Java Springboot

REST API (Spring MVC)

Verified on Boot 4.1.1 (Framework 7) and, where noted, Boot 3.5.16 (Framework 6.2). Security of endpoints: spring-security. Test slices (@WebMvcTest, MockMvcTester, @RestClientTest): spring-boot-testing.

Starters and packages (Boot 3 vs 4)

Jackson 2 databind may still be on the classpath transitively (for example through Spring for Apache Kafka), which is not a reason to code against it in Boot 4. A temporary Jackson 2 bridge exists (spring-boot-jackson2, see the Boot 4 migration guide and spring-boot-4-migration).

Controllers and validation

public record CreateOrderRequest(@NotBlank String customer, @Positive BigDecimal total) {}

@RestController
@RequestMapping("/api/orders")
class OrderController {
	private final OrderService orders;
	OrderController(OrderService orders) { this.orders = orders; }

	@PostMapping
	ResponseEntity<OrderSummary> create(@Valid @RequestBody CreateOrderRequest req) {
		var created = orders.create(req);
		return ResponseEntity.created(URI.create("/api/orders/" + created.id())).body(created);
	}
}

• Nouns for resources, plural, HTTP verbs for actions; 201 + Location on create, 204 on delete, 404 via exception, not null.

• Request/response types are records, separate from entities. Bean Validation (jakarta.validation.constraints.*) goes on the record components. In Framework 6.1+ (Boot 3.2+), put constraints directly on @RequestParam/@PathVariable parameters and omit class-level @Validated: built-in MVC method validation raises HandlerMethodValidationException. Class-level @Validated instead enables AOP validation, which raises ConstraintViolationException by default and needs its own error handler. On older Framework versions, use that AOP approach and map the exception to HTTP 400 explicitly.

• Never accept unbounded input: cap size for pagination, limit request body size at the server/proxy.

• Return Page<T> only if exposing Spring Data's page JSON is acceptable; for a stable public contract return your own record PageResponse<T>(List<T> content, long total, int page, int size). Pagination parameters and stable sorting: data.md.

Errors: ProblemDetail (RFC 9457)

@RestControllerAdvice
class ApiExceptionHandler extends ResponseEntityExceptionHandler {

	@ExceptionHandler(OrderNotFoundException.class)
	ProblemDetail notFound(OrderNotFoundException ex) {
		var pd = ProblemDetail.forStatusAndDetail(HttpStatus.NOT_FOUND, ex.getMessage());
		pd.setTitle("Order not found");
		return pd;
	}
}

• Extending ResponseEntityExceptionHandler turns Spring MVC's own exceptions (validation MethodArgumentNotValidException and, in Framework 6.1+, HandlerMethodValidationException, malformed JSON, unsupported media type, invalid API version) into application/problem+json (verified: validation errors give status 400 problem details; the unknown-API-version error gives 400). Test invalid query/path parameters as well as invalid request bodies; class-level @Validated must not divert them to an unhandled AOP exception.

• Verified the same advice works unchanged on Boot 3.5.16. Do not return exception messages that contain internals (SQL, class names) in detail; log them, return a stable message, and add a custom property (pd.setProperty("code", "ORDER_NOT_FOUND")) for clients.

• Test with jsonPath("$.status"), jsonPath("$.title") and content().contentTypeCompatibleWith(MediaType.APPLICATION_PROBLEM_JSON).

API versioning (Boot 4 / Framework 7 only)

Framework 7 has first-class API versioning; it does not exist in Framework 6 (verified: @GetMapping(version = ...) does not compile on Boot 3.5.16, use URL prefixes such as /api/v1/... there).

spring.mvc.apiversion.use.header=X-API-Version
spring.mvc.apiversion.default=1
spring.mvc.apiversion.supported=1,2

@GetMapping(value = "/{id}", version = "1")  OrderSummary getV1(@PathVariable Long id) { ... }
@GetMapping(value = "/{id}", version = "2")  OrderSummary getV2(@PathVariable Long id) { ... }

Alternatives to the header are ...use.path-segment, ...use.query-parameter and ...use.media-type-parameter. Verified: a request without the header uses the default version, and an unsupported version (3) returns a 400 problem detail ("Invalid API version"). Version only when a breaking change is unavoidable; additive changes do not need a new version.

