So, I am trying to implement an annotation in Kotlin, and it works, but I definitely feel like I could do a lot better. Below is the code and some areas I think could be improved. The purpose of this validator is to be applied to a field/getter and fail if it's not of a certain type (please don't ask why I need to do this)


  1. For starters, It's not clear to me how I can get it to have a default error message of ${field} must be of type {type} When I try that, I get no output
  2. If I look at the case of @NotNull I can see that it has no validator code at all, and yet still works as well as has proper interpolation.

The annotation:

package com.company.validators

import javax.validation.Constraint
import javax.validation.Payload
import kotlin.reflect.KClass

@Target(AnnotationTarget.FIELD, AnnotationTarget.VALUE_PARAMETER, AnnotationTarget.PROPERTY_GETTER)
@Constraint(validatedBy = [InstanceOfValidator::class])
annotation class InstanceOf(
    val type: KClass<*>,
    val message: String = "{com.company.validators.InstanceOf.message}",
    val groups: Array<KClass<*>> = [],
    val payload: Array<KClass<out Payload>> = []

The validator:

package com.company.validators

import javax.validation.ConstraintValidator
import javax.validation.ConstraintValidatorContext
import kotlin.reflect.KClass

class InstanceOfValidator : ConstraintValidator<InstanceOf,Any> {

    private var type: KClass<*>? = null

    override fun initialize(annotation: InstanceOf) {
        this.type = annotation.type

    override fun isValid(value: Any?, context: ConstraintValidatorContext?): Boolean {
        return value != null && (type?.isInstance(value) ?: false)


And here is an example of it's usage:

class PSRRaw {

    var bugId: Any? = null


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