Kotlin Symbol Processing (KSP) is an API that you can use to develop lightweight compiler plugins. KSP provides a simplified compiler plugin API that leverages the power of Kotlin while keeping the learning curve at a minimum. Compared to KAPT, annotation processors that use KSP can run up to 2x faster.

To learn more about how KSP compares to KAPT, check out why KSP. To get started writing a KSP processor, take a look at the KSP quickstart.

The KSP API processes Kotlin programs idiomatically. KSP understands Kotlin-specific features, such as extension functions, declaration-site variance, and local functions. KSP also models types explicitly and provides basic type checking, such as equivalence and assign-compatibility.

The API models Kotlin program structures at the symbol level according to Kotlin grammar. When KSP-based plugins process source programs, constructs like classes, class members, functions, and associated parameters are easily accessible for the processors, while things like if blocks and for loops are not.

Conceptually, KSP is similar to KType in Kotlin reflection. The API allows processors to navigate from class declarations to corresponding types with specific type arguments and vice-versa. Substituting type arguments, specifying variances, applying star projections, and marking nullabilities of types are also possible.

Another way to think of KSP is as a pre-processor framework of Kotlin programs. If we refer to KSP-based plugins as symbol processors, or simply processors, then the data flow in a compilation can be described in the following steps:

1. Processors read and analyze source programs and resources.
2. Processors generate code or other forms of output.
3. The Kotlin compiler compiles the source programs together with the generated code.

Unlike a full-fledged compiler plugin, processors cannot modify the code. A compiler plugin that changes language semantics can sometimes be very confusing. KSP avoids that by treating the source programs as read-only.

Most processors navigate through the various program structures of the input source code. Before diving into usage of the API, let's look at how a file might look from KSP's point of view:

KSFile
  packageName: KSName
  fileName: String
  annotations: List<KSAnnotation>  (File annotations)
  declarations: List<KSDeclaration>
    KSClassDeclaration // class, interface, object
      simpleName: KSName
      qualifiedName: KSName
      containingFile: String
      typeParameters: KSTypeParameter
      parentDeclaration: KSDeclaration
      classKind: ClassKind
      primaryConstructor: KSFunctionDeclaration
      superTypes: List<KSTypeReference>
      // contains inner classes, member functions, properties, etc.
      declarations: List<KSDeclaration>
    KSFunctionDeclaration // top level function
      simpleName: KSName
      qualifiedName: KSName
      containingFile: String
      typeParameters: KSTypeParameter
      parentDeclaration: KSDeclaration
      functionKind: FunctionKind
      extensionReceiver: KSTypeReference?
      returnType: KSTypeReference
      parameters: List<KSValueParameter>
      // contains local classes, local functions, local variables, etc.
      declarations: List<KSDeclaration>
    KSPropertyDeclaration // global variable
      simpleName: KSName
      qualifiedName: KSName
      containingFile: String
      typeParameters: KSTypeParameter
      parentDeclaration: KSDeclaration
      extensionReceiver: KSTypeReference?
      type: KSTypeReference
      getter: KSPropertyGetter
        returnType: KSTypeReference
      setter: KSPropertySetter
        parameter: KSValueParameter

This view lists common things that are declared in the file--classes, functions, properties, and so on.

KSP expects an implementation of the SymbolProcessorProvider interface to instantiate SymbolProcessor:

interface SymbolProcessorProvider {
    fun create(environment: SymbolProcessorEnvironment): SymbolProcessor
}

While SymbolProcessor is defined as:

interface SymbolProcessor {
    fun process(resolver: Resolver): List<KSAnnotated> // Let's focus on this
    fun finish() {}
    fun onError() {}
}

A Resolver provides SymbolProcessor with access to compiler details such as symbols. A processor that finds all top-level functions and non-local functions in top-level classes might look something like this:

class HelloFunctionFinderProcessor : SymbolProcessor() {
    ...
    val functions = mutableListOf<String>()
    val visitor = FindFunctionsVisitor()

    override fun process(resolver: Resolver) {
        resolver.getAllFiles().map { it.accept(visitor, Unit) }
    }

    inner class FindFunctionsVisitor : KSVisitorVoid() {
        override fun visitClassDeclaration(classDeclaration: KSClassDeclaration, data: Unit) {
            classDeclaration.getDeclaredFunctions().map { it.accept(this, Unit) }
        }

        override fun visitFunctionDeclaration(function: KSFunctionDeclaration, data: Unit) {
            functions.add(function)
        }

        override fun visitFile(file: KSFile, data: Unit) {
            file.declarations.map { it.accept(this, Unit) }
        }
    }
    ...
    
    class Provider : SymbolProcessorProvider {
        override fun create(environment: SymbolProcessorEnvironment): SymbolProcessor = ...
    }
}

Some handy links:

• Quickstart
• Why use KSP?
• Examples
• How KSP models Kotlin code
• Reference for Java annotation processor authors
• Incremental processing notes
• Multiple round processing notes
• KSP on multiplatform projects
• Running KSP from command line
• Contributor guide
• FAQ

Please let us know what you think about KSP by filing a Github issue or connecting with our team in the #ksp channel in the Kotlin Slack workspace!

Here are some planned features that have not yet been completely implemented:

• Support new Kotlin compiler
• Improve support to multiplatform. E.g., running KSP on a subset of targets / sharing computations between targets
• Improve performance. There a bunch of optimizations to be done!
• Make the IDE aware of the generated code
• Keep fixing bugs!

The table below includes a list of popular libraries on Android and their various stages of support for KSP. If your library is missing, please feel free to submit a pull request.