Calling other services

RestClient (Boot 3.2+/4)

RestClient and RestClientAdapter require Framework 6.1+; Boot's builder auto-configuration starts in 3.2. On Boot 3.0/3.1, retain the project's existing RestTemplate rather than generating this example or implicitly upgrading dependencies.

On Boot 4 add spring-boot-starter-restclient (it was in the verified build, which injected the builder; Boot 3.2+: RestClient.Builder comes with the web starter). Inject the builder (RestClient.Builder), never create clients without it, so that observability and customizers apply (spring-observability). Set base URL and timeouts through properties/customizers, not per call. Prefer RestClient over RestTemplate (maintenance mode) and over WebClient unless you are reactive.

@Bean
RestClient ratesClient(RestClient.Builder builder) {
	return builder.baseUrl("https://rates.example.com").build();
}
// usage: ratesClient.get().uri("/rates/{c}", "EUR").retrieve().body(Rate.class);

retrieve() throws RestClientResponseException on 4xx/5xx; map to your own exceptions at the boundary.

HTTP interfaces (@HttpExchange)

@HttpExchange("/rates")
public interface RatesClient {
	record Rate(String currency, double value) {}

	@GetExchange("/{currency}")
	Rate get(@PathVariable String currency);
}

• Boot 4 / Framework 7: register with @ImportHttpServices and configure the URL in properties (verified):

  Copy

  @Configuration(proxyBeanMethods = false)
  @ImportHttpServices(group = "rates", types = RatesClient.class)
  class HttpClientsConfig {}

  Copy

  spring.http.serviceclient.rates.base-url=https://rates.example.com

  then inject RatesClient like any bean.

• Boot 3.2+ / Framework 6.1+: @ImportHttpServices does not exist (verified). Build the proxy manually:

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  var client = HttpServiceProxyFactory.builderFor(RestClientAdapter.create(restClient)).build().createClient(RatesClient.class);

  (org.springframework.web.client.support.RestClientAdapter, same on Boot 4.)

• Boot 3.0/3.1 / Framework 6.0: if WebClient is already available, HTTP interfaces can use WebClientAdapter with the Framework 6.0 HttpServiceProxyFactory API. Otherwise retain the existing RestTemplate setup; do not introduce RestClientAdapter or add WebFlux implicitly.

• Tests (Boot 3.2+/4): MockRestServiceServer.bindTo(RestClient.Builder) records and answers requests without a network (verified for both the manual and the Boot 4 setups); use WireMock only for contract-style tests with real HTTP.

OpenAPI (springdoc)

springdoc is not Spring-Boot-managed: pick the line that matches the Boot major from the springdoc compatibility table and pin it. Verified: org.springdoc:springdoc-openapi-starter-webmvc-ui 3.1.1 on Boot 4.1.1 serves /v3/api-docs including controller paths (springdoc 2.x targets Boot 3). Expose Swagger UI (/swagger-ui.html) only in non-production profiles or behind authentication; allow the paths in spring-security. Document with @Operation/@ApiResponse sparingly; the controller signature and validation annotations already feed the schema.

Null-safety with JSpecify (Framework 7 / Boot 4)

Spring Framework 7 and Boot 4 annotate their APIs with JSpecify (org.jspecify.annotations): everything is non-null unless marked @Nullable. In your code:

• Add @NullMarked in package-info.java (verified to compile on Boot 4.1.1 where org.jspecify:jspecify is already on the classpath) and mark nullable parameters, returns and fields with org.jspecify.annotations.Nullable.

• Replace org.springframework.lang.Nullable/NonNull (deprecated in Framework 7) with JSpecify when touching those files.

• JSpecify is documentation for tools (IDE, NullAway, Error Prone) and does not validate at runtime. For request validation keep using jakarta.validation.constraints.NotNull/@NotBlank. Optional: add NullAway to the build only if the project asks for it.