springboot-patterns

# Spring Boot Development Patterns

Spring Boot architecture and API patterns for scalable, production-grade services.

## When to Activate

- Building REST APIs with Spring MVC or WebFlux
- Structuring adapters → use cases → domain (hexagonal / ports & adapters)
- Configuring Spring Data JPA, caching, or async processing
- Adding validation, exception handling, or pagination
- Setting up profiles for dev/staging/production environments
- Implementing event-driven patterns with Spring Events or Kafka

## REST API Structure

```java
@RestController
@RequestMapping("/api/markets")
@Validated
class MarketController {
  private final MarketService marketService;

  MarketController(MarketService marketService) {
    this.marketService = marketService;
  }

  @GetMapping
  ResponseEntity<Page<MarketResponse>> list(
      @RequestParam(defaultValue = "0") int page,
      @RequestParam(defaultValue = "20") int size) {
    Page<Market> markets = marketService.list(PageRequest.of(page, size));
    return ResponseEntity.ok(markets.map(MarketResponse::from));
  }

  @PostMapping
  ResponseEntity<MarketResponse> create(@Valid @RequestBody CreateMarketRequest request) {
    Market market = marketService.create(request);
    return ResponseEntity.status(HttpStatus.CREATED).body(MarketResponse.from(market));
  }
}
```

## Repository Pattern (Spring Data JPA)

```java
public interface MarketRepository extends JpaRepository<MarketEntity, Long> {
  @Query("select m from MarketEntity m where m.status = :status order by m.volume desc")
  List<MarketEntity> findActive(@Param("status") MarketStatus status, Pageable pageable);
}
```

## Use Case (Application Service) with Transactions

Use cases implement input ports and depend on output ports — no JPA or Spring framework imports in domain:

```java
// domain/port/in/CreateMarketUseCase.java
public interface CreateMarketUseCase {
  Market create(CreateMarketCommand command);
}

// domain/port/out/MarketRepository.java
public interface MarketRepository {
  Market save(Market market);
  Optional<Market> findBySlug(String slug);
}

// application/usecase/CreateMarketService.java — implements input port, uses output port
@Transactional
public class CreateMarketService implements CreateMarketUseCase {
  private final MarketRepository marketRepository;  // output port interface

  public CreateMarketService(MarketRepository marketRepository) {
    this.marketRepository = marketRepository;
  }

  @Override
  public Market create(CreateMarketCommand command) {
    var market = Market.create(command.name(), command.slug());
    return marketRepository.save(market);
  }
}

// adapter/out/persistence/JpaMarketRepository.java — implements output port
@Repository
class JpaMarketRepository implements MarketRepository {
  private final MarketJpaRepository jpaRepo;

  JpaMarketRepository(MarketJpaRepository jpaRepo) {
    this.jpaRepo = jpaRepo;
  }

  @Override
  public Market save(Market market) {
    return MarketMapper.toDomain(jpaRepo.save(MarketMapper.toEntity(market)));
  }

  @Override
  public Optional<Market> findBySlug(String slug) {
    return jpaRepo.findBySlug(slug).map(MarketMapper::toDomain);
  }
}
```

## DTOs and Validation

```java
public record CreateMarketRequest(
    @NotBlank @Size(max = 200) String name,
    @NotBlank @Size(max = 2000) String description,
    @NotNull @FutureOrPresent Instant endDate,
    @NotEmpty List<@NotBlank String> categories) {}

public record MarketResponse(Long id, String name, MarketStatus status) {
  static MarketResponse from(Market market) {
    return new MarketResponse(market.id(), market.name(), market.status());
  }
}
```

## Exception Handling (RFC 7807 / RFC 9457 Problem Details)

Spring Boot 4 has native RFC 7807 support via `ProblemDetail`. Enable it in `application.yml`:

```yaml
spring:
  mvc:
    problemdetails:
      enabled: true   # auto-maps built-in exceptions to ProblemDetail
```

