KSP
- Kotlin course
- Expert
- Lesson 45/47
- 6 min
Write code generation with KSP.
Code generation with KSP
The third lesson represents code generation with KSP (Kotlin Symbol Processing). KSP reads annotations at compile time and generates code.
In our model the @AutoBuilder annotation marks a class; KSP generates a builder for it. In this lesson we model the concept by writing the builder by hand: annotation class AutoBuilder and class NoteBuilder. The builder creates a Note step by step. This is the hand-written version of the real KSP flow.
The NoteBuilder model
NoteBuilder is the hand-written version of the builder KSP would generate. var fields are filled step by step; the build() method produces the final Note.
Example: class NoteBuilder { var id = 0; var title = ""; fun build(): Note = Note(id, title) }. Usage: val b = NoteBuilder(); b.title = "KSP"; b.build(). This is the basis of the fluent builder pattern. Real KSP generates this class automatically; we wrote it by hand as a model.
Practice Add AutoBuilder and NoteBuilder
Define annotation class AutoBuilder. Define class NoteBuilder with var id = 0, var title = "", fun build(): Note = Note(id, title). In main, do val b = NoteBuilder(); b.title = "KSP" and write println(b.build().title).
Starter code
object App {
fun run(): String = "Expert Notes"
}
data class Note(val id: Int, val title: String)
// add the @AutoBuilder annotation and NoteBuilder class
fun main() {
// build a Note with NoteBuilder and print it
}Sample solution
object App {
fun run(): String = "Expert Notes"
}
data class Note(val id: Int, val title: String)
annotation class AutoBuilder
class NoteBuilder {
var id = 0
var title = ""
fun build(): Note = Note(id, title)
}
fun main() {
val b = NoteBuilder()
b.title = "KSP"
println(b.build().title)
}Expected output
KSP
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 When does KSP run?
KSP runs during compilation.- Runtime
- Load time
- Compile time Correct answer
- Shutdown
Explanation: KSP processes annotations at compile time.
2 Complete the annotation declaration
annotation class ____annotation class AutoBuilderExplanation: annotation class AutoBuilder marks a class.
3 Match the KSP parts
| Term | Meaning |
|---|---|
KSP |
Symbol processing |
@AutoBuilder |
Marker |
NoteBuilder |
Builder model |
build() |
Build method |