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Kotlin-friendly Annotation Processing Sergey Ryabov

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JSR-269

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• Annotation processing works inside javac JSR-269

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• Annotation processing works inside javac • Separate Annotation Processors are pluggable JSR-269

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• Annotation processing works inside javac • Separate Annotation Processors are pluggable • Gradle’s annotationProcessor dependencies configuration JSR-269

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• Annotation processing works inside javac • Separate Annotation Processors are pluggable • Gradle’s annotationProcessor dependencies configuration • /META-INF/services/javax.annotation.processing.Processor 
 declares your AP’s main entry point JSR-269

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• Annotation processing works inside javac • Separate Annotation Processors are pluggable • Gradle’s annotationProcessor dependencies configuration • /META-INF/services/javax.annotation.processing.Processor 
 declares your AP’s main entry point • Multiple rounds of processing JSR-269

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*.java JSR-269

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*.java apt JSR-269

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*.java apt javac JSR-269

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Kotlin-way

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• APT can’t process Kotlin sources — build KAPT Kotlin-way

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• APT can’t process Kotlin sources — build KAPT • Kotlin’s own JSR-269 implementation? Kotlin-way

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• APT can’t process Kotlin sources — build KAPT • Kotlin’s own JSR-269 implementation? • Generate Java code/stubs from Kotlin? Kotlin-way

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• APT can’t process Kotlin sources — build KAPT • Kotlin’s own JSR-269 implementation? • Generate Java code/stubs from Kotlin? • Precompile Kotlin classes and feed them to javac? Kotlin-way

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• APT can’t process Kotlin sources — build KAPT • Kotlin’s own JSR-269 implementation? • Generate Java code/stubs from Kotlin? • Precompile Kotlin classes and feed them to javac? Kotlin-way

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• APT can’t process Kotlin sources — build KAPT • Kotlin’s own JSR-269 implementation? • Generate Java code/stubs from Kotlin? • Precompile Kotlin classes and feed them to javac? Kotlin-way

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• APT can’t process Kotlin sources — build KAPT • Kotlin’s own JSR-269 implementation? • Generate Java code/stubs from Kotlin? • Precompile Kotlin classes and feed them to javac? Kotlin-way

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• APT can’t process Kotlin sources — build KAPT • Kotlin’s own JSR-269 implementation? • Generate Java code/stubs from Kotlin? • Precompile Kotlin classes and feed them to javac? • No multiple APT rounds over generated Kotlin files Kotlin-way

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*.kt Kotlin-way

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*.kt kapt Kotlin-way

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*.kt kapt *.java Kotlin-way

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*.kt kapt apt *.java Kotlin-way

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*.kt kapt kotlinc apt *.java Kotlin-way

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class KotlinProcessor : AbstractProcessor() { } JSR-269

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class KotlinProcessor : AbstractProcessor() { override fun getSupportedOptions(): Set } JSR-269

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class KotlinProcessor : AbstractProcessor() { override fun getSupportedOptions(): Set override fun getSupportedAnnotationTypes(): Set } JSR-269

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class KotlinProcessor : AbstractProcessor() { override fun getSupportedOptions(): Set override fun getSupportedAnnotationTypes(): Set override fun getSupportedSourceVersion(): SourceVersion } JSR-269

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class KotlinProcessor : AbstractProcessor() { override fun getSupportedOptions(): Set override fun getSupportedAnnotationTypes(): Set override fun getSupportedSourceVersion(): SourceVersion override fun init(processingEnv: ProcessingEnvironment) } JSR-269

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class KotlinProcessor : AbstractProcessor() { override fun getSupportedOptions(): Set override fun getSupportedAnnotationTypes(): Set override fun getSupportedSourceVersion(): SourceVersion override fun init(processingEnv: ProcessingEnvironment) override fun process( annotations: Set, roundEnv: RoundEnvironment ): Boolean } JSR-269

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class KotlinProcessor : AbstractProcessor() { override fun getSupportedOptions(): Set override fun getSupportedAnnotationTypes(): Set override fun getSupportedSourceVersion(): SourceVersion override fun init(processingEnv: ProcessingEnvironment) override fun process( annotations: Set, roundEnv: RoundEnvironment ): Boolean } JSR-269

