kapt

Kapt, short for Kotlin Annotation Processing Tool, is a compiler plugin that enables annotation processing for Kotlin code by leveraging Java annotation processors. It allows Kotlin projects to use libraries and frameworks that rely on annotation processing without rewriting their processors in Kotlin.

How it works: The Kotlin compiler with kapt generates Java-compatible stubs from Kotlin sources and runs standard Java annotation processors against those stubs. The processors may generate new source files, resources, or metadata. Kapt then collects these outputs and feeds the generated sources back into the Kotlin compiler for final compilation. This arrangement lets existing Java-based processors function in Kotlin projects.

Usage: In Gradle-based projects, kapt is typically enabled by applying the Kotlin kapt plugin and declaring processors in the kapt configuration. You would add dependencies for the annotation processors (for example, dagger-compiler or room-compiler) alongside regular Kotlin dependencies. In Android projects, kapt is widely used to integrate libraries like Dagger, Room, and other code-generating annotation processors.

Limitations and alternatives: Not all Java annotation processors are fully compatible with kapt, and kapt can introduce build-time overhead. Some processors rely on Java-specific features or require additional configuration. Since the rise of Kotlin Symbol Processing (KSP), some projects migrate to KSP for a more native Kotlin processing experience, improved incremental processing, and streamlined pipelines. KSP provides a similar capability through a separate tool and API, offering an alternative to kapt for Kotlin annotation processing.