What is the annotation processing tool in Kotlin and how does it work?

The annotation processing tool (Annotation Processor) in Kotlin is a mechanism that allows analyzing and processing annotations in source code during compilation. It is used for generating additional code, verifying correct annotation usage, or creating metadata.

It works as follows: during compilation, the compiler invokes a special annotation processor that accesses code elements marked with annotations. The processor can read annotation data and generate new files (e.g., classes or resources), which are then included in the final build.

An example of usage is the Dagger library for dependency injection, which generates code for creating dependency graphs using annotation processors.

In Kotlin, annotation processors are often written using KAPT (Kotlin Annotation Processing Tool), which integrates with the Java Annotation Processing API.