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README.md

Kotlin Symbol Processing API

Compiler plugins are powerful metaprogramming tools that can greatly enhance how you write code. Compiler plugins call compilers directly as libraries to analyze and edit input programs. These plugins can also generate output for various uses. For example, they can generate boilerplate code, and they can even generate full implementations for specially-marked program elements, such as Parcelable. Plugins have a variety of other uses and can even be used to implement and fine-tune features that are not provided directly in a language.

While compiler plugins are powerful, this power comes at a price. To write even the simplest plugin, you need to have some compiler background knowledge, as well as a certain level of familiarity with the implementation details of your specific compiler. Another practical issue is that plugins are often closely tied to specific compiler versions, meaning you might need to update your plugin each time you want to support a newer version of the compiler.

KSP makes creating lightweight compiler plugins easier

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. KSP is designed to hide compiler changes, minimizing maintenance efforts for processors that use it. KSP is designed not to be tied to the JVM so that it can be adapted to other platforms more easily in the future. KSP is also designed to minimize build times. For some processors, such as Glide, KSP reduces full compilation times by up to 25% when compared to KAPT.

KSP is itself implemented as a compiler plugin. There are prebuilt packages on Google's Maven repository that you can download and use without having to build the project yourself. For more information, see Try it out!

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.

How KSP looks at source files

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
    KSEnumEntryDeclaration
      // same as KSClassDeclaration

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

SymbolProcessor: The entry point

Every processor in KSP implements SymbolProcessor:

interface SymbolProcessor {
    fun init(options: Map<String, String>,
             kotlinVersion: KotlinVersion,
             codeGenerator: CodeGenerator,
             logger: KSPLogger)
    fun process(resolver: Resolver) // Let's focus on this
    fun finish()
}

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) }
        }
    }
    ...
}

Examples

Get all member functions that are declared directly within a class:

fun KSClassDeclaration.getDeclaredFunctions(): List<KSFunctionDeclaration> {
    return this.declarations.filterIsInstance<KSFunctionDeclaration>()
}

Determine whether a class or function is local to another function:

fun KSDeclaration.isLocal(): Boolean {
    return this.parentDeclaration != null && this.parentDeclaration !is KSClassDeclaration
}

Determine whether a class member is visible to another:

fun KSDeclaration.isVisibleFrom(other: KSDeclaration): Boolean {
    return when {
        // locals are limited to lexical scope
        this.isLocal() -> this.parentDeclaration == other
        // file visibility or member
        this.isPrivate() -> {
            this.parentDeclaration == other.parentDeclaration
                    || this.parentDeclaration == other
                    || (
                        this.parentDeclaration == null
                            && other.parentDeclaration == null
                            && this.containingFile == other.containingFile
                    )
        }
        this.isPublic() -> true
        this.isInternal() && other.containingFile != null && this.containingFile != null -> true
        else -> false
    }
}

Example annotations

// Find out suppressed names in a file annotation:
// @file:kotlin.Suppress("Example1", "Example2")
fun KSFile.suppressedNames(): List<String> {
    val ignoredNames = mutableListOf<String>()
    annotations.forEach {
        if (it.shortName.asString() == "Suppress" && it.annotationType.resolve()?.declaration?.qualifiedName?.asString() == "kotlin.Suppress") {
            it.arguments.forEach {
                (it.value as List<String>).forEach { ignoredNames.add(it) }
            }
        }
    }
    return ignoredNames
}

Additional details

The API definition can be found here. The diagram below is an overview of how Kotlin is modeled in KSP: class diagram

Type and resolution

In KSP, references to types are designed to be resolved by processors explicitly (with a few exceptions) because most of the cost of the underlying API implementation is in resolution. When a type is referenced, such as KSFunctionDeclaration.returnType or KSAnnotation.annotationType, it is always a KSTypeReference, which is a KSReferenceElement with annotations and modifiers.

interface KSFunctionDeclaration : ... {
  val returnType: KSTypeReference?
  ...
}

interface KSTypeReference : KSAnnotated, KSModifierListOwner {
  val type: KSReferenceElement
}

A KSTypeReference can be resolved to a KSType, which refers to a type in Kotlin's type system.

A KSTypeReference has a KSReferenceElement, which models Kotlin‘s program structure: namely, how the reference is written. It corresponds to the type element in Kotlin's grammar.

