Agent Skills: Spring Boot Web API Layer

Spring Boot Web API Layer

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

Technology Selection

| Choose | When | |--------|------| | Spring MVC | JPA/JDBC backend, simpler debugging, team knows imperative style | | Spring WebFlux | High concurrency (10k+ connections), streaming, reactive DB (R2DBC) |

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

Core Workflow

1. Create controller → @RestController with @RequestMapping base path
2. Define endpoints → @GetMapping, @PostMapping, etc.
3. Add validation → @Valid on request body, custom validators
4. Handle exceptions → @RestControllerAdvice with ProblemDetail
5. Configure versioning → Native API versioning (Spring Boot 4)

Quick Patterns

See 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

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

Anti-Pattern Checklist

| Anti-Pattern | Fix | |--------------|-----| | Business logic in controllers | Delegate to application services | | Returning entities directly | Convert to DTOs | | Generic error messages | Use typed ProblemDetail with error URIs | | Missing validation | Add @Valid on @RequestBody | | Blocking calls in WebFlux | Use reactive operators only | | Catching exceptions silently | Let propagate to @RestControllerAdvice |

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