProcessOrder() {
    doReturn(true).when(paymentClient).processPayment(any());
    
    // Test with real service but mocked payment client
  }
}

@TestBean for Custom Test Beans

Register a custom bean instance in the test context:

@SpringBootTest
class OrderServiceTest {
  
  @TestBean
  private PaymentGatewayClient paymentClient() {
    return new FakePaymentClient();
  }
}

Scoping: Singleton vs Prototype

Spring Framework 7+ (Spring Boot 4+) supports mocking non-singleton beans:

@Component
@Scope("prototype")
public class OrderProcessor {
  public String process() { return "real"; }
}

@SpringBootTest
class OrderServiceTest {
  @MockitoBean
  private OrderProcessor orderProcessor;
  
  @Test
  void shouldWorkWithPrototype() {
    given(orderProcessor.process()).willReturn("mocked");
    // Test code
  }
}

Common Patterns

Mocking Repository in Service Test

@SpringBootTest
class OrderServiceTest {
  @MockitoBean
  private OrderRepository orderRepository;
  
  @Autowired
  private OrderService orderService;
  
  @Test
  void shouldCreateOrder() {
    given(orderRepository.save(any())).willReturn(new Order(1L));
    
    Long id = orderService.createOrder(new OrderRequest());
    
    assertThat(id).isEqualTo(1L);
    verify(orderRepository).save(any(Order.class));
  }
}

Multiple Mocks of Same Type

Use bean names:

@MockitoBean(name = "primaryDataSource")
private DataSource primaryDataSource;

@MockitoBean(name = "secondaryDataSource")
private DataSource secondaryDataSource;

Migration from @MockBean

Before (Deprecated)

@MockBean
private OrderService orderService;

After (Spring Boot 4+)

@MockitoBean
private OrderService orderService;

Key Differences from Mockito @Mock

Best Practices

1. Use @MockitoBean only when Spring context is involved 2. For pure unit tests, use Mockito's @Mock or Mockito.mock() 3. Always verify interactions that have side effects 4. Don't verify simple queries (stubbing is enough) 5. Reset mocks if test modifies shared mock state

MockMvc Classic

Traditional MockMvc API for Spring MVC controller tests (pre-Spring Boot 3.2 or legacy codebases).

When to Use This Reference

• The project uses Spring Boot < 3.2 (no MockMvcTester available)
• Existing tests use mvc.perform(...) and you are maintaining or extending them
• You need to migrate classic MockMvc tests to MockMvcTester (see migration section below)
• The user explicitly asks about ResultActions, andExpect(), or Hamcrest-style web assertions

For new tests on Spring Boot 3.2+, prefer mockmvc-tester.md instead.

Setup

@WebMvcTest(OrderController.class)
class OrderControllerTest {

  @Autowired
  private MockMvc mvc;

  @MockBean
  private OrderService orderService;
}

Basic GET Request

@Test
void shouldReturnOrder() throws Exception {
  given(orderService.findById(1L)).willReturn(new Order(1L, "PENDING", 99.99));

  mvc.perform(get("/orders/1"))
    .andExpect(status().isOk())
    .andExpect(content().contentType(MediaType.APPLICATION_JSON))
    .andExpect(jsonPath("$.id").value(1))
    .andExpect(jsonPath("$.status").value("PENDING"))
    .andExpect(jsonPath("$.totalToPay").value(99.99));
}

POST with Request Body

@Test
void shouldCreateOrder() throws Exception {
  given(orderService.create(any(OrderRequest.class))).willReturn(1L);

  mvc.perform(post("/orders")
      .contentType(MediaType.APPLICATION_JSON)
      .content("{\"product\": \"Laptop\", \"quantity\": 2}"))
    .andExpect(status().isCreated())
    .andExpect(header().string("Location", "/orders/1"));
}

PUT Request

@Test
void shouldUpdateOrder() throws Exception {
  mvc.perform(put("/orders/1")
      .contentType(MediaType.APPLICATION_JSON)
      .content("{\"status\": \"COMPLETED\"}"))
    .andExpect(status().isOk());
}

DELETE Request

@Test
void shouldDeleteOrder() throws Exception {
  mvc.perform(delete("/orders/1"))
    .andExpect(status().isNoContent());
}

Status Matchers

.andExpect(status().isOk())           // 200
.andExpect(status().isCreated())      // 201
.andExpect(status().isNoContent())    // 204
.andExpect(status().isBadRequest())   // 400
.andExpect(status().isUnauthorized()) // 401
.andExpect(status().isForbidden())    // 403
.andExpect(status().isNotFound())     // 404
.andExpect(status().is(422))          // arbitrary code

JSON Path Assertions

// Exact value
.andExpect(jsonPath("$.status").value("PENDING"))

// Existence
.andExpect(jsonPath("$.id").exists())
.andExpect(jsonPath("$.deletedAt").doesNotExist())

// Array size
.andExpect(jsonPath("$.items").isArray())
.andExpect(jsonPath("$.items", hasSize(3)))

// Nested field
.andExpect(jsonPath("$.customer.name").value("John Doe"))
.andExpect(jsonPath("$.customer.address.city").value("Berlin"))