A KSReferenceElement can be a KSClassifierReference or KSCallableReference, which contains a lot of useful information without the need for resolution. For example, KSClassifierReference has referencedName, while KSCallableReference has receiverType, functionArguments, and returnType.

If the original declaration referenced by a KSTypeReference is needed, it can usually be found by resolving to KSType and accessing through KSType.declaration. Moving from where a type is mentioned to where its class is defined looks like this:

KSTypeReference -> .resolve() -> KSType -> .declaration -> KSDeclaration

Type resolution is costly and is therefore made explicit. Some of the information obtained from resolution is already available in KSReferenceElement. For example, KSClassifierReference.referencedName can filter out a lot of elements that are not interesting. You should resolve type only if you need specific information from KSDeclaration or KSType.

Note that a KSTypeReference pointing to a function type has most of its information in its element. Although it can be resolved to the family of Function0, Function1, and so on, these resolutions don‘t bring any more information than KSCallableReference. One use case for resolving function type references is dealing with the identity of the function's prototype.

Comparison to kotlinc compiler plugins

kotlinc compiler plugins have access to almost everything from the compiler and therefore have maximum power and flexibility. On the other hand, because these plugins can potentially depend on anything in the compiler, they are sensitive to compiler changes and need to be maintained frequently. These plugins also require a deep understanding of kotlinc’s implementation, so the learning curve can be steep.

KSP aims to hide most compiler changes through a well-defined API, though major changes in compiler or even the Kotlin language might still require to be exposed to API users.

KSP tries to fulfill common use cases by providing an API that trades power for simplicity. Its capability is a strict subset of a general kotlinc plugin. For example, while kotlinc can examine expressions and statements and can even modify code, KSP cannot.

While writing a kotlinc plugin can be a lot of fun, it can also take a lot of time. If you aren't in a position to learn kotlinc’s implementation and do not need to modify source code or read expressions, KSP might be a good fit.

Comparison to reflection

KSP's API looks similar to kotlin.reflect. The major difference between them is that type references in KSP need to be resolved explicitly. This is one of the reasons why the interfaces are not shared.

Comparison to KAPT

KAPT is a remarkable solution which makes a large amount of Java annotation processors work for Kotlin programs out-of-box. The major advantages of KSP over KAPT are improved build performance, not tied to JVM, a more idiomatic Kotlin API, and the ability to understand Kotlin-only symbols.

To run Java annotation processors unmodified, KAPT compiles Kotlin code into Java stubs that retain information that Java annotation processors care about. To create these stubs, KAPT needs to resolve all symbols in the Kotlin program. The stub generation costs roughly 1/3 of a full kotlinc analysis and the same order of kotlinc code-generation. For many annotation processors, this is much longer than the time spent in the processors themselves. For example, Glide looks at a very limited number of classes with a predefined annotation, and its code generation is fairly quick. Almost all of the build overhead resides in the stub generation phase. Switching to KSP would immediately reduce the time spent in the compiler by 25%.

For performance evaluation, we implemented a simplified version of Glide in KSP to make it generate code for the Tachiyomi project. While the total Kotlin compilation time of the project is 21.55 seconds on our test device, it took 8.67 seconds for KAPT to generate the code, and it took 1.15 seconds for our KSP implementation to generate the code.

Unlike KAPT, processors in KSP do not see input programs from Java's point of view. The API is more natural to Kotlin, especially for Kotlin-specific features such as top-level functions. Because KSP doesn't delegate to javac like KAPT, it doesn't assume JVM-specific behaviors and can be used with other platforms potentially.

Limitations

While KSP tries to be a simple solution for most common use cases, it has made several trade-offs compared to other plugin solutions. The following are not goals of KSP:

  • Examining expression-level information of source code.
  • Modifying source code.
  • 100% compatibility with the Java Annotation Processing API.

We are also exploring several additional features. Note that these features are currently unavailable:

  • IDE integration: Currently IDEs know nothing about the generated code.
  • We are still investigating how to provide interoperability between processors.

For Java Annotation Processor Authors

If you are familiar with the Java Annotation Processing API, see the Java annotation processing to KSP reference for how to implement most functions and data structures that are provided by Java's API.

