REST API Conventions

REST API Conventions

Project Contract

Inspect existing controllers, tests and OpenAPI before choosing a response contract. Preserve the project's IDs, response shape and versioning strategy. Do not migrate unrelated endpoints while adding one route. The envelope below is an optional convention for a project that uses it; plain success DTOs are equally valid. A 204 response has no body.

Success and error formats are independent: an existing success envelope can coexist with RFC 9457 errors. Use one consistent error policy; choose the legacy error examples below only when the project already requires that format.

Example success 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 — route with native API versioning (below), don't duplicate controllers per version
• Nested resources max 2 levels: /orders/{id}/items ✅, /orders/{id}/items/{itemId}/notes ❌ — flatten to /order-item-notes/{id}
• IDs: preserve the existing ID type. UUIDs are an option, not an authorization mechanism.

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

API Versioning (Native in Boot 4)

Spring Boot 4 / Framework 7 route requests by API version natively — never hand-roll it with duplicated V1/V2 controllers, custom RequestConditions, or header if checks.

Pick ONE resolution strategy per API (path segment, header, query param, or media-type param):

spring:
  mvc:                        # WebFlux: same keys under spring.webflux.apiversion.*
    apiversion:
      use:
        path-segment: 1       # index of the path segment holding the version: /api/v1.1/orders
        # header: X-API-Version
        # query-parameter: version
      supported: [1.0, 1.1, 2.0]
      default: 1.0

Route with the version attribute on any mapping annotation:

@GetMapping("/{id}")                            // no version — matches any
public OrderResponse getById(@PathVariable UUID id) { ... }

@GetMapping(value = "/{id}", version = "1.1")   // fixed: matches 1.1 only
public OrderResponseV1_1 getByIdV1_1(@PathVariable UUID id) { ... }

@GetMapping(value = "/{id}", version = "1.2+")  // baseline: 1.2 and supported versions above
public OrderResponseV2 getByIdV2(@PathVariable UUID id) { ... }

The most specific matching version wins. Unsupported version → 400 (InvalidApiVersionException); missing required version → 400 (MissingApiVersionException).

• Deprecate old versions with StandardApiVersionDeprecationHandler (register via WebMvcConfigurer#configureApiVersioning(ApiVersionConfigurer)) — it emits RFC 9745 Deprecation/Sunset and Link response headers
• Clients: RestClient/WebClient and HTTP interface clients send versions too — configure .apiVersionInserter(ApiVersionInserter.fromHeader("X-API-Version").build()) and .defaultVersion("1.2") on the builder, matching the server's strategy

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<PageResponse<OrderResponse>> list(Pageable pageable) {
    return ApiResponse.ok(PageResponse.from(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

The compiled PageResponse fixes the JSON pagination contract. Mapping entities to DTOs alone does not stabilize Spring Data PageImpl serialization. Spring Data's org.springframework.data.web.PagedModel is another option when its shape fits the API. Validate sort fields against an allowlist and add an ID tie-breaker to non-unique sorts.

Legacy Envelope 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 imposes a new envelope - preserve the existing success contract.
• Agent invents competing error handlers - use the project's established error contract.
• Agent puts exception handlers in controllers — always use @RestControllerAdvice
• Agent changes ID types while adding an endpoint - retain the existing ID scheme.
• Agent accepts unbounded Pageable — set spring.data.web.pageable.max-page-size or one request can pull the whole table
• Agent serializes PageImpl directly - map entities to DTOs, then wrap in PageResponse.
• Agent hand-rolls versioning with duplicated /v1//v2 controllers — Boot 4 has native API versioning: version attribute on mappings + spring.mvc.apiversion.*
• Agent adds spring-boot-starter-web — renamed spring-boot-starter-webmvc in Boot 4 (MockMvc tests: spring-boot-starter-webmvc-test)
• Agent uses @JsonComponent or Jackson2ObjectMapperBuilderCustomizer to tune serialization — Jackson 3 renames: @JacksonComponent, JsonMapperBuilderCustomizer; declare JsonMapper beans, not generic ObjectMapper
• Agent tests controllers with bare @SpringBootTest expecting MockMvc — Boot 4 no longer auto-provides it; add @AutoConfigureMockMvc (or the new RestTestClient via @AutoConfigureRestTestClient)