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Kartik Dhiman e4fc57f204 feat: add spring-boot-testing skill for Spring Boot 4.0 (#1085 )

- Introduced MockMvcTester for AssertJ-style assertions in Spring MVC testing.
- Added @RestClientTest for testing REST clients with MockRestServiceServer.
- Implemented RestTestClient as a modern alternative to TestRestTemplate.
- Documented migration steps from Spring Boot 3.x to 4.0, including dependency and annotation changes.
- Created an overview of test slices to guide testing strategies.
- Included Testcontainers setup for JDBC testing with PostgreSQL and MySQL.
- Enhanced @WebMvcTest documentation with examples for various HTTP methods and validation.

2026-03-20 10:24:37 +11:00

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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.

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