Development status

The API is still under development and is likely to change in the future. Please do not use it in production yet. The purpose of this preview is to get your feedback. 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 implemented:

  • Passing options to processors in Gradle.
  • Incremental processing.
  • Improved support for reading Java source code.
  • Make the IDE aware of the generated code.

Try it out!

Here's a sample processor that you can check out: https://github.com/google/ksp/releases/download/1.4.10-dev-experimental-20201023/playground-ksp-1.4.10-dev-experimental-20201023.zip

Create a processor of your own

  • Create an empty gradle project.

  • Specify version 1.4.10 of the Kotlin plugin in the root project for use in other project modules.

    plugins {
        kotlin("jvm") version "1.4.10" apply false
    }
    
    buildscript {
        dependencies {
            classpath(kotlin("gradle-plugin", version = "1.4.10"))
        }
    }
    
  • Add a module for hosting the processor.

  • In the module's build.gradle.kts file, do the following:

    • Add google() to repositories so that Gradle can find our plugins.
    • Apply Kotlin plugin
    • Add the KSP API to the dependencies block.
    plugins {
        kotlin("jvm")
    }
    
    repositories {
        google()
        mavenCentral()
    }
    
    dependencies {
        implementation("com.google.devtools.ksp:symbol-processing-api:1.4.10-dev-experimental-20201023")
    }
    
  • The processor you're writing needs to implement com.google.devtools.ksp.processing.SymbolProcessor. Note the following:

    • Your main logic should be in the process() method.
    • Use CodeGenerator in the init() method for code generation. You can also save the CodeGenerator instance for later use in either process() or finish().
    • Use resolver.getSymbolsWithAnnotation() to get the symbols you want to process, given the fully-qualified name of an annotation.
    • A common use case for KSP is to implement a customized visitor (interface com.google.devtools.ksp.symbol.KSVisitor) for operating on symbols. A simple template visitor is com.google.devtools.ksp.symbol.KSDefaultVisitor.
    • For sample implementations of the SymbolProcessor interface, see the following files in the sample project.
      • src/main/kotlin/BuilderProcessor.kt
      • src/main/kotlin/TestProcessor.kt
    • After writing your own processor, register your processor to the package by including the fully-qualified name of that processor in resources/META-INF/services/com.google.devtools.ksp.processing.SymbolProcessor.

Use your own processor in a project

  • Create another module that contains a workload where you want to try out your processor.

  • In the project's setting.gradle.kts, override resolutionStrategy for the KSP plugin. This is necessary because KSP is still in preview and there is no plugin marker published yet.

    pluginManagement {
        resolutionStrategy {
            eachPlugin {
                when (requested.id.id) {
                    "symbol-processing" ->
                        useModule("com.google.devtools.ksp:symbol-processing:${requested.version}")
                }
            }
        }
    
        repositories {
                gradlePluginPortal()
                google()
        }
    }
    
  • In the new module's build.gradle.kts, do the following:

    • Apply the symbol-processing plugin with the specified version.
    • Add ksp(<your processor>) to the list of dependencies.
  • Run ./gradlew build. You can find the generated code under build/generated/source/ksp.

  • Here's a sample build.gradle.kts to apply the KSP plugin to a workload.

    plugins {
        id("symbol-processing") version "1.4.10-dev-experimental-20201023"
        kotlin("jvm") 
    }
    
    version = "1.0-SNAPSHOT"
    
    repositories {
        mavenCentral()
        google()
    }
    
    dependencies {
        implementation(kotlin("stdlib-jdk8"))
        implementation(project(":test-processor"))
        ksp(project(":test-processor"))
    }
    

Pass Options to Processors

Processor options in SymbolProcessor.init(options: Map<String, String>, ...) are specified in gradle build scripts:

  ksp {
    arg("option1", "value1")
    arg("option2", "value2")
    ...
  }

Make IDE Aware Of Generated Code

By default, IntelliJ or other IDEs don't know about the generated code and therefore references to those generated symbols will be marked unresolvable. To make, for example, IntelliJ be able to reason about the generated symbols, the following paths need to be marked as generated source root:

build/generated/ksp/src/main/kotlin/
build/generated/ksp/src/main/java/

and perhaps also resource directory if your IDE supports them:

build/generated/ksp/src/main/resources

Migration from old KSP releases

See Migration from old KSP releases

How to contribute

See CONTRIBUTING.md

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