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Problems

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Problems • No info about Kotlin-specifics is available for Annotation Processors

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Problems • No info about Kotlin-specifics is available for Annotation Processors • No Nullability

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Problems • No info about Kotlin-specifics is available for Annotation Processors • No Nullability • No default arguments

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Problems • No info about Kotlin-specifics is available for Annotation Processors • No Nullability • No default arguments • No sealed & data classes

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Problems • No info about Kotlin-specifics is available for Annotation Processors • No Nullability • No default arguments • No sealed & data classes • …

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Problems • No info about Kotlin-specifics is available for Annotation Processors • No Nullability • No default arguments • No sealed & data classes • … • No nothing!

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No content

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Dad! APT can’t get any Kotlin info. And StackOverflow can’t help!

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Dad! APT can’t get any Kotlin info. And StackOverflow can’t help! If even StackOverflow keeps silence…

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Dad! APT can’t get any Kotlin info. And StackOverflow can’t help! If even StackOverflow keeps silence… B-b-b-but compiler..?

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Dad! APT can’t get any Kotlin info. And StackOverflow can’t help! If even StackOverflow keeps silence… B-b-b-but compiler..? Well, this city has a Hero!

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@Metadata

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@Metadata( mv = {1, 1, 11}, bv = {1, 0, 2}, k = 1, d1 = {"\u0000\u0080\u0001\n\u0002\u0018\u0002\n\u0002\u0018\u0002\n\u00… d2 = {""Lxyz/ryabov/gsonkot/User;", "", "id", "", "username", "", "isAdmin ) public final class User { ... }

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@Metadata

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@Metadata • Provides Kotlin-specific info

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@Metadata • Provides Kotlin-specific info • Class-level annotation

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@Metadata • Provides Kotlin-specific info • Class-level annotation • Accessible both at compile time and runtime

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@Metadata • Provides Kotlin-specific info • Class-level annotation • Accessible both at compile time and runtime • Used by compiler as well

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@Metadata • mv • bv • k • d1 • d2 • xs • xi • pn

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@Metadata • metadataVersion • bytecodeVersion • kind • data1 • data2 • extraString • extraInt • packageName

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What can we do with that?

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What can we do with that? Parse data1 Protocol Buffers using kind to define its format

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What can we do with that? Parse data1 Protocol Buffers using kind to define its format https://github.com/JetBrains/kotlin/blob/master/core/metadata/src/metadata.proto

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Protocol Buffers

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Protocol Buffers

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What tools do we have?

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What tools do we have? • Hands

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What tools do we have? • Hands

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What tools do we have? • Hands • Use kotlin-metadata by Eugenio Marletti
 https://github.com/Takhion/kotlin-metadata

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What tools do we have? • Hands • Use kotlin-metadata by Eugenio Marletti
 https://github.com/Takhion/kotlin-metadata • Or…


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What tools do we have? • Hands • Use kotlin-metadata by Eugenio Marletti
 https://github.com/Takhion/kotlin-metadata • Or… kotlinx-metadata by JetBrains
 https://github.com/JetBrains/kotlin/tree/master/libraries/kotlinx-metadata/jvm

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Two patterns

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Two patterns • Visitors API

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Two patterns • Visitors API • DOM-like API

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Visitors API

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Visitors API public interface Element { R accept(ElementVisitor v, P p); }

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Visitors API public interface ElementVisitor { R visit(Element e, P p); }

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Visitors API public interface ElementVisitor { R visit(Element e, P p); R visitPackage(PackageElement e, P p); }

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Visitors API public interface ElementVisitor { R visit(Element e, P p); R visitPackage(PackageElement e, P p); R visitType(TypeElement e, P p); }

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Visitors API public interface ElementVisitor { R visit(Element e, P p); R visitPackage(PackageElement e, P p); R visitType(TypeElement e, P p); R visitVariable(VariableElement e, P p); }

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Visitors API public interface ElementVisitor { R visit(Element e, P p); R visitPackage(PackageElement e, P p); R visitType(TypeElement e, P p); R visitVariable(VariableElement e, P p); R visitExecutable(ExecutableElement e, P p); }

