ValidationDemoController.java

@RestController
public class ValidationDemoController {

    @PostMapping("/format")
    public Mono<ResponseEntity<FormattedNameResponse>> format(@Valid @RequestBody Mono<FormatNameRequest> request) {
        return request
                .map(res -> ResponseEntity.status(HttpStatus.OK).body(FormattedNameResponseMapper.fromFormatNameRequest(res)))
                .onErrorResume(WebExchangeBindException.class,
                        ex -> Mono.just(ResponseEntity.status(HttpStatus.BAD_REQUEST)
                                        .body(FormattedNameResponseMapper.fromWebExchangeBindException(ex))));
    }
}