// With Hamcrest matchers
.andExpect(jsonPath("$.total", greaterThan(0.0)))
.andExpect(jsonPath("$.description", containsString("order")))

Content Assertions

.andExpect(content().contentType(MediaType.APPLICATION_JSON))
.andExpect(content().contentTypeCompatibleWith(MediaType.APPLICATION_JSON))
.andExpect(content().string(containsString("PENDING")))
.andExpect(content().json("{\"status\":\"PENDING\"}"))

Header Assertions

.andExpect(header().string("Location", "/orders/1"))
.andExpect(header().string("Content-Type", containsString("application/json")))
.andExpect(header().exists("X-Request-Id"))
.andExpect(header().doesNotExist("X-Deprecated"))

Request Parameters and Headers

// Query parameters
mvc.perform(get("/orders").param("status", "PENDING").param("page", "0"))
  .andExpect(status().isOk());

// Path variables
mvc.perform(get("/orders/{id}", 1L))
  .andExpect(status().isOk());

// Request headers
mvc.perform(get("/orders/1").header("X-Api-Key", "secret"))
  .andExpect(status().isOk());

Capturing the Response

@Test
void shouldReturnCreatedId() throws Exception {
  given(orderService.create(any())).willReturn(42L);

  MvcResult result = mvc.perform(post("/orders")
      .contentType(MediaType.APPLICATION_JSON)
      .content("{\"product\": \"Laptop\", \"quantity\": 1}"))
    .andExpect(status().isCreated())
    .andReturn();

  String location = result.getResponse().getHeader("Location");
  assertThat(location).isEqualTo("/orders/42");
}

Chaining with andDo

mvc.perform(get("/orders/1"))
  .andDo(print())              // prints request/response to console (debug)
  .andExpect(status().isOk());

Static Imports

import org.springframework.boot.test.mock.mockito.MockBean;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
import static org.springframework.test.web.servlet.result.MockMvcResultHandlers.*;
import static org.hamcrest.Matchers.*;

Migration to MockMvcTester

See mockmvc-tester.md for the full modern API.

Key Points

1. Every test method must declare `throws Exception` — perform() throws checked exceptions 2. Use `andDo(print())` during debugging — remove before committing 3. Prefer `jsonPath()` over `content().string()` — more precise field-level assertions 4. Static imports are required — IDE can auto-add them 5. Migrate to MockMvcTester when upgrading to Spring Boot 3.2+ for better readability

MockMvcTester

AssertJ-style testing for Spring MVC controllers (Spring Boot 3.2+).

Overview

MockMvcTester provides fluent, AssertJ-style assertions for web layer testing. More readable and type-safe than traditional MockMvc.

Recommended Pattern: Convert JSON to real objects and assert with AssertJ:

assertThat(mvc.get().uri("/orders/1"))
  .hasStatus(HttpStatus.OK)
  .bodyJson()
  .convertTo(OrderResponse.class)
  .satisfies(response -> {
    assertThat(response.getTotalToPay()).isEqualTo(expectedAmount);
    assertThat(response.getItems()).isNotEmpty();
  });

Basic Usage

@WebMvcTest(OrderController.class)
class OrderControllerTest {
  
  @Autowired
  private MockMvcTester mvc;
  
  @MockitoBean
  private OrderService orderService;
}

Recommended: Object Conversion Pattern

Single Object Response

@Test
void shouldGetOrder() {
  given(orderService.findById(1L)).willReturn(new Order(1L, "PENDING", 99.99));
  
  assertThat(mvc.get().uri("/orders/1"))
    .hasStatus(HttpStatus.OK)
    .bodyJson()
    .convertTo(OrderResponse.class)
    .satisfies(response -> {
      assertThat(response.getId()).isEqualTo(1L);
      assertThat(response.getStatus()).isEqualTo("PENDING");
      assertThat(response.getTotalToPay()).isEqualTo(new BigDecimal("99.99"));
    });
}

List Response

@Test
void shouldGetAllOrders() {
  given(orderService.findAll()).willReturn(Arrays.asList(
    new Order(1L, "PENDING"),
    new Order(2L, "COMPLETED")
  ));
  
  assertThat(mvc.get().uri("/orders"))
    .hasStatus(HttpStatus.OK)
    .bodyJson()
    .convertTo(new TypeReference<List<OrderResponse>>() {})
    .satisfies(orders -> {
      assertThat(orders).hasSize(2);
      assertThat(orders.get(0).getStatus()).isEqualTo("PENDING");
      assertThat(orders.get(1).getStatus()).isEqualTo("COMPLETED");
    });
}

Nested Objects

@Test
void shouldGetOrderWithCustomer() {
  assertThat(mvc.get().uri("/orders/1"))
    .hasStatus(HttpStatus.OK)
    .bodyJson()
    .convertTo(OrderResponse.class)
    .satisfies(response -> {
      assertThat(response.getCustomer()).isNotNull();
      assertThat(response.getCustomer().getName()).isEqualTo("John Doe");
      assertThat(response.getCustomer().getAddress().getCity()).isEqualTo("Berlin");
    });
}