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Visitors API public interface ElementVisitor { R visit(Element e, P p); R visitPackage(PackageElement e, P p); R visitType(TypeElement e, P p); R visitVariable(VariableElement e, P p); R visitExecutable(ExecutableElement e, P p); R visitTypeParameter(TypeParameterElement e, P p); }

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Visitors API public interface ElementVisitor { R visit(Element e, P p); R visitPackage(PackageElement e, P p); R visitType(TypeElement e, P p); R visitVariable(VariableElement e, P p); R visitExecutable(ExecutableElement e, P p); R visitTypeParameter(TypeParameterElement e, P p); R visitUnknown(Element e, P p); }

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Visitors API

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Visitors API class KotlinProcessor : AbstractProcessor() { override fun process( annotations: Set, roundEnv: RoundEnvironment ): Boolean { return false } }

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Visitors API class KotlinProcessor : AbstractProcessor() { override fun process( annotations: Set, roundEnv: RoundEnvironment ): Boolean { for (element in roundEnv.getElementsAnnotatedWith(Builder::class.java)) { }. return false } }

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Visitors API class KotlinProcessor : AbstractProcessor() { override fun process( annotations: Set, roundEnv: RoundEnvironment ): Boolean { for (element in roundEnv.getElementsAnnotatedWith(Builder::class.java)) { element.accept(visitor, null) }.. return false } }

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Visitors API class KotlinProcessor : AbstractProcessor() { override fun process( annotations: Set, roundEnv: RoundEnvironment ): Boolean { for (element in roundEnv.getElementsAnnotatedWith(Builder::class.java)) { val visitor = object : ElementScanner7() { override fun visitVariable(e: VariableElement, p: Unit?) { handleVariable(e) } } element.accept(visitor, null) }. return false } }

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Visitors API class KotlinProcessor : AbstractProcessor() { override fun process( annotations: Set, roundEnv: RoundEnvironment ): Boolean { for (element in roundEnv.getElementsAnnotatedWith(Builder::class.java)) { val visitor = object : ElementScanner7() { override fun visitVariable(e: VariableElement, p: Unit?) { handleVariable(e) } } element.accept(visitor, null) }. return false } }

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Two patterns • Visitors API • DOM-like API

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DOM-like API

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DOM-like API public interface Element { }

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DOM-like API public interface Element { Element getEnclosingElement(); }

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DOM-like API public interface Element { Element getEnclosingElement(); List getEnclosedElements(); }

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DOM-like API public interface Element { Element getEnclosingElement(); List getEnclosedElements(); ElementKind getKind(); }

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DOM-like API class KotlinProcessor : AbstractProcessor() { override fun process( annotations: Set, roundEnv: RoundEnvironment ): Boolean { for (element in roundEnv.getElementsAnnotatedWith(BindView::class.java)) { } return false } }

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DOM-like API class KotlinProcessor : AbstractProcessor() { override fun process( annotations: Set, roundEnv: RoundEnvironment ): Boolean { for (element in roundEnv.getElementsAnnotatedWith(BindView::class.java)) { } return false } }

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DOM-like API class KotlinProcessor : AbstractProcessor() { override fun process( annotations: Set, roundEnv: RoundEnvironment ): Boolean { for (element in roundEnv.getElementsAnnotatedWith(BindView::class.java)) { if (element.kind == ElementKind.FIELD) { handleVariable(element as VariableElement) }. } return false } }

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DOM-like API class KotlinProcessor : AbstractProcessor() { override fun process( annotations: Set, roundEnv: RoundEnvironment ): Boolean { for (element in roundEnv.getElementsAnnotatedWith(BindView::class.java)) { if (element.kind == ElementKind.FIELD) { handleVariable(element as VariableElement) }. } return false } }

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kotlinx-metadata

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kotlinx-metadata Has only low-level Visitors API

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Visitors API

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Visitors API abstract class KmFunctionVisitor abstract class KmPropertyVisitor abstract class KmPackageVisitor abstract class KmClassVisitor abstract class KmConstructorVisitor abstract class KmValueParameterVisitor abstract class KmLambdaVisitor abstract class KmTypeVisitor abstract class KmTypeAliasVisitor abstract class KmTypeParameterVisitor

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Visitors API abstract class KmClassVisitor { open fun visit*(...) }.

