KSP with Kotlin Multiplatform

For a quick start, see a sample Kotlin Multiplatform project defining a KSP processor.

Starting from KSP 1.0.1, applying KSP on a multiplatform project is similar to that on a single platform, JVM project. The main difference is that, instead of writing the ksp(...) configuration in dependencies, add(ksp<Target>) or add(ksp<SourceSet>) is used to specify which compilation targets need symbol processing, before compilation.

plugins {
    kotlin("multiplatform")
    id("com.google.devtools.ksp")
}

kotlin {
    jvm {
        withJava()
    }
    linuxX64() {
        binaries {
            executable()
        }
    }
    sourceSets {
        val commonMain by getting
        val linuxX64Main by getting
        val linuxX64Test by getting
    }
}

dependencies {
    add("kspCommonMainMetadata", project(":test-processor"))
    add("kspJvm", project(":test-processor"))
    add("kspJvmTest", project(":test-processor")) // Not doing anything because there's no test source set for JVM
    // There is no processing for the Linux x64 main source set, because kspLinuxX64 isn't specified
    add("kspLinuxX64Test", project(":test-processor"))
}

Compilation and processing

In a multiplatform project, Kotlin compilation may happen multiple times (main, test, or other build flavors) for each platform. So is symbol processing. A symbol processing task is created whenever there is a Kotlin compilation task and a corresponding ksp<Target> or ksp<SourceSet> configuration is specified.

For example, in the above build.gradle.kts, there are 4 compilation tasks: common/metadata, JVM main, Linux x64 main, Linux x64 test, and 3 symbol processing tasks: common/metadata, JVM main, Linux x64 test.

Avoid the ksp(...) configuration on KSP 1.0.1+

Before KSP 1.0.1, there is only one, unified ksp(...) configuration available. Therefore, processors either applies to all compilation targets, or nothing at all. Note that the ksp(...) configuration not only applies to the main source set, but also the test source set if it exists, even on traditional, non-multiplatform projects. This brought unnecessary overheads to build time.

Starting from KSP 1.0.1, per-target configurations are provided as shown in the above example. In the future:

For multiplatform projects, the ksp(...) configuration will be deprecated and removed.
For single platform projects, the ksp(...) configuration will only apply to the main, default compilation. Other targets like test will need to specify kspTest(...) in order to apply processors.

Starting from KSP 1.0.1, there is an early access flag -DallowAllTargetConfiguration=false to switch to the more efficient behavior. If the current behavior is causing performance issues, please give it a try. The default value of the flag will be flipped from true to false on KSP 2.0.

Last modified: 25 September 2024