Kotlin Spring Boot bean validation not working

Question

I have quite a few projects that is slowly being migrated from Java to Kotlin, but I'm facing a problem when changing from Java POJO to Kotlin data classes. Bean validation stops working in REST controllers. I have created a very simple project directly from https://start.spring.io to demonstrate the failure.

@SpringBootApplication
class RestValidationApplication

fun main(args: Array<String>) {
    runApplication<RestValidationApplication>(*args)
}

@RestController
class Controller {

    @PostMapping
    fun test(@Valid @RequestBody request: Test) {
        println(request)
    }
}

data class Test(@field:NotNull val id: String)

and gradle:

import org.jetbrains.kotlin.gradle.tasks.KotlinCompile

plugins {
    id("org.springframework.boot") version "2.6.1"
    id("io.spring.dependency-management") version "1.0.11.RELEASE"
    kotlin("jvm") version "1.6.0"
    kotlin("plugin.spring") version "1.6.0"
}

group = "com.example"
version = "0.0.1-SNAPSHOT"
java.sourceCompatibility = JavaVersion.VERSION_17

repositories {
    mavenCentral()
}

dependencies {
    implementation("org.springframework.boot:spring-boot-starter-validation")
    implementation("org.springframework.boot:spring-boot-starter-web")
    implementation("com.fasterxml.jackson.module:jackson-module-kotlin")
    implementation("org.jetbrains.kotlin:kotlin-reflect")
    implementation("org.jetbrains.kotlin:kotlin-stdlib-jdk8")
    testImplementation("org.springframework.boot:spring-boot-starter-test")
}

tasks.withType<KotlinCompile> {
    kotlinOptions {
        freeCompilerArgs = listOf("-Xjsr305=strict")
        jvmTarget = "17"
    }
} 

Sending a request does not trigger the bean validation, but it throws a HttpMessageNotReadableException because id is NULL.

curl -X POST http://localhost:8080 -H "Content-Type: application/json" -d "{}"

I have tried both @Valid and @Validated and using @get:NotNull on the attributes, but nothing works. I see many others have the same problem and using a Java class from a Kotlin REST controller works. Also changing to a non data class makes validation works. Anyone know if it's possible to get bean validation working with Kotlin data classes?

Answer 1

Make the field nullable even its not, and then add @field:NotNull

data class Test(@field:NotNull val id: String?) 

This will stop kotlin validation to happen before javax