REST API Conventions

Response Envelope

All endpoints return a consistent envelope:

{
  "success": true,
  "data": { },
  "error": null,
  "timestamp": "2026-04-13T10:00:00Z"
}

Error response:

{
  "success": false,
  "data": null,
  "error": {
    "code": "ORDER_NOT_FOUND",
    "message": "Order with id 123 not found",
    "details": []
  },
  "timestamp": "2026-04-13T10:00:00Z"
}

ApiResponse Wrapper

@JsonInclude(JsonInclude.Include.NON_NULL)
public record ApiResponse<T>(
    boolean success,
    T data,
    ApiError error,
    Instant timestamp
) {
    public static <T> ApiResponse<T> ok(T data) {
        return new ApiResponse<>(true, data, null, Instant.now());
    }

    public static <T> ApiResponse<T> error(String code, String message) {
        return new ApiResponse<>(false, null, new ApiError(code, message, List.of()), Instant.now());
    }
}

public record ApiError(String code, String message, List<String> details) {}

HTTP Status Mapping

URL Conventions

• Plural nouns for resources: /orders, /users, /products
• Kebab-case for multi-word: /order-items, not /orderItems
• Versioning in path: /api/v1/orders
• Nested resources max 2 levels: /orders/{id}/items ✅, /orders/{id}/items/{itemId}/notes ❌ — flatten to /order-item-notes/{id}
• IDs as UUIDs in path, never auto-increment integers exposed in URL

GET    /api/v1/orders              → list (paginated)
POST   /api/v1/orders              → create
GET    /api/v1/orders/{id}         → get one
PUT    /api/v1/orders/{id}         → full update
PATCH  /api/v1/orders/{id}         → partial update
DELETE /api/v1/orders/{id}         → delete
GET    /api/v1/orders/{id}/items   → nested resource

Pagination

{
  "success": true,
  "data": {
    "content": [...],
    "page": 0,
    "size": 20,
    "totalElements": 150,
    "totalPages": 8,
    "last": false
  }
}

Query params: ?page=0&size=20&sort=createdAt,desc

Use Spring Data Pageable in controllers:

@GetMapping
public ApiResponse<Page<OrderResponse>> list(Pageable pageable) {
    return ApiResponse.ok(orderService.findAll(pageable).map(OrderResponse::from));
}

Cap the page size. A bare Pageable accepts ?size=100000 from any client — one request can drag your whole table into memory. Spring's default cap is 2000, still too high for most APIs:

spring:
  data:
    web:
      pageable:
        default-page-size: 20
        max-page-size: 100   # requests above this are silently clamped

Global Exception Handler

@RestControllerAdvice
@RequiredArgsConstructor
public class GlobalExceptionHandler {

    @ExceptionHandler(EntityNotFoundException.class)
    public ResponseEntity<ApiResponse<Void>> handleNotFound(EntityNotFoundException ex) {
        return ResponseEntity.status(404).body(ApiResponse.error("NOT_FOUND", ex.getMessage()));
    }

    @ExceptionHandler(MethodArgumentNotValidException.class)
    public ResponseEntity<ApiResponse<Void>> handleValidation(MethodArgumentNotValidException ex) {
        List<String> details = ex.getBindingResult().getFieldErrors().stream()
            .map(e -> e.getField() + ": " + e.getDefaultMessage()).toList();
        return ResponseEntity.status(400)
            .body(new ApiResponse<>(false, null, new ApiError("VALIDATION_FAILED", "Invalid input", details), Instant.now()));
    }

    @ExceptionHandler(Exception.class)
    public ResponseEntity<ApiResponse<Void>> handleGeneric(Exception ex) {
        return ResponseEntity.status(500).body(ApiResponse.error("INTERNAL_ERROR", "An unexpected error occurred"));
    }
}

Gotchas

• Agent returns raw objects without envelope — always wrap in ApiResponse.ok(...)
• Agent uses ResponseEntity<Map<String, Object>> for errors — use ApiResponse
• Agent puts exception handlers in controllers — always use @RestControllerAdvice
• Agent uses Long IDs in URLs — use UUID
• Agent accepts unbounded Pageable — set spring.data.web.pageable.max-page-size or one request can pull the whole table
• Agent returns Page<Entity> serialized directly — exposes Hibernate internals; map to DTOs first