Welcome to VnnTools!

Server-side rendering with Thymeleaf 3.1.5 in Spring Boot 4.1.1, checked on a running application: when SSR beats a JSON API, how a view name becomes classpath:/templates/products/list.html, the five standard expressions, th:text versus th:utext and XSS, th:each with #numbers and #temporals, a validated create form with th:field and th:errors, where BindingResult must go, Post/Redirect/Get with flash attributes, fragments, static CSS, what spring.thymeleaf.cache really changes, and HTML error pages.

Calling external HTTP APIs from Spring Boot 4.1.1 with RestClient, checked against a local stub: RestClient vs RestTemplate, WebClient and @HttpExchange, spring-boot-starter-restclient and the auto-configured RestClient.Builder, GET into records and lists, toEntity, query parameter encoding, POST, PUT and DELETE, the real HttpClientErrorException messages, onStatus, defaultStatusHandler and exchange, measured default and configured connect and read timeouts with spring.http.clients, a logging ClientHttpRequestInterceptor, and turning upstream failures into 502, 503 and 504.

springdoc-openapi 3.1.1 on Spring Boot 4.1.1, checked on a running jar: the OpenAPI 3.1 document at /v3/api-docs, Swagger UI and Try it out, what springdoc infers from controllers, DTO records and Bean Validation constraints, which @RestControllerAdvice responses it adds, @Tag, @Operation, @ApiResponse, @Parameter and @Schema on records, a global OpenAPI bean and customizer, GroupedOpenApi, springdoc properties and switching the docs off in a prod profile.

Layered architecture in Spring Boot 4.1.1, refactored and verified on a real catalogue API: the controller that does everything and the startup crash when it is reused, what controller, service and repository each own, where DTO mapping belongs, a step-by-step refactor proven unchanged with curl, a service interface or a concrete class checked with Mockito 5.23, five layering anti-patterns, and package by layer versus package by feature measured on one change, with package-private beans injected and the compiler error that keeps features apart.

Global exception handling in Spring Boot 4.1.1, verified on a real project: the default /error body and BasicErrorController, spring.web.error.* replacing server.error.*, @ResponseStatus and ResponseStatusException, @ExceptionHandler in a controller and in @RestControllerAdvice, how Spring picks one handler by type distance, controller, @Order and cause, ProblemDetail (RFC 9457) and application/problem+json, ErrorResponseException, spring.mvc.problemdetails.enabled, ResponseEntityExceptionHandler with a 422 field error list, and a catch-all that keeps framework 4xx responses.

Bean Validation in Spring Boot 4.1.1 with Hibernate Validator 9.1.3, checked against real runs: spring-boot-starter-validation, @NotNull vs @NotEmpty vs @NotBlank, @Size, @DecimalMin, @Digits, @Email and @Pattern on request DTO records, @Valid on @RequestBody and the default 400, nested objects and lists, @PathVariable and @RequestParam validation and the @Validated 500 trap, validation groups, ValidationMessages.properties and Accept-Language, custom ConstraintValidator and cross-field constraints, and validation in the service layer.