Complex Assertions

@Test
void shouldCalculateOrderTotal() {
  assertThat(mvc.get().uri("/orders/1/calculate"))
    .hasStatus(HttpStatus.OK)
    .bodyJson()
    .convertTo(CalculationResponse.class)
    .satisfies(calc -> {
      assertThat(calc.getSubtotal()).isEqualTo(new BigDecimal("100.00"));
      assertThat(calc.getTax()).isEqualTo(new BigDecimal("19.00"));
      assertThat(calc.getTotalToPay()).isEqualTo(new BigDecimal("119.00"));
      assertThat(calc.getItems()).allMatch(item -> item.getPrice().compareTo(BigDecimal.ZERO) > 0);
    });
}

HTTP Methods

POST with Request Body

@Test
void shouldCreateOrder() {
  given(orderService.create(any())).willReturn(1L);
  
  assertThat(mvc.post().uri("/orders")
    .contentType(MediaType.APPLICATION_JSON)
    .content("{\"product\": \"Laptop\", \"quantity\": 2}"))
    .hasStatus(HttpStatus.CREATED)
    .hasHeader("Location", "/orders/1");
}

PUT Request

@Test
void shouldUpdateOrder() {
  assertThat(mvc.put().uri("/orders/1")
    .contentType(MediaType.APPLICATION_JSON)
    .content("{\"status\": \"COMPLETED\"}"))
    .hasStatus(HttpStatus.OK);
}

DELETE Request

@Test
void shouldDeleteOrder() {
  assertThat(mvc.delete().uri("/orders/1"))
    .hasStatus(HttpStatus.NO_CONTENT);
}

Status Assertions

assertThat(mvc.get().uri("/orders/1"))
  .hasStatusOk()                    // 200
  .hasStatus(HttpStatus.OK)         // 200
  .hasStatus2xxSuccessful()         // 2xx
  .hasStatusBadRequest()            // 400
  .hasStatusNotFound()              // 404
  .hasStatusUnauthorized()          // 401
  .hasStatusForbidden()             // 403
  .hasStatus(HttpStatus.CREATED);   // 201

Content Type Assertions

assertThat(mvc.get().uri("/orders/1"))
  .hasContentType(MediaType.APPLICATION_JSON)
  .hasContentTypeCompatibleWith(MediaType.APPLICATION_JSON);

Header Assertions

assertThat(mvc.post().uri("/orders"))
  .hasHeader("Location", "/orders/123")
  .hasHeader("X-Request-Id", matchesPattern("[a-z0-9-]+"));

Alternative: JSON Path (Use Sparingly)

Only use when you cannot convert to a typed object:

assertThat(mvc.get().uri("/orders/1"))
  .hasStatusOk()
  .bodyJson()
  .extractingPath("$.customer.address.city")
  .asString()
  .isEqualTo("Berlin");

Request Parameters

// Query parameters
assertThat(mvc.get().uri("/orders?status=PENDING&page=0"))
  .hasStatusOk();

// Path parameters
assertThat(mvc.get().uri("/orders/{id}", 1L))
  .hasStatusOk();

// Headers
assertThat(mvc.get().uri("/orders/1")
  .header("X-Api-Key", "secret"))
  .hasStatusOk();

Request Body with JacksonTester

@Autowired
private JacksonTester<OrderRequest> json;

@Test
void shouldCreateOrder() {
  OrderRequest request = new OrderRequest("Laptop", 2);
  
  assertThat(mvc.post().uri("/orders")
    .contentType(MediaType.APPLICATION_JSON)
    .content(json.write(request).getJson()))
    .hasStatus(HttpStatus.CREATED);
}

Error Responses

@Test
void shouldReturnValidationErrors() {
  given(orderService.findById(999L))
    .willThrow(new OrderNotFoundException(999L));
  
  assertThat(mvc.get().uri("/orders/999"))
    .hasStatus(HttpStatus.NOT_FOUND)
    .bodyJson()
    .convertTo(ErrorResponse.class)
    .satisfies(error -> {
      assertThat(error.getMessage()).isEqualTo("Order 999 not found");
      assertThat(error.getCode()).isEqualTo("ORDER_NOT_FOUND");
    });
}

Validation Error Testing

@Test
void shouldRejectInvalidOrder() {
  OrderRequest invalidRequest = new OrderRequest("", -1);
  
  assertThat(mvc.post().uri("/orders")
    .contentType(MediaType.APPLICATION_JSON)
    .content(json.write(invalidRequest).getJson()))
    .hasStatus(HttpStatus.BAD_REQUEST)
    .bodyJson()
    .convertTo(ValidationErrorResponse.class)
    .satisfies(errors -> {
      assertThat(errors.getFieldErrors()).hasSize(2);
      assertThat(errors.getFieldErrors())
        .extracting("field")
        .contains("product", "quantity");
    });
}

Comparison: MockMvcTester vs Classic MockMvc

Migration from Classic MockMvc

Before (Classic)

mvc.perform(get("/orders/1"))
  .andExpect(status().isOk())
  .andExpect(jsonPath("$.status").value("PENDING"))
  .andExpect(jsonPath("$.totalToPay").value(99.99));

After (Tester with Object Conversion)

assertThat(mvc.get().uri("/orders/1"))
  .hasStatus(HttpStatus.OK)
  .bodyJson()
  .convertTo(OrderResponse.class)
  .satisfies(response -> {
    assertThat(response.getStatus()).isEqualTo("PENDING");
    assertThat(response.getTotalToPay()).isEqualTo(new BigDecimal("99.99"));
  });

Key Points

1. Prefer `convertTo()` over `extractingPath()` - Type-safe, refactorable 2. Use `satisfies()` for multiple assertions - Keeps tests readable 3. Import static `org.assertj.core.api.Assertions.assertThat` 4. Works with generics via `TypeReference` - For List<T> responses 5. IDE refactoring friendly - Rename fields, IDE updates tests

@RestClientTest

Testing REST clients in isolation with MockRestServiceServer.