This automatically handles `MethodArgumentNotValidException`, `NoResourceFoundException`, etc.
For domain exceptions, add a `@RestControllerAdvice`:

```java
@RestControllerAdvice
class ProblemDetailsAdvice {

  @ExceptionHandler(MarketNotFoundException.class)
  ProblemDetail handleNotFound(MarketNotFoundException ex, HttpServletRequest req) {
    ProblemDetail pd = ProblemDetail.forStatusAndDetail(HttpStatus.NOT_FOUND, ex.getMessage());
    pd.setType(URI.create("https://api.example.com/problems/not-found"));
    pd.setTitle("Not Found");
    pd.setProperty("instance", req.getRequestURI());
    return pd;  // Spring sets Content-Type: application/problem+json automatically
  }

  @ExceptionHandler(ConstraintViolationException.class)
  ProblemDetail handleValidation(ConstraintViolationException ex, HttpServletRequest req) {
    ProblemDetail pd = ProblemDetail.forStatus(HttpStatus.UNPROCESSABLE_ENTITY);
    pd.setType(URI.create("https://api.example.com/problems/validation-failed"));
    pd.setTitle("Validation Failed");
    pd.setDetail("One or more fields failed validation.");
    pd.setProperty("instance", req.getRequestURI());
    pd.setProperty("errors", ex.getConstraintViolations().stream()
        .map(v -> Map.of("field", v.getPropertyPath().toString(), "detail", v.getMessage()))
        .toList());
    return pd;
  }

  @ExceptionHandler(AccessDeniedException.class)
  ProblemDetail handleAccessDenied(HttpServletRequest req) {
    ProblemDetail pd = ProblemDetail.forStatus(HttpStatus.FORBIDDEN);
    pd.setType(URI.create("https://api.example.com/problems/forbidden"));
    pd.setTitle("Forbidden");
    pd.setProperty("instance", req.getRequestURI());
    return pd;
  }

  @ExceptionHandler(Exception.class)
  ProblemDetail handleGeneric(Exception ex, HttpServletRequest req) {
    log.error("unhandled exception uri={}", req.getRequestURI(), ex);
    ProblemDetail pd = ProblemDetail.forStatus(HttpStatus.INTERNAL_SERVER_ERROR);
    pd.setType(URI.create("about:blank"));
    pd.setTitle("Internal Server Error");
    return pd;
  }
}
```

Response example:
```
HTTP/1.1 404 Not Found
Content-Type: application/problem+json

{
  "type": "https://api.example.com/problems/not-found",
  "title": "Not Found",
  "status": 404,
  "detail": "Market 'crypto-btc' not found.",
  "instance": "/api/markets/crypto-btc"
}
```

See skill: `problem-details` for the full RFC 7807/9457 reference and multi-language examples.

## Caching

Requires `@EnableCaching` on a configuration class.

```java
@Service
public class MarketCacheService {
  private final MarketRepository repo;

  public MarketCacheService(MarketRepository repo) {
    this.repo = repo;
  }

  @Cacheable(value = "market", key = "#id")
  public Market getById(Long id) {
    return repo.findById(id)
        .map(Market::from)
        .orElseThrow(() -> new EntityNotFoundException("Market not found"));
  }

  @CacheEvict(value = "market", key = "#id")
  public void evict(Long id) {}
}
```

## Async Processing

Requires `@EnableAsync` on a configuration class.

```java
@Service
public class NotificationService {
  @Async
  public CompletableFuture<Void> sendAsync(Notification notification) {
    // send email/SMS
    return CompletableFuture.completedFuture(null);
  }
}
```

## Logging (SLF4J)

```java
@Service
public class ReportService {
  private static final Logger log = LoggerFactory.getLogger(ReportService.class);

  public Report generate(Long marketId) {
    log.info("generate_report marketId={}", marketId);
    try {
      // logic
    } catch (Exception ex) {
      log.error("generate_report_failed marketId={}", marketId, ex);
      throw ex;
    }
    return new Report();
  }
}
```

## Middleware / Filters

```java
@Component
public class RequestLoggingFilter extends OncePerRequestFilter {
  private static final Logger log = LoggerFactory.getLogger(RequestLoggingFilter.class);

  @Override
  protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response,
      FilterChain filterChain) throws ServletException, IOException {
    long start = System.currentTimeMillis();
    try {
      filterChain.doFilter(request, response);
    } finally {
      long duration = System.currentTimeMillis() - start;
      log.info("req method={} uri={} status={} durationMs={}",
          request.getMethod(), request.getRequestURI(), response.getStatus(), duration);
    }
  }
}
```

## Pagination and Sorting

```java
PageRequest page = PageRequest.of(pageNumber, pageSize, Sort.by("createdAt").descending());
Page<Market> results = marketService.list(page);
```

