spring-boot-web-api

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Spring Boot 4 REST API implementation patterns. Use when creating REST controllers, request validation, exception handlers with ProblemDetail (RFC 9457), API versioning, content negotiation, or WebFlux reactive endpoints. Covers @RestController patterns, Bean Validation 3.1, global error handling, and Jackson 3 configuration.

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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

Create controller → @RestController with @RequestMapping base path
Define endpoints → @GetMapping, @PostMapping, etc.
Add validation → @Valid on request body, custom validators
Handle exceptions → @RestControllerAdvice with ProblemDetail
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

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

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