Overview

@RestClientTest auto-configures:

• RestTemplate/RestClient with mock server support
• Jackson ObjectMapper
• MockRestServiceServer

Basic Setup

@RestClientTest(WeatherService.class)
class WeatherServiceTest {
  
  @Autowired
  private WeatherService weatherService;
  
  @Autowired
  private MockRestServiceServer server;
}

Testing RestTemplate

@RestClientTest(WeatherService.class)
class WeatherServiceTest {
  
  @Autowired
  private WeatherService weatherService;
  
  @Autowired
  private MockRestServiceServer server;
  
  @Test
  void shouldFetchWeather() {
    // Given
    server.expect(requestTo("https://api.weather.com/v1/current"))
      .andExpect(method(HttpMethod.GET))
      .andExpect(queryParam("city", "Berlin"))
      .andRespond(withSuccess()
        .contentType(MediaType.APPLICATION_JSON)
        .body("{\"temperature\": 22, \"condition\": \"Sunny\"}"));
    
    // When
    Weather weather = weatherService.getCurrentWeather("Berlin");
    
    // Then
    assertThat(weather.getTemperature()).isEqualTo(22);
    assertThat(weather.getCondition()).isEqualTo("Sunny");
  }
}

Testing RestClient (Spring 6.1+)

@RestClientTest(WeatherService.class)
class WeatherServiceTest {
  
  @Autowired
  private WeatherService weatherService;
  
  @Autowired
  private MockRestServiceServer server;
  
  @Test
  void shouldFetchWeatherWithRestClient() {
    server.expect(requestTo("https://api.weather.com/v1/current"))
      .andRespond(withSuccess()
        .body("{\"temperature\": 22}"));
    
    Weather weather = weatherService.getCurrentWeather("Berlin");
    
    assertThat(weather.getTemperature()).isEqualTo(22);
  }
}

Request Matching

Exact URL

server.expect(requestTo("https://api.example.com/users/1"))
  .andRespond(withSuccess());

URL Pattern

server.expect(requestTo(matchesPattern("https://api.example.com/users/\\d+")))
  .andRespond(withSuccess());

HTTP Method

server.expect(ExpectedCount.once(), 
  requestTo("https://api.example.com/users"))
  .andExpect(method(HttpMethod.POST))
  .andRespond(withCreatedEntity(URI.create("/users/1")));

Request Body

server.expect(requestTo("https://api.example.com/users"))
  .andExpect(content().contentType(MediaType.APPLICATION_JSON))
  .andExpect(content().json("{\"name\": \"John\"}"))
  .andRespond(withSuccess());

Headers

server.expect(requestTo("https://api.example.com/users"))
  .andExpect(header("Authorization", "Bearer token123"))
  .andExpect(header("X-Api-Key", "secret"))
  .andRespond(withSuccess());

Response Types

Success with Body

server.expect(requestTo("/users/1"))
  .andRespond(withSuccess()
    .contentType(MediaType.APPLICATION_JSON)
    .body("{\"id\": 1, \"name\": \"John\"}"));

Success from Resource

server.expect(requestTo("/users/1"))
  .andRespond(withSuccess()
    .body(new ClassPathResource("user-response.json")));

Created

server.expect(requestTo("/users"))
  .andExpect(method(HttpMethod.POST))
  .andRespond(withCreatedEntity(URI.create("/users/1")));

Error Response

server.expect(requestTo("/users/999"))
  .andRespond(withResourceNotFound());

server.expect(requestTo("/users"))
  .andRespond(withServerError()
    .body("Internal Server Error"));

server.expect(requestTo("/users"))
  .andRespond(withStatus(HttpStatus.BAD_REQUEST)
    .body("{\"error\": \"Invalid input\"}"));

Verifying Requests

@Test
void shouldCallApi() {
  server.expect(ExpectedCount.once(), 
    requestTo("https://api.example.com/data"))
    .andRespond(withSuccess());
  
  service.fetchData();
  
  server.verify(); // Verify all expectations met
}

Ignoring Extra Requests

@Test
void shouldHandleMultipleCalls() {
  server.expect(ExpectedCount.manyTimes(),
    requestTo(matchesPattern("/api/.*")))
    .andRespond(withSuccess());
  
  // Multiple calls allowed
  service.callApi();
  service.callApi();
  service.callApi();
}

Reset Between Tests

@BeforeEach
void setUp() {
  server.reset();
}

Testing Timeouts

server.expect(requestTo("/slow-endpoint"))
  .andRespond(withSuccess()
    .body("{\"data\": \"test\"}")
    .delay(100, TimeUnit.MILLISECONDS));

// Test timeout handling

Best Practices

1. Always verify server.verify() at end of test 2. Use resource files for large JSON responses 3. Match on minimal set of request attributes 4. Reset server in @BeforeEach 5. Test error responses, not just success 6. Verify request body for POST/PUT calls

RestTestClient

Modern REST client testing with Spring Boot 4+ (replaces TestRestTemplate).

