KAPT

Learn KAPT annotation processing.

What is KAPT?

KAPT (Kotlin Annotation Processing Tool) is the legacy annotation processing path for Kotlin. It first converts Kotlin to Java, then runs Java's annotation processor.

KAPT uses the javax.annotation.processing.Processor interface. Libraries like Dagger, ButterKnife and older Room use KAPT. The Kotlin-to-Java conversion step is slow, so KAPT is slow. For new projects KSP is recommended. If KAPT is still needed for old dependencies, add the kapt(...) dependency and enable the kapt plugin.

A KAPT processor example

A KAPT processor extends AbstractProcessor and implements process(annotations, roundEnv). This structure comes from Java.

Example: class EntityProcessor : AbstractProcessor() { override fun process(annotations: Set<TypeElement>, roundEnv: RoundEnvironment): Boolean { ... } }. Here annotated elements are found via roundEnv. The processor writes a source file with Filer for each element. This structure follows Java standards; it is not Kotlin-specific. That is why KSP is both faster and more Kotlin-friendly than KAPT.

Exercise Write KaptProcessor

Define class KaptProcessor. Method fun process(annotations: List<String>) should call println("apt: $it") for each annotation. In main, call it with listOf("Entity", "Dao").

Show example solution

class KaptProcessor {
    fun process(annotations: List<String>) {
        for (it in annotations) println("apt: $it")
    }
}
fun main() {
    KaptProcessor().process(listOf("Entity", "Dao"))
}

Test yourself

Match KAPT parts

KAPT

Legacy processing path
javax.annotation.processing

Java APT interface
kapt plugin

Gradle plugin
kapt(...)

Dependency
Why is KAPT slow?
- It uses reflection
- It launches coroutines
- It opens a network connection each time
- It first converts Kotlin to Java
Show answer

It first converts Kotlin to Java — KAPT is slow because it converts Kotlin to Java first.

Complete the KAPT processor class

class EntityProcessor : ____() {
    override fun process(annotations: Set<TypeElement>, roundEnv: RoundEnvironment): Boolean {
        return true
    }
}

Show answer

class EntityProcessor : AbstractProcessor() {
    override fun process(annotations: Set<TypeElement>, roundEnv: RoundEnvironment): Boolean {
        return true
    }
}

Next lesson: KSP vs KAPT

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