Kotlin & Spring MVC - HTTP Status 400 - Null - spring

Question

I have a Kotlin #RestController, and I would expect it to return 400 Bad Request in a situation where a null is passed for a #RequestParam argument.
Example:
#RestController
class Api() {
#PostMapping("endpoint")
fun endpoint(#DateTimeFormat(iso = DATE) #RequestParam date: LocalDate) {
//do something
}
}
If I were to make a request to POST /endpoint?date I get a 500 Internal Server Error with the following (shortened) body:
{
"timestamp": "2020-09-14T20:39:38.102+0000",
"status": 500,
"error": "Internal Server Error",
"message": "Parameter specified as non-null is null: method Api.endpoint, parameter date",
"trace": "java.lang.IllegalArgumentException: Parameter specified as non-null is null: method Api.endpoint, parameter date\r\n\tat Api.endpoint(Api.kt)\r\n\t
...
...
atjava.base/java.lang.Thread.run(Thread.java:834)\r\n",
"path": "/campaigns/contributions/unfunded/retries"
}
Is there any way to fix this using either some additional library, a configuration, or some custom code that does not have other side effects that keeps everything the same except that the status code will be 400 Bad Request

Answer 1

The following works for Kotlin 1.4.*. This is not necessarily the best answer because there is no guarantee that Kotlin will not change the excption type of message in future releases. For eaxample, when I originally asked the question I was using 1.3.*, and the exception was IllegalArgumentException. In 1.4.* it has been changed to NullPointerException.
#ControllerAdvice
class KotlinNonNullParamHandling {
#ExceptionHandler
protected fun handleKotlinNonNullViolation(
exception: NullPointerException,
response: HttpServletResponse
) {
val nullParameter = exception.message?.startsWith("Parameter specified as non-null is null: ") == true
val restController = exception
.stackTrace
.getOrNull(0)
?.let { Class.forName(it.className) }
?.getAnnotation(RestController::class.java) != null
val status =
if (nullParameter && restController) HttpStatus.BAD_REQUEST
else HttpStatus.INTERNAL_SERVER_ERROR
response.sendError(status.value(), exception.message)
}
}
Before assuming that it is 400, we are checking that the exception
is a NullPointerException
has a message starting with "Parameter specified as non-null is null: "
it was thrown from a class the is RestController (otherwise it might be some other problem, such as a null being sent to a method by reflection having nothing to do with the web layer.)

Answer 2

Spring will throw the IllegalArgumentException by default when a validation fails here. Every exception inside your Spring-Boot application will be treated by default as InternalServerError.
You can modify your response code by adding a function like this:
#RestController
class Api() {
#ExceptionHandler(IllegalArgumentException.class)
#ResponseStatus(HttpStatus.BAD_REQUEST)
public void onIllegalArgumentException(IllegalArgumentException e) {
}
#PostMapping("endpoint")
fun endpoint(#DateTimeFormat(iso = DATE) #RequestParam date: LocalDate) {
//do something
}
}
Afterwards you'll get the 400 status code on invalid request param.

Answer 3

I've tested your code, and there's nothing obviously wrong with it - works as expected.
curl -v -d "date=2020-10-01" http://localhost:8080/endpoint
* Trying ::1:8080...
* TCP_NODELAY set
* Connected to localhost (::1) port 8080 (#0)
> POST /endpoint HTTP/1.1
> Host: localhost:8080
> User-Agent: curl/7.68.0
> Accept: */*
> Content-Length: 15
> Content-Type: application/x-www-form-urlencoded
>
* upload completely sent off: 15 out of 15 bytes
* Mark bundle as not supporting multiuse
< HTTP/1.1 200
< Content-Length: 0
< Date: Tue, 13 Oct 2020 07:04:21 GMT
<
* Connection #0 to host localhost left intact
So I would:
Verify that the client is properly formatting the request - e.g. try explicitly setting the Content-Type header to application/x-www-form-urlencoded, check if there's no typo in the param name etc.
That you have Jackson properly configured to work with Kotlin (com.fasterxml.jackson.module:jackson-module-kotlin - dependency)