Overview

RestTestClient is the modern alternative to TestRestTemplate in Spring Boot 4.0+. It provides a fluent, reactive API for testing REST endpoints.

Setup

Dependency (Spring Boot 4+)

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-restclient-test</artifactId>
  <scope>test</scope>
</dependency>

Basic Configuration

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
@AutoConfigureRestTestClient
class OrderIntegrationTest {
  
  @Autowired
  private RestTestClient restClient;
}

HTTP Methods

GET Request

@Test
void shouldGetOrder() {
  restClient
    .get()
    .uri("/orders/1")
    .exchange()
    .expectStatus()
    .isOk()
    .expectBody(Order.class)
    .value(order -> {
      assertThat(order.getId()).isEqualTo(1L);
      assertThat(order.getStatus()).isEqualTo("PENDING");
    });
}

POST Request

@Test
void shouldCreateOrder() {
  OrderRequest request = new OrderRequest("Laptop", 2);
  
  restClient
    .post()
    .uri("/orders")
    .contentType(MediaType.APPLICATION_JSON)
    .body(request)
    .exchange()
    .expectStatus()
    .isCreated()
    .expectHeader()
    .location("/orders/1")
    .expectBody(Long.class)
    .isEqualTo(1L);
}

PUT Request

@Test
void shouldUpdateOrder() {
  restClient
    .put()
    .uri("/orders/1")
    .body(new OrderUpdate("COMPLETED"))
    .exchange()
    .expectStatus()
    .isOk();
}

DELETE Request

@Test
void shouldDeleteOrder() {
  restClient
    .delete()
    .uri("/orders/1")
    .exchange()
    .expectStatus()
    .isNoContent();
}

Response Assertions

Status Codes

restClient
  .get()
  .uri("/orders/1")
  .exchange()
  .expectStatus()
  .isOk()           // 200
  .isCreated()      // 201
  .isNoContent()    // 204
  .isBadRequest()   // 400
  .isNotFound()     // 404
  .is5xxServerError() // 5xx
  .isEqualTo(200);  // Specific code

Headers

restClient
  .post()
  .uri("/orders")
  .exchange()
  .expectHeader()
  .location("/orders/1")
  .contentType(MediaType.APPLICATION_JSON)
  .exists("X-Request-Id")
  .valueEquals("X-Api-Version", "v1");

Body Assertions

restClient
  .get()
  .uri("/orders/1")
  .exchange()
  .expectBody(Order.class)
  .value(order -> assertThat(order.getId()).isEqualTo(1L))
  .returnResult();

JSON Path

restClient
  .get()
  .uri("/orders")
  .exchange()
  .expectBody()
  .jsonPath("$.content[0].id").isEqualTo(1)
  .jsonPath("$.content[0].status").isEqualTo("PENDING")
  .jsonPath("$.totalElements").isNumber();

Request Configuration

Headers

restClient
  .get()
  .uri("/orders/1")
  .header("Authorization", "Bearer token")
  .header("X-Api-Key", "secret")
  .exchange();

Query Parameters

restClient
  .get()
  .uri(uriBuilder -> uriBuilder
    .path("/orders")
    .queryParam("status", "PENDING")
    .queryParam("page", 0)
    .queryParam("size", 10)
    .build())
  .exchange();

Path Variables

restClient
  .get()
  .uri("/orders/{id}", 1L)
  .exchange();

With MockMvc

RestTestClient can also work with MockMvc (no server startup):

@SpringBootTest
@AutoConfigureMockMvc
@AutoConfigureRestTestClient
class OrderMockMvcTest {
  
  @Autowired
  private RestTestClient restClient;
  
  @Test
  void shouldWorkWithMockMvc() {
    // Uses MockMvc under the hood - no server startup
    restClient
      .get()
      .uri("/orders/1")
      .exchange()
      .expectStatus()
      .isOk();
  }
}

Comparison: RestTestClient vs TestRestTemplate

Migration from TestRestTemplate

Before (Deprecated)

@Autowired
private TestRestTemplate restTemplate;

@Test
void shouldGetOrder() {
  ResponseEntity<Order> response = restTemplate
    .getForEntity("/orders/1", Order.class);
  
  assertThat(response.getStatusCode()).isEqualTo(HttpStatus.OK);
  assertThat(response.getBody().getId()).isEqualTo(1L);
}

After (RestTestClient)

@Autowired
private RestTestClient restClient;

@Test
void shouldGetOrder() {
  restClient
    .get()
    .uri("/orders/1")
    .exchange()
    .expectStatus()
    .isOk()
    .expectBody(Order.class)
    .value(order -> assertThat(order.getId()).isEqualTo(1L));
}

Best Practices

1. Use with @SpringBootTest(WebEnvironment.RANDOM_PORT) for real HTTP 2. Use with @AutoConfigureMockMvc for faster tests without server 3. Leverage fluent assertions for readability 4. Test both success and error scenarios 5. Verify headers for security/API versioning

Spring Boot 4.0 Migration

Key testing changes when migrating from Spring Boot 3.x to 4.0.

