KSP vs KAPT: Why KSP Is Faster
Compare KSP and KAPT processing models and understand the performance benefits.
KSP vs KAPT: Why KSP Is Faster is a free Kotlin Academy lesson on CoddyKit — lesson 1 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Kotlin Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
The Problem with Annotation Processing
Annotation processors generate code at build time. For years, Kotlin projects used KAPT — Kotlin Annotation Processing Tool — which works by converting Kotlin source to Java stubs before passing them to Java annotation processors. This conversion is costly.
How KAPT Works
KAPT runs the Kotlin compiler to produce Java stub files, then runs Java annotation processors (APT) on those stubs, then compiles everything together. Every build step adds latency:
- Kotlin → Java stubs compilation
- Java APT processing on stubs
- Final Kotlin + generated-source compilation
How KSP Works
KSP (Kotlin Symbol Processing) runs directly on Kotlin's compiler plugin API. It reads the Kotlin abstract syntax tree (AST) natively — no stub generation, no Java intermediary. Processors implement SymbolProcessor and traverse KSNode objects.
Speed Comparison
Benchmark data from Google shows KSP is up to 2× faster than KAPT on incremental builds and significantly faster on clean builds. The savings come from eliminating stub generation, which can account for 30–50% of KAPT's build time in large projects.
Incremental Processing
KSP supports incremental processing natively: only symbols that changed since the last build are reprocessed. KAPT has limited incremental support. For large code bases with frequent small changes, this difference is dramatic.
API Differences
KAPT processors implement javax.annotation.processing.AbstractProcessor (Java API). KSP processors implement com.google.devtools.ksp.processing.SymbolProcessor (Kotlin API with Kotlin-native constructs like sealed classes and extension functions).
Kotlin-Native Type Information
Because KSP reads the Kotlin AST directly, it sees Kotlin-specific constructs that KAPT stubs cannot represent faithfully: nullability, default parameter values, extension functions, expect/actual declarations, and inline classes.
Multiplatform Support
KSP 2.x supports Kotlin Multiplatform. A single KSP processor can generate code for JVM, JS, and Native targets. KAPT is JVM-only and cannot process KMP commonMain sources.
Migration Path from KAPT to KSP
Most popular libraries have KSP variants: Room (KSP), Hilt (KSP), Moshi (KSP), Dagger (partial). Check the library's documentation. Replace kapt("...") with ksp("...") in your build and remove the KAPT plugin once all processors are migrated.
plugins {
id("com.google.devtools.ksp") version "2.0.0-1.0.21"
}
dependencies {
ksp("androidx.room:room-compiler:2.6.1")
}When KAPT Is Still Needed
Use KAPT when a library does not yet have a KSP processor variant. Mixing KAPT and KSP in the same project is supported but does not eliminate KAPT's stub-generation overhead for the KAPT processors.
Tooling and Debugging
KSP processors can log messages at different severity levels (KSPLogger). Generated files appear in build/generated/ksp/. IDE support (IntelliJ / Android Studio) highlights generated sources and allows navigating to them.
Quick Check
Why is KSP faster than KAPT on incremental builds?
Recap: KSP vs KAPT
Key takeaways:
- KAPT: converts Kotlin to Java stubs, then runs Java APT — slow and JVM-only
- KSP: reads Kotlin AST natively via compiler plugin — up to 2× faster
- KSP supports incremental processing and Kotlin Multiplatform
- KSP sees Kotlin-specific constructs KAPT stubs cannot represent
- Migrate by replacing
kapt()withksp()when the library supports it
Frequently asked questions
Is the “KSP vs KAPT: Why KSP Is Faster” lesson free?
Yes — the full text of “KSP vs KAPT: Why KSP Is Faster” is free to read here on the web, and the Kotlin Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Kotlin Academy course, upgrade to CoddyKit PRO.
What will I learn in “KSP vs KAPT: Why KSP Is Faster”?
Compare KSP and KAPT processing models and understand the performance benefits. You practise Kotlin Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start Kotlin Academy?
No prior experience is required. Kotlin Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “KSP vs KAPT: Why KSP Is Faster” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this Kotlin Academy lesson?
Yes. Every Kotlin Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.