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Visitors API abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }.

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Visitors API class Type abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }.

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Visitors API class Type abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }.

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Visitors API class Type abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }.

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Visitors API class Type abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }.

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Visitors API class Type abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }.

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Visitors API class Type abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }.

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Visitors API class Type abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }.

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Visitors API abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }.

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Visitors API abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }. interface ElementVisitor { R visitTypeParameter( TypeParameterElement e, P p ); }

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Visitors API abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }. interface ElementVisitor { R visitTypeParameter( TypeParameterElement e, P p ); }

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Visitors API abstract class KmClassVisitor { open fun visitTypeParameter( flags: Flags, name: String, id: Int, variance: KmVariance ): KmTypeParameterVisitor? }. interface ElementVisitor { R visitTypeParameter( TypeParameterElement e, P p ); }

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GsonTypeAdapter

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GsonTypeAdapter @GsonAdapter data class User( val id: Long, val username: String, val isAdmin: Boolean )

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val annotation = GsonAdapter::class.java for (element in roundEnv.getElementsAnnotatedWith(annotation)) { val input = getInputFrom(element) ?: continue if (!input.generateAndWrite()) return true }

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val annotation = GsonAdapter::class.java for (element in roundEnv.getElementsAnnotatedWith(annotation)) { val input = getInputFrom(element) ?: continue if (!input.generateAndWrite()) return true }

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val metadata = getMetadata(element)

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fun getMetadata(element: Element): KotlinClassMetadata? { val annotation = element.getAnnotation(Metadata::class.java) val header = annotation.run { KotlinClassHeader(k, mv, bv, d1, d2, xs, pn, xi) }. return KotlinClassMetadata.read(header) }.

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fun getMetadata(element: Element): KotlinClassMetadata? { val annotation = element.getAnnotation(Metadata::class.java) val header = annotation.run { KotlinClassHeader(k, mv, bv, d1, d2, xs, pn, xi) }. return KotlinClassMetadata.read(header) }.

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fun getMetadata(element: Element): KotlinClassMetadata? { val annotation = element.getAnnotation(Metadata::class.java) val header = annotation.run { KotlinClassHeader(k, mv, bv, d1, d2, xs, pn, xi) }. return KotlinClassMetadata.read(header) }.

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fun getMetadata(element: Element): KotlinClassMetadata? { val annotation = element.getAnnotation(Metadata::class.java) val header = annotation.run { KotlinClassHeader(k, mv, bv, d1, d2, xs, pn, xi) }. return KotlinClassMetadata.read(header) }.

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sealed class KotlinClassMetadata { class Class : KotlinClassMetadata class FileFacade : KotlinClassMetadata class SyntheticClass : KotlinClassMetadata class MultiFileClassFacade : KotlinClassMetadata class MultiFileClassPart : KotlinClassMetadata class Unknown : KotlinClassMetadata }

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sealed class KotlinClassMetadata { class Class : KotlinClassMetadata class FileFacade : KotlinClassMetadata class SyntheticClass : KotlinClassMetadata class MultiFileClassFacade : KotlinClassMetadata class MultiFileClassPart : KotlinClassMetadata class Unknown : KotlinClassMetadata }

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val metadata = getMetadata(element)

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val metadata = getMetadata(element) if (metadata !is KotlinClassMetadata.Class) { errorMustBeKotlinClass(element) return null }.

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val metadata = getMetadata(element) if (metadata !is KotlinClassMetadata.Class) { errorMustBeKotlinClass(element) return null }. metadata.accept(object : KmClassVisitor() { })

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val metadata = getMetadata(element) if (metadata !is KotlinClassMetadata.Class) { errorMustBeKotlinClass(element) return null }. metadata.accept(object : KmClassVisitor() { override fun visit(flags: Flags, name: ClassName) { } })

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val metadata = getMetadata(element) if (metadata !is KotlinClassMetadata.Class) { errorMustBeKotlinClass(element) return null }. metadata.accept(object : KmClassVisitor() { override fun visit(flags: Flags, name: ClassName) { pkg = name.substringBeforeLast('/').fqName() className = name.substringAfterLast('/') fqClassName = name.fqName() } })