Dependency Changes

Modular Test Starters

Spring Boot 4.0 introduces modular test starters:

Before (3.x):

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-test</artifactId>
  <scope>test</scope>
</dependency>

After (4.0) - WebMvc Testing:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-webmvc-test</artifactId>
  <scope>test</scope>
</dependency>

After (4.0) - REST Client Testing:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-restclient-test</artifactId>
  <scope>test</scope>
</dependency>

Annotation Migration

@MockBean → @MockitoBean

Deprecated (3.x):

@MockBean
private OrderService orderService;

New (4.0):

@MockitoBean
private OrderService orderService;

@SpyBean → @MockitoSpyBean

Deprecated (3.x):

@SpyBean
private PaymentGatewayClient paymentClient;

New (4.0):

@MockitoSpyBean
private PaymentGatewayClient paymentClient;

New Testing Features

RestTestClient

Replaces TestRestTemplate (deprecated):

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
@AutoConfigureRestTestClient
class OrderIntegrationTest {
  
  @Autowired
  private RestTestClient restClient;
  
  @Test
  void shouldCreateOrder() {
    restClient
      .post()
      .uri("/orders")
      .body(new OrderRequest("Product", 2))
      .exchange()
      .expectStatus()
      .isCreated()
      .expectHeader()
      .location("/orders/1");
  }
}

JUnit 6 Support

Spring Boot 4.0 uses JUnit 6 by default:

• JUnit 4 is deprecated (use JUnit Vintage temporarily)
• All JUnit 5 features still work
• Remove JUnit 4 dependencies for clean migration

Testcontainers 2.0

Module naming changed:

Before (1.x):

<artifactId>postgresql</artifactId>

After (2.0):

<artifactId>testcontainers-postgresql</artifactId>

Non-Singleton Bean Mocking

Spring Framework 7 allows mocking prototype-scoped beans:

@Component
@Scope("prototype")
public class OrderProcessor { }

@SpringBootTest
class OrderServiceTest {
  @MockitoBean
  private OrderProcessor orderProcessor; // Now works!
}

SpringExtension Context Changes

Extension context is now test-method scoped by default.

If tests fail with @Nested classes:

@SpringExtensionConfig(useTestClassScopedExtensionContext = true)
@SpringBootTest
class OrderTest {
  // Use old behavior
}

Migration Checklist

• [ ] Replace @MockBean with @MockitoBean
• [ ] Replace @SpyBean with @MockitoSpyBean
• [ ] Update Testcontainers dependencies to 2.0 naming
• [ ] Add modular test starters as needed
• [ ] Migrate TestRestTemplate to RestTestClient
• [ ] Remove JUnit 4 dependencies
• [ ] Update custom TestExecutionListener implementations
• [ ] Test @Nested class behavior

Backward Compatibility

Use "classic" starters for gradual migration:

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-test-classic</artifactId>
  <scope>test</scope>
</dependency>

This provides old behavior while you migrate incrementally.

Test Slices Overview

Quick reference for selecting the right Spring Boot test slice.

Decision Matrix

Selection Guide

Use NO Annotation (Plain Unit Test)

class PriceCalculatorTest {
  private PriceCalculator calculator = new PriceCalculator();
  
  @Test
  void shouldApplyDiscount() {
    var result = calculator.applyDiscount(100, 0.1);
    assertThat(result).isEqualTo(new BigDecimal("90.00"));
  }
}

When: Pure business logic, no dependencies or simple dependencies mockable via constructor injection.

Use @WebMvcTest

@WebMvcTest(OrderController.class)
class OrderControllerTest {
  @Autowired private MockMvcTester mvc;
  @MockitoBean private OrderService orderService;
}

When: Testing request mapping, validation, JSON mapping, security, filters.

What you get: MockMvc, ObjectMapper, Spring Security (if present), exception handlers.

Use @DataJpaTest

@DataJpaTest
@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
@Testcontainers
class OrderRepositoryTest {
  @Container
  static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18");
}

When: Testing custom JPA queries, entity mappings, transaction behavior, cascade operations.

What you get: Repository beans, EntityManager, TestEntityManager, transaction support.

Use @RestClientTest

@RestClientTest(WeatherService.class)
class WeatherServiceTest {
  @Autowired private WeatherService weatherService;
  @Autowired private MockRestServiceServer server;
}

When: Testing REST clients that call external APIs.

What you get: MockRestServiceServer to stub HTTP responses.

Use @JsonTest

@JsonTest
class OrderJsonTest {
  @Autowired private JacksonTester<Order> json;
}

When: Testing custom serializers/deserializers, complex JSON mapping.

Use @SpringBootTest

@SpringBootTest(webEnvironment = WebEnvironment.RANDOM_PORT)
@AutoConfigureRestTestClient
class OrderIntegrationTest {
  @Autowired private RestTestClient restClient;
}

When: Testing full request flow, security filters, database interactions together.

What you get: Full application context, embedded server (optional), real beans.

