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interop:kotlinx-metadata is an API for working with Kotlin @Metadata annotations. Its API sits atop kotlinx-metadata, offering extensions for its types + JVM metadata information. This can be used to read Kotlin language semantics off of Class or TypeElement @Metadata annotations.


data class Taco(val seasoning: String, val soft: Boolean) {
  fun prepare() {


val kmClass = Taco::class.toKmClass()

// Now you can access misc information about Taco from a Kotlin lens
println( { println( }
kmClass.functions.forEach { println( }


There are a number of boolean flags available to types as well under Flags.kt. These read the underlying kotlinx-metadata Flags property.

Using the Taco example above, we can glean certain information:

println("Is class? ${kmClass.isClass}")
println("Is data class? ${kmClass.isData}")

Interop with KotlinPoet

interop:kotlinx-metadata offers an API for converting core kotlinx-metadata Km types to KotlinPoet source representations of their APIs. This includes full type resolution, signatures, enclosed elements, and general stub source representations of the underlying API.


data class Taco(val seasoning: String, val soft: Boolean) {
  fun prepare() {

val typeSpec = Taco::class.toTypeSpec()

// Or FileSpec
val fileSpec = Taco::class.toFileSpec()

Source representation

The generated representations are a best effort representation of the underlying source code. This means that synthetic elements will be excluded from generation. Kotlin-specific language features like lambdas or delegation will be coerced to their idiomatic source form.

To aid with this, toTypeSpec() and toFileSpec() accept optional ClassInspector instances to assist in parsing/understanding the underlying JVM code. This is important for things like annotations, companion objects, certain JVM modifiers, overrides, and more. While it is optional, represented sources can be incomplete without this information available. Reflective and javax Elements implementations are available under the com.squareup.kotlinpoet.metadata.classinspectors package.

Generated sources are solely stub implementations, meaning implementation details of elements like functions, property getters, and delegated properties are simply stubbed with TODO() placeholders.

Known limitations

  • Only KotlinClassMetadata.Class and KotlinClassMetadata.FileFacade are supported for now. No support for SyntheticClass, MultiFileClassFacade, or MultiFileClassPart
  • @JvmOverloads annotations are only supported with ElementsClassInspector and not reflection.
  • Non-const literal values are only supported with ElementsClassInspector and not reflection.
  • ClassInspector data sourced from synthetic constructs are only supported with ReflectiveClassInspector and not elements. This is because the javax Elements API does not model synthetic constructs. This can yield some missing information, like static companion object properties or property: site target annotations.
  • Annotations annotated with AnnotationRetention.SOURCE are not parsable in reflection nor javax elements.