Ktor: How can I validate JSON request?

Question

I already know how to receive a JSON object and automatically deserialize it into the required format (e.g. with a data class). Also look here: How to receive JSON object in Ktor?

My problem now is that I want to validate the JSON request and return BadRequest if it's not in the desired format, something like that in Django: https://stackoverflow.com/a/44085405/5005715

How can I do that in Ktor/Kotlin? Unfortunately, I couldn't find a solution in the docs. Also, required/optional fields would be nice.

Answer 1

You can use hibernate-validator for input validations. Refer below:

Add Dependency (Gradle):

compile "org.hibernate.validator:hibernate-validator:6.1.1.Final"

Annotate your data class (DTO):

data class SampleDto(
    @field:Min(value=100)
    val id: Int,
    @field:Max(value=99)
    val age: Int
)

Add Validator in Routing:

import javax.validation.Validation

fun Application.module() {

    val service = SampleService()
    val validator = Validation.buildDefaultValidatorFactory().validator

    routing {
        post("/sample/resource/") {
            val sampleDto = call.receive<SampleDto>()
            sampleDto.validate(validator)
            service.process(sampleDto)
            call.respond(HttpStatusCode.OK)
        }
    }
}

@Throws(BadRequestException::class)
fun <T : Any> T.validate(validator: Validator) {
    validator.validate(this)
        .takeIf { it.isNotEmpty() }
        ?.let { throw BadRequestException(it.first().messageWithFieldName()) }
}

fun <T : Any> ConstraintViolation<T>.messageWithFieldName() = "${this.propertyPath} ${this.message}"

Bonus Step (Optional) - Add Exception Handler:

fun Application.exceptionHandler() {

    install(StatusPages) {
        exception<BadRequestException> { e ->
            call.respond(HttpStatusCode.BadRequest, ErrorDto(e.message, HttpStatusCode.BadRequest.value))
            throw e
        }
    }

}

data class ErrorDto(val message: String, val errorCode: Int)

Answer 2

Here is a quick example of how to validate and respond with 400 if needed.

fun main(args: Array<String>) {
    embeddedServer(Netty, 5000) {
        install(CallLogging)
        install(ContentNegotiation) { gson { } }
        install(Routing) {
            post("test") {
                val sample = call.receive<Sample>()
                if (!sample.validate()) {
                    call.respond(HttpStatusCode.BadRequest, "Sample did not pass validation")
                }
                call.respond("Ok")
            }
        }
    }.start()
}

fun Sample.validate(): Boolean = id > 5

data class Sample(val id: Int)

Did you have something else in mind?

There are no inbuilt annotations or the like.