Common Mistakes

1. Using @SpringBootTest for everything - Slows down your test suite unnecessarily 2. @WebMvcTest without mocking services - Causes context loading failures 3. @DataJpaTest with @MockBean - Defeats the purpose (you want real repositories) 4. Multiple slices in one test - Each slice is a separate test class

Java 25 Features in Tests

Records for Test Data

record OrderRequest(String product, int quantity) {}
record OrderResponse(Long id, String status, BigDecimal total) {}

Pattern Matching in Tests

@Test
void shouldHandleDifferentOrderTypes() {
  var order = orderService.create(new OrderRequest("Product", 2));
  
  switch (order) {
    case PhysicalOrder po -> assertThat(po.getShippingAddress()).isNotNull();
    case DigitalOrder do_ -> assertThat(do_.getDownloadLink()).isNotNull();
    default -> throw new IllegalStateException("Unknown order type");
  }
}

Text Blocks for JSON

@Test
void shouldParseComplexJson() {
  var json = """
    {
      "id": 1,
      "status": "PENDING",
      "items": [
        {"product": "Laptop", "price": 999.99},
        {"product": "Mouse", "price": 29.99}
      ]
    }
    """;
  
  assertThat(mvc.post().uri("/orders")
    .contentType(APPLICATION_JSON)
    .content(json))
    .hasStatus(CREATED);
}

Sequenced Collections

@Test
void shouldReturnOrdersInSequence() {
  var orders = orderRepository.findAll();
  
  assertThat(orders.getFirst().getStatus()).isEqualTo("NEW");
  assertThat(orders.getLast().getStatus()).isEqualTo("COMPLETED");
  assertThat(orders.reversed().getFirst().getStatus()).isEqualTo("COMPLETED");
}

Dependencies by Slice

<!-- WebMvcTest -->
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-webmvc-test</artifactId>
  <scope>test</scope>
</dependency>

<!-- DataJpaTest -->
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-data-jpa</artifactId>
</dependency>

<!-- RestClientTest -->
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-starter-restclient-test</artifactId>
  <scope>test</scope>
</dependency>

<!-- Testcontainers -->
<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-testcontainers</artifactId>
  <scope>test</scope>
</dependency>

Testcontainers JDBC

Testing JPA repositories with real databases using Testcontainers.

Overview

Testcontainers provides real database instances in Docker containers for integration testing. More reliable than H2 for production parity.

PostgreSQL Setup

Dependencies

<dependency>
  <groupId>org.springframework.boot</groupId>
  <artifactId>spring-boot-testcontainers</artifactId>
  <scope>test</scope>
</dependency>
<dependency>
  <groupId>org.testcontainers</groupId>
  <artifactId>testcontainers-postgresql</artifactId>
  <scope>test</scope>
</dependency>

Basic Test

@DataJpaTest
@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
@Testcontainers
class OrderRepositoryPostgresTest {
  
  @Container
  @ServiceConnection
  static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18");
  
  @Autowired
  private OrderRepository orderRepository;
  
  @Autowired
  private TestEntityManager entityManager;
}

MySQL Setup

<dependency>
  <groupId>org.testcontainers</groupId>
  <artifactId>testcontainers-mysql</artifactId>
  <scope>test</scope>
</dependency>

@Container
@ServiceConnection
static MySQLContainer<?> mysql = new MySQLContainer<>("mysql:8.4");

Multiple Databases

@DataJpaTest
@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
@Testcontainers
class MultiDatabaseTest {
  
  @Container
  @ServiceConnection(name = "primary")
  static PostgreSQLContainer<?> primaryDb = new PostgreSQLContainer<>("postgres:18");
  
  @Container
  @ServiceConnection(name = "analytics")
  static PostgreSQLContainer<?> analyticsDb = new PostgreSQLContainer<>("postgres:18");
}

Container Reuse (Speed Optimization)

Add to ~/.testcontainers.properties:

testcontainers.reuse.enable=true

Then enable reuse in code:

@Container
@ServiceConnection
static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18")
  .withReuse(true);

Database Initialization

With SQL Scripts

@Container
@ServiceConnection
static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18")
  .withInitScript("schema.sql");

With Flyway

@SpringBootTest
@Testcontainers
@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
class MigrationTest {
  
  @Container
  @ServiceConnection
  static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18");
  
  @Autowired
  private Flyway flyway;
  
  @Test
  void shouldApplyMigrations() {
    flyway.migrate();
    // Test code
  }
}

Advanced Configuration

Custom Database/Schema

@Container
@ServiceConnection
static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18")
  .withDatabaseName("testdb")
  .withUsername("testuser")
  .withPassword("testpass")
  .withInitScript("init-schema.sql");

Wait Strategies

@Container
static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18")
  .waitingFor(Wait.forLogMessage(".*database system is ready.*", 1));

Test Example

@DataJpaTest
@AutoConfigureTestDatabase(replace = AutoConfigureTestDatabase.Replace.NONE)
@Testcontainers
class OrderRepositoryTest {
  
  @Container
  @ServiceConnection
  static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18");
  
  @Autowired
  private OrderRepository orderRepository;
  
  @Autowired
  private TestEntityManager entityManager;
  
  @Test
  void shouldFindOrdersByStatus() {
    // Given
    entityManager.persist(new Order("PENDING"));
    entityManager.persist(new Order("COMPLETED"));
    entityManager.flush();
    