## Error-Resilient External Calls

```java
public <T> T withRetry(Supplier<T> supplier, int maxRetries) {
  int attempts = 0;
  while (true) {
    try {
      return supplier.get();
    } catch (Exception ex) {
      attempts++;
      if (attempts >= maxRetries) {
        throw ex;
      }
      try {
        Thread.sleep((long) Math.pow(2, attempts) * 100L);
      } catch (InterruptedException ie) {
        Thread.currentThread().interrupt();
        throw ex;
      }
    }
  }
}
```

## Rate Limiting (Filter + Bucket4j)

**Security Note**: The `X-Forwarded-For` header is untrusted by default because clients can spoof it.
Only use forwarded headers when:
1. Your app is behind a trusted reverse proxy (nginx, AWS ALB, etc.)
2. You have registered `ForwardedHeaderFilter` as a bean
3. You have configured `server.forward-headers-strategy=NATIVE` or `FRAMEWORK` in application properties
4. Your proxy is configured to overwrite (not append to) the `X-Forwarded-For` header

When `ForwardedHeaderFilter` is properly configured, `request.getRemoteAddr()` will automatically
return the correct client IP from the forwarded headers. Without this configuration, use
`request.getRemoteAddr()` directly—it returns the immediate connection IP, which is the only
trustworthy value.

```java
@Component
public class RateLimitFilter extends OncePerRequestFilter {
  private final Map<String, Bucket> buckets = new ConcurrentHashMap<>();

  /*
   * SECURITY: This filter uses request.getRemoteAddr() to identify clients for rate limiting.
   *
   * If your application is behind a reverse proxy (nginx, AWS ALB, etc.), you MUST configure
   * Spring to handle forwarded headers properly for accurate client IP detection:
   *
   * 1. Set server.forward-headers-strategy=NATIVE (for cloud platforms) or FRAMEWORK in
   *    application.properties/yaml
   * 2. If using FRAMEWORK strategy, register ForwardedHeaderFilter:
   *
   *    @Bean
   *    ForwardedHeaderFilter forwardedHeaderFilter() {
   *        return new ForwardedHeaderFilter();
   *    }
   *
   * 3. Ensure your proxy overwrites (not appends) the X-Forwarded-For header to prevent spoofing
   * 4. Configure server.tomcat.remoteip.trusted-proxies or equivalent for your container
   *
   * Without this configuration, request.getRemoteAddr() returns the proxy IP, not the client IP.
   * Do NOT read X-Forwarded-For directly—it is trivially spoofable without trusted proxy handling.
   */
  @Override
  protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response,
      FilterChain filterChain) throws ServletException, IOException {
    // Use getRemoteAddr() which returns the correct client IP when ForwardedHeaderFilter
    // is configured, or the direct connection IP otherwise. Never trust X-Forwarded-For
    // headers directly without proper proxy configuration.
    String clientIp = request.getRemoteAddr();

    Bucket bucket = buckets.computeIfAbsent(clientIp,
        k -> Bucket.builder()
            .addLimit(Bandwidth.classic(100, Refill.greedy(100, Duration.ofMinutes(1))))
            .build());

    if (bucket.tryConsume(1)) {
      filterChain.doFilter(request, response);
    } else {
      response.setStatus(HttpStatus.TOO_MANY_REQUESTS.value());
    }
  }
}
```

## Background Jobs

Use Spring’s `@Scheduled` or integrate with queues (e.g., Kafka, SQS, RabbitMQ). Keep handlers idempotent and observable.

## Observability

- Structured logging (JSON) via Logback encoder
- Metrics: Micrometer + Prometheus/OTel
- Tracing: Micrometer Tracing with OpenTelemetry or Brave backend

## Production Defaults

- Prefer constructor injection, avoid field injection
- Enable `spring.mvc.problemdetails.enabled=true` for RFC 7807 errors (Spring Boot 4+)
- Enable virtual threads for I/O-bound throughput (Spring Boot 4+):
  ```yaml
  spring:
    threads:
      virtual:
        enabled: true   # Tomcat + @Async + @Scheduled all use virtual threads
  ```
- Configure HikariCP pool sizes for workload, set timeouts
- Use `@Transactional(readOnly = true)` for queries
- Enforce null-safety via `@NonNull` and `Optional` where appropriate

**Remember**: Keep domain framework-free, use cases focused on business logic, adapters thin (map only), and errors handled centrally. Dependency arrows always point inward — toward domain. Optimize for testability and replaceability of adapters.