Spring Boot Web API Layer

REST API implementation patterns for Spring Boot 4 with Spring MVC and WebFlux.

Technology Selection

With Virtual Threads (Java 21+), MVC handles high concurrency without WebFlux complexity.

Core Workflow

1. Create controller → 2. Define endpoints → 3. Add validation → 4. Handle exceptions → 5. Configure versioning

See [WORKFLOW.md](WORKFLOW.md) for detailed step-by-step instructions with code examples.

Quick Patterns

See [EXAMPLES.md](EXAMPLES.md) for complete working examples including:

• REST Controller with CRUD operations and pagination (Java + Kotlin)
• Request/Response DTOs with Bean Validation 3.1
• Global Exception Handler using ProblemDetail (RFC 9457)
• Native API Versioning with header configuration
• Jackson 3 Configuration for custom serialization
• Controller Testing with @WebMvcTest

Spring Boot 4 Specifics

• Jackson 3 uses tools.jackson package (not com.fasterxml.jackson)
• ProblemDetail enabled by default: spring.mvc.problemdetails.enabled=true
• API Versioning via version attribute in mapping annotations
• @MockitoBean replaces @MockBean in tests
• @HttpExchange declarative HTTP clients (replaces RestTemplate patterns)
• RestTestClient new fluent API for testing REST endpoints

@HttpExchange Declarative Client (Spring 7)

New declarative HTTP client interface (alternative to RestTemplate/WebClient):

@HttpExchange(url = "/users", accept = "application/json")
public interface UserClient {

    @GetExchange("/{id}")
    User getUser(@PathVariable Long id);

    @PostExchange
    User createUser(@RequestBody CreateUserRequest request);

    @DeleteExchange("/{id}")
    void deleteUser(@PathVariable Long id);
}

// Configuration
@Configuration
class ClientConfig {
    @Bean
    UserClient userClient(RestClient.Builder builder) {
        RestClient restClient = builder.baseUrl("https://api.example.com").build();
        return HttpServiceProxyFactory
            .builderFor(RestClientAdapter.create(restClient))
            .build()
            .createClient(UserClient.class);
    }
}

Benefits: Type-safe, annotation-driven, works with both RestClient and WebClient.

Detailed References

• Workflow: See [WORKFLOW.md](WORKFLOW.md) for detailed step-by-step web API implementation
• Examples: See [EXAMPLES.md](EXAMPLES.md) for complete working code examples
• Troubleshooting: See [TROUBLESHOOTING.md](TROUBLESHOOTING.md) for common issues and Boot 4 migration
• Controllers & Validation: See [references/CONTROLLERS.md](references/CONTROLLERS.md) for validation groups, custom validators, content negotiation
• Error Handling: See [references/ERROR-HANDLING.md](references/ERROR-HANDLING.md) for ProblemDetail patterns, exception hierarchy
• WebFlux Patterns: See [references/WEBFLUX.md](references/WEBFLUX.md) for reactive endpoints, functional routers, WebTestClient

Related Skills

Anti-Pattern Checklist

Critical Reminders

1. Controllers are thin — Delegate to services, no business logic
2. Validate at the boundary — @Valid on all request bodies
3. Use ProblemDetail — Structured errors for all exceptions
4. Version from day one — Easier than retrofitting
5. @MockitoBean not @MockBean — Spring Boot 4 change