    // When
    List<Order> pending = orderRepository.findByStatus("PENDING");
    
    // Then
    assertThat(pending).hasSize(1);
    assertThat(pending.get(0).getStatus()).isEqualTo("PENDING");
  }
  
  @Test
  void shouldSupportPostgresSpecificFeatures() {
    // Can use Postgres-specific features like:
    // - JSONB columns
    // - Array types
    // - Full-text search
  }
}

@DynamicPropertySource Alternative

If not using @ServiceConnection:

@SpringBootTest
@Testcontainers
class OrderServiceTest {
  
  @Container
  static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:18");
  
  @DynamicPropertySource
  static void configureProperties(DynamicPropertyRegistry registry) {
    registry.add("spring.datasource.url", postgres::getJdbcUrl);
    registry.add("spring.datasource.username", postgres::getUsername);
    registry.add("spring.datasource.password", postgres::getPassword);
  }
}

Supported Databases

Best Practices

1. Use @ServiceConnection when possible (Spring Boot 3.1+) 2. Enable container reuse for faster local builds 3. Use specific versions (postgres:18) not latest 4. Keep container config in static field 5. Use @DataJpaTest with AutoConfigureTestDatabase.Replace.NONE

@WebMvcTest

Testing Spring MVC controllers with focused slice tests.

Basic Structure

@WebMvcTest(OrderController.class)
class OrderControllerTest {
  
  @Autowired
  private MockMvcTester mvc;
  
  @MockitoBean
  private OrderService orderService;
  
  @MockitoBean
  private UserService userService;
}

What Gets Loaded

• The specified controller(s)
• Spring MVC infrastructure (HandlerMapping, HandlerAdapter)
• Jackson ObjectMapper (for JSON)
• Exception handlers (@ControllerAdvice)
• Spring Security filters (if on classpath)
• Validation (if on classpath)

Testing GET Endpoints

@Test
void shouldReturnOrder() {
  var order = new Order(1L, "PENDING", BigDecimal.valueOf(99.99));
  given(orderService.findById(1L)).willReturn(order);
  
  assertThat(mvc.get().uri("/orders/1"))
    .hasStatusOk()
    .hasContentType(MediaType.APPLICATION_JSON)
    .bodyJson()
    .extractingPath("$.status")
    .isEqualTo("PENDING");
}

Testing POST with Request Body

Using Text Blocks (Java 25)

@Test
void shouldCreateOrder() {
  given(orderService.create(any(OrderRequest.class))).willReturn(1L);
  
  var json = """
    {
      "product": "Product A",
      "quantity": 2
    }
    """;
  
  assertThat(mvc.post().uri("/orders")
    .contentType(MediaType.APPLICATION_JSON)
    .content(json))
    .hasStatus(HttpStatus.CREATED)
    .hasHeader("Location", "/orders/1");
}

Using Records

record OrderRequest(String product, int quantity) {}

@Test
void shouldCreateOrderWithRecord() {
  var request = new OrderRequest("Product A", 2);
  given(orderService.create(any())).willReturn(1L);
  
  assertThat(mvc.post().uri("/orders")
    .contentType(MediaType.APPLICATION_JSON)
    .content(json.write(request).getJson()))
    .hasStatus(HttpStatus.CREATED);
}

Testing Validation Errors

@Test
void shouldRejectInvalidOrder() {
  var invalidJson = """
    {
      "product": "",
      "quantity": -1
    }
    """;
  
  assertThat(mvc.post().uri("/orders")
    .contentType(MediaType.APPLICATION_JSON)
    .content(invalidJson))
    .hasStatus(HttpStatus.BAD_REQUEST)
    .bodyJson()
    .hasPath("$.errors");
}

Testing Query Parameters

@Test
void shouldFilterOrdersByStatus() {
  assertThat(mvc.get().uri("/orders?status=PENDING"))
    .hasStatusOk();
  
  verify(orderService).findByStatus(OrderStatus.PENDING);
}

Testing Path Variables

@Test
void shouldCancelOrder() {
  assertThat(mvc.put().uri("/orders/123/cancel"))
    .hasStatusOk();
  
  verify(orderService).cancel(123L);
}

Testing with Security

@Test
@WithMockUser(roles = "ADMIN")
void adminShouldDeleteOrder() {
  assertThat(mvc.delete().uri("/orders/1"))
    .hasStatus(HttpStatus.NO_CONTENT);
}

@Test
void anonymousUserShouldBeForbidden() {
  assertThat(mvc.delete().uri("/orders/1"))
    .hasStatus(HttpStatus.UNAUTHORIZED);
}

Multiple Controllers

@WebMvcTest({OrderController.class, ProductController.class})
class WebLayerTest {
  // Tests multiple controllers in one slice
}

Excluding Auto-Configuration

@WebMvcTest(OrderController.class)
@AutoConfigureMockMvc(addFilters = false) // Skip security filters
class OrderControllerWithoutSecurityTest {
  // Tests without security filters
}

Key Points

1. Always mock services with @MockitoBean 2. Use MockMvcTester for AssertJ-style assertions 3. Test HTTP semantics (status, headers, content-type) 4. Verify service method calls when side effects matter 5. Don't test business logic here - that's for unit tests 6. Leverage Java 25 text blocks for JSON payloads