@GetMapping("/newest")
Flux<ThreadsPerBoardGame> getThreads(@RequestParam String user, @RequestParam(defaultValue = "PT1H") Duration since) {
    return threadService.findNewestThreads(user, since);
}

curl -i localhost:8080/threads/newest
HTTP/1.1 400 Bad Request
Content-Type: application/json
Content-Length: 189
{"timestamp":"2020-04-15T03:40:00.460+0000","path":"/threads/newest","status":400,"error":"Bad Request","message":"Required String parameter 'user' is not present","requestId":"98427b15-7"}
curl -i "localhost:8080/threads/newest?user=chrigu&since=a"
HTTP/1.1 400 Bad Request
Content-Type: application/json
Content-Length: 156
{"timestamp":"2020-04-15T03:40:06.952+0000","path":"/threads/newest","status":400,"error":"Bad Request","message":"Type mismatch.","requestId":"7600c788-8"}

server:
  error:
    include-stacktrace: never

@GetMapping("/newest/obj")
Flux<ThreadsPerBoardGame> getThreads(@Valid ThreadRequest params) {
    return threadService.findNewestThreads(params.user, params.since);
}
static class ThreadRequest {
    @NotNull
    private final String user;
    @NotNull
    private final Duration since;
    public ThreadRequest(String user, Duration since) {
        this.user = user;
        this.since = since == null ? Duration.ofHours(1) : since;
    }
}

curl "localhost:8080/java/threads/newest/obj" -i
HTTP/1.1 400 Bad Request
Content-Type: application/json
Content-Length: 1138
{"timestamp":"2020-04-17T13:52:39.500+0000","path":"/java/threads/newest/obj","status":400,"error":"Bad Request","message":"Validation failed for argument at index 0 in method: reactor.core.publisher.Flux<ch.chrigu.bgg.service.ThreadsPerBoardGame> ch.chrigu.bgg.infrastructure.web.JavaThreadController.getThreads(ch.chrigu.bgg.infrastructure.web.JavaThreadController$ThreadRequest), with 1 error(s): [Field error in object 'threadRequest' on field 'user': rejected value [null]; codes [NotNull.threadRequest.user,NotNull.user,NotNull.java.lang.String,NotNull]; arguments [org.springframework.context.support.DefaultMessageSourceResolvable: codes [threadRequest.user,user]; arguments []; default message [user]]; default message [darf nicht null sein]] ","requestId":"c87c7cbb-17","errors":[{"codes":["NotNull.threadRequest.user","NotNull.user","NotNull.java.lang.String","NotNull"],"arguments":[{"codes":["threadRequest.user","user"],"arguments":null,"defaultMessage":"user","code":"user"}],"defaultMessage":"darf nicht null sein","objectName":"threadRequest","field":"user","rejectedValue":null,"bindingFailure":false,"code":"NotNull"}]}

curl "localhost:8080/java/threads/newest/obj?user=chrigu&since=a" -i
HTTP/1.1 500 Internal Server Error
Content-Type: application/json
Content-Length: 513
{"timestamp":"2020-04-17T13:56:42.922+0000","path":"/java/threads/newest/obj","status":500,"error":"Internal Server Error","message":"Failed to convert value of type 'java.lang.String' to required type 'java.time.Duration'; nested exception is org.springframework.core.convert.ConversionFailedException: Failed to convert from type [java.lang.String] to type [java.time.Duration] for value 'a'; nested exception is java.lang.IllegalArgumentException: Parse attempt failed for value [a]","requestId":"4c0dc6bd-21"}

@ControllerAdvice
class GlobalExceptionHandler {
    @ExceptionHandler(TypeMismatchException::class)
    fun handleTypeMismatchException(e: TypeMismatchException): HttpStatus {
        throw ResponseStatusException(HttpStatus.BAD_REQUEST, "Invalid value '${e.value}'", e)
    }
    @ExceptionHandler(WebExchangeBindException::class)
    fun handleWebExchangeBindException(e: WebExchangeBindException): HttpStatus {
        throw object : WebExchangeBindException(e.methodParameter!!, e.bindingResult) {
            override val message = "${fieldError?.field} has invalid value '${fieldError?.rejectedValue}'"
        }
    }
}