Kotlin Academy · Lesson

Reflection-Based Validation and Mapping

Build a simple validator or object mapper using annotation-driven reflection.

Lesson 4 of 413 steps

Reflection-Based Validation and Mapping is a free Kotlin Academy lesson on CoddyKit. This is lesson 4 of 4. Any 3 lessons of this learning path are free to read in full — after that, CoddyKit PRO unlocks every lesson, plus hands-on practice with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Academy learning path, and your progress syncs across the web and the CoddyKit app. The Kotlin Academy course includes 4 lessons in total.

What This Pattern Solves

Reflection-based validation reads annotations from a class's properties at runtime and applies validation rules automatically — no manual checks per field. The same technique powers ORM mappers that convert between objects and database rows.

The Validate Annotation

Define a runtime-retained annotation targeting properties:

@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.PROPERTY)
annotation class Validate(
    val minLength: Int = 0,
    val maxLength: Int = Int.MAX_VALUE,
    val pattern: String = ""
)

Annotating a Data Class

Apply @Validate to data class properties. The validator will scan these at runtime:

data class UserForm(
    @Validate(minLength = 2, maxLength = 50) val name: String,
    @Validate(pattern = "^[\\w.]+@[\\w]+\\.[a-z]{2,}$") val email: String,
    @Validate(minLength = 8) val password: String
)

The Validator Function

Use KClass.declaredMemberProperties to iterate properties, find annotations, and collect errors:

fun validate(obj: Any): List<String> {
    val errors = mutableListOf<String>()
    for (prop in obj::class.declaredMemberProperties) {
        val ann = prop.findAnnotation<Validate>() ?: continue
        val value = (prop as KProperty1<Any, *>).get(obj)?.toString() ?: ""
        if (value.length < ann.minLength) errors += "${prop.name}: too short"
        if (value.length > ann.maxLength) errors += "${prop.name}: too long"
        if (ann.pattern.isNotEmpty() && !value.matches(Regex(ann.pattern)))
            errors += "${prop.name}: invalid format"
    }
    return errors
}

Running the Validator

Call validate() with any annotated object. The function returns a list of error messages; an empty list means valid:

val form = UserForm("A", "not-an-email", "short")
val errors = validate(form)
errors.forEach(::println)
// name: too short
// email: invalid format
// password: too short

Object-to-Map Mapping with @Column

Another reflection pattern maps an object to a Map using a @Column annotation to provide column names:

@Retention(AnnotationRetention.RUNTIME)
@Target(AnnotationTarget.PROPERTY)
annotation class Column(val name: String)

fun toMap(obj: Any): Map<String, Any?> =
    obj::class.declaredMemberProperties
        .mapNotNull { prop ->
            val col = prop.findAnnotation<Column>() ?: return@mapNotNull null
            col.name to (prop as KProperty1<Any, *>).get(obj)
        }.toMap()

Using the Mapper

Annotate the data class and call toMap() to get a column-keyed map ready for an INSERT statement:

data class User(
    @Column("user_name") val name: String,
    @Column("user_email") val email: String
)

val user = User("Alice", "alice@example.com")
println(toMap(user))
// {user_name=Alice, user_email=alice@example.com}

Caching Reflection Results

Reflection scans are expensive. Cache the property-to-annotation mapping at startup using a ConcurrentHashMap keyed by KClass:

val cache = ConcurrentHashMap<KClass<*>, List<Pair<KProperty1<Any,*>, Column>>>()

fun columnsOf(klass: KClass<*>) = cache.getOrPut(klass) {
    klass.declaredMemberProperties
        .mapNotNull { p ->
            p.findAnnotation<Column>()?.let { (p as KProperty1<Any,*>) to it }
        }
}

Combining Validation and Mapping

You can stack annotations on a single property and process each independently. First validate, then map — or do both in a single pass by collecting each annotation type:

data class Product(
    @Validate(minLength = 1) @Column("product_name") val name: String,
    @Column("price") val price: Double
)

Limitations

Reflection-based validation is powerful but has trade-offs: it adds runtime overhead, it relies on annotation retention, and it cannot catch errors at compile time. For performance-critical or compile-time-safe validation, consider KSP-generated code instead.

When to Use This Pattern

Reflection-based validation shines in framework code that must handle arbitrary user-defined classes: ORMs, serializers, form validators, configuration binders. Application code that owns its types should prefer explicit validation for speed and clarity.

Quick Check

Which annotation retention policy is required for a validation annotation to be readable at runtime?

Answer it in the interactive lesson

Recap: Reflection-Based Validation and Mapping

Key takeaways:

Declare runtime-retained annotations targeting properties
Use declaredMemberProperties + findAnnotation to drive validation or mapping
Cache results per KClass to avoid repeated reflection overhead
Stack multiple annotations on a property for combined behavior
Consider KSP for compile-time alternatives in performance-critical paths

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Frequently Asked Questions

Is the “Reflection-Based Validation and Mapping” lesson free?

Yes — any 3 lessons of the Kotlin Academy learning path, including “Reflection-Based Validation and Mapping”, are free to read in full here on the web. After that, CoddyKit PRO unlocks every lesson, plus interactive practice with a built-in code editor and a 24/7 AI tutor. The Kotlin Academy course includes 4 lessons in total.

What will I learn in “Reflection-Based Validation and Mapping”?

Build a simple validator or object mapper using annotation-driven reflection. You practise Kotlin Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Kotlin Academy?

No prior experience is required. Kotlin Academy on CoddyKit is structured for beginners through advanced learners, so you can start here or from the beginning and move at your own pace. This is lesson 4 of 4.

How long does the “Reflection-Based Validation and Mapping” lesson take?

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All lessons in this course

Kotlin Reflection: KClass, KFunction, KProperty
Creating and Targeting Custom Annotations
Reading Annotations at Runtime
Reflection-Based Validation and Mapping

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