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return null }. metadata.accept(object : KmClassVisitor() { override fun visit(flags: Flags, name: ClassName) { pkg = name.substringBeforeLast('/').fqName() className = name.substringAfterLast('/') fqClassName = name.fqName() } override fun visitConstructor(flags: Flags): KmConstructorVisitor? { } })

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return null }. metadata.accept(object : KmClassVisitor() { override fun visit(flags: Flags, name: ClassName) { pkg = name.substringBeforeLast('/').fqName() className = name.substringAfterLast('/') fqClassName = name.fqName() } override fun visitConstructor(flags: Flags): KmConstructorVisitor? { if (!flags.isPrimaryConstructor) return null } })

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val Flags.isPrimaryConstructor: Boolean get() = Flag.Constructor.IS_PRIMARY(this)

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val Flags.isNullableType: Boolean get() = Flag.Type.IS_NULLABLE(this) val Flags.isDataClass: Boolean get() = Flag.Class.IS_DATA(this) val Flags.isPrimaryConstructor: Boolean get() = Flag.Constructor.IS_PRIMARY(this) val Flags.hasDefaultValue: Boolean get() = Flag.ValueParameter.DECLARES_DEFAULT_VALUE(this)

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return null }. metadata.accept(object : KmClassVisitor() { override fun visit(flags: Flags, name: ClassName) { pkg = name.substringBeforeLast('/').fqName() className = name.substringAfterLast('/') fqClassName = name.fqName() } override fun visitConstructor(flags: Flags): KmConstructorVisitor? { if (!flags.isPrimaryConstructor) return null } })

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pkg = name.substringBeforeLast('/').fqName() className = name.substringAfterLast('/') fqClassName = name.fqName() } override fun visitConstructor(flags: Flags): KmConstructorVisitor? { if (!flags.isPrimaryConstructor) return null return object : KmConstructorVisitor() { override fun visitValueParameter( flags: Flags, name: String ): KmValueParameterVisitor? { }. }. } })

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} override fun visitConstructor(flags: Flags): KmConstructorVisitor? { if (!flags.isPrimaryConstructor) return null return object : KmConstructorVisitor() { override fun visitValueParameter( flags: Flags, name: String ): KmValueParameterVisitor? { return object : KmValueParameterVisitor() { override fun visitType(flags: Flags): KmTypeVisitor? { return ... } } }. }. } })

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return object :.KmTypeVisitor().{. }.

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return object :.KmTypeVisitor().{. override fun visitAbbreviatedType(flags: Flags): KmTypeVisitor? { } override fun visitArgument( flags: Flags, variance: KmVariance ): KmTypeVisitor? { } }.

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class KmTypeInfoVisitor() :.KmTypeVisitor().{. }.

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class KmTypeInfoVisitor( private val flags: Flags, private val onVisitEnd: (KmTypeInfoVisitor) -> Unit ) : KmTypeVisitor() {. }.

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class KmTypeInfoVisitor( private val flags: Flags, private val onVisitEnd: (KmTypeInfoVisitor) -> Unit ) : KmTypeVisitor() {. lateinit var fqName: String var isNullable: Boolean = false private val typeArgs = arrayListOf() }.

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class KmTypeInfoVisitor( private val flags: Flags, private val onVisitEnd: (KmTypeInfoVisitor) -> Unit ) : KmTypeVisitor() { lateinit var fqName: String var isNullable: Boolean = false private val typeArgs = arrayListOf() override fun visitEnd() { } }.

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private val onVisitEnd: (KmTypeInfoVisitor) -> Unit ) : KmTypeVisitor() { lateinit var fqName: String var isNullable: Boolean = false private val typeArgs = arrayListOf() override fun visitEnd() { if (typeArgs.isNotEmpty()) { fqName += typeArgs.joinToString(prefix = "<", postfix = ">") typeArgs.clear() }, isNullable = flags.isNullableType onVisitEnd(this) } }.

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fqName += typeArgs.joinToString(prefix = "<", postfix = ">") typeArgs.clear() }, isNullable = flags.isNullableType onVisitEnd(this) } override fun visitClass(name: ClassName) { fqName = name.fqName() }. }.

