Boost Kotlin Build Speed: A Complete Guide to Kotlin Symbol Processing (KSP)

1. Introduction to KSP

Kotlin Symbol Processing (KSP) is an API for lightweight compiler plugins that handles annotations more efficiently than KAPT, offering up to twice the processing speed and full multi‑platform support.

2. Why Choose KSP

It delivers better performance, tighter Kotlin integration, and cross‑platform compatibility, simplifying metaprogramming, automatic code analysis, and generation.

3. Comparison with KAPT

KAPT generates Java stubs, adding significant compilation overhead; KSP works directly with Kotlin symbols, reducing build time by roughly 25 % in many cases (e.g., Glide). Performance charts illustrate the advantage.

4. What KSP Exposes

KSP provides access to KSFile, KSClassDeclaration, KSFunctionDeclaration, KSPropertyDeclaration and their related metadata such as names, types, modifiers and parent declarations.

KSFile
  packageName: KSName
  fileName: String
  annotations: List<KSAnnotation>
  declarations: List<KSDeclaration>
    KSClassDeclaration // class, interface, object
      simpleName: KSName
      qualifiedName: KSName
      ...
    KSFunctionDeclaration // top level function
      simpleName: KSName
      qualifiedName: KSName
      ...
    KSPropertyDeclaration // global variable
      simpleName: KSName
      qualifiedName: KSName
      ...

5. Migrating from KAPT to KSP

Typical migration steps: verify library support (many libraries such as Room, Moshi, RxHttp already support KSP), add the KSP plugin and symbol-processing-api dependency, replace kapt with ksp in Gradle, configure source sets, and remove kapt‑related configuration. A table lists supported libraries.

6. Using KSP

Create a dedicated KSP module, implement SymbolProcessorProvider and SymbolProcessor, register the provider via SPI, define custom annotations, and generate source files with KotlinPoet‑KSP. Example code shows how to retrieve symbols with resolver.getSymbolsWithAnnotation, log them, and write new files using

FileSpec.writeTo(environment.codeGenerator, Dependencies(...))

.

class DemoProcessorProvider: SymbolProcessorProvider {
    override fun create(environment: SymbolProcessorEnvironment) = DemoSymbolProcessor(environment)
}

class DemoSymbolProcessor(private val env: SymbolProcessorEnvironment) : SymbolProcessor {
    override fun process(resolver: Resolver): List<KSAnnotated>> {
        resolver.getSymbolsWithAnnotation(TestClassAnno::class.qualifiedName!!)
            .filterIsInstance<KSClassDeclaration>()
            .forEach { env.logger.warn("DemoSymbolProcessor:" + it) }
        return emptyList()
    }
}

7. Incremental Compilation

KSP enables incremental processing by default; it can be disabled with ksp.incremental=false. The Dependencies object controls aggregating vs isolating modes, allowing fine‑grained regeneration of only changed files.

file.addType(classes)
    .build()
    .writeTo(environment.codeGenerator, Dependencies(aggregating = true))

8. Java Developers

KSP also offers documentation for Java annotation‑processor migration, ensuring Java developers can adopt the same workflow.

Conclusion

Adding KSP to a project is straightforward but requires careful Gradle version upgrades, library compatibility checks, and proper incremental settings; the payoff is a noticeable reduction in compilation time.