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onVisitEnd(this) } override fun visitClass(name: ClassName) { fqName = name.fqName() }. override fun visitAbbreviatedType(flags: Flags): KmTypeVisitor? { return KmTypeInfoVisitor(flags, typeResolver) { fqName = it.fqName typeArgs.clear() }. }, }.

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onVisitEnd(this) } override fun visitClass(name: ClassName) { fqName = name.fqName() }. override fun visitAbbreviatedType(flags: Flags): KmTypeVisitor? { return KmTypeInfoVisitor(flags, typeResolver) { fqName = it.fqName typeArgs.clear() }. }, }.

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fqName = name.fqName() }. override fun visitAbbreviatedType(flags: Flags): KmTypeVisitor? { return KmTypeInfoVisitor(flags, typeResolver) { fqName = it.fqName typeArgs.clear() }. },’ override fun visitTypeAlias(name: ClassName) { fqName = name.fqName() }. }.

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return KmTypeInfoVisitor(flags, typeResolver) { fqName = it.fqName typeArgs.clear() }. }, override fun visitTypeAlias(name: ClassName) { fqName = name.fqName() }. override fun visitTypeParameter(id: Int) { fqName = typeResolver(id) ?: throw error("Missing param: id=$id") }, }.

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override fun visitTypeAlias(name: ClassName) { fqName = name.fqName() }. override fun visitTypeParameter(id: Int) { fqName = typeResolver(id) ?: throw error("Missing param: id=$id") }, override fun visitArgument( flags: Flags, variance: KmVariance ): KmTypeVisitor? { return KmTypeInfoVisitor(flags, typeResolver) { typeArgs += it.fqName }, }, }.

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override fun visitTypeAlias(name: ClassName) { fqName = name.fqName() }. override fun visitTypeParameter(id: Int) { fqName = typeResolver(id) ?: throw error("Missing param: id=$id") }, override fun visitArgument( flags: Flags, variance: KmVariance ): KmTypeVisitor? { return KmTypeInfoVisitor(flags, typeResolver) { typeArgs += it.fqName },. }, }.

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} override fun visitConstructor(flags: Flags): KmConstructorVisitor? { if (!flags.isPrimaryConstructor) return null return object : KmConstructorVisitor() { override fun visitValueParameter( flags: Flags, name: String ): KmValueParameterVisitor? { return object : KmValueParameterVisitor() { override fun visitType(flags: Flags): KmTypeVisitor? { return ... }. }. }. }. } })

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} override fun visitConstructor(flags: Flags): KmConstructorVisitor? { if (!flags.isPrimaryConstructor) return null return object : KmConstructorVisitor() { override fun visitValueParameter( flags: Flags, name: String ): KmValueParameterVisitor? { return object : KmValueParameterVisitor() { override fun visitType(flags: Flags): KmTypeVisitor? { return KmTypeInfoVisitor(flags, typeArgsRegistry::get) { parameters += Parameter(name, it.fqName, it.isNullable) } }. }. }. }. } })

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SHOW ME THE CODE

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Worth noting

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Worth noting • Metadata has only partial info

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Worth noting • Metadata has only partial info • Separate code model from generation

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Worth noting • Metadata has only partial info • Separate code model from generation • Both Kotlin & Java can be supported in one place

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Worth noting • Metadata has only partial info • Separate code model from generation • Both Kotlin & Java can be supported in one place • Multiplatform support will follow

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Other applications?

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Other applications? • Annotation processing with codegen

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Other applications? • Annotation processing with codegen • Bytecode weaving

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Other applications? • Annotation processing with codegen • Bytecode weaving • Code validation

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Other applications? • Annotation processing with codegen • Bytecode weaving • Code validation • Generating headers for interop with other languages: TypeScript, C/C++

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Links • Metadata ProtoBuffer spec: github.com/JetBrains/kotlin/blob/master/core/metadata/src/metadata.proto • kotlin-metadata by Eugenio Marletti: github.com/Takhion/kotlin-metadata • kotlinx-metadata by JetBrains: github.com/JetBrains/kotlin/tree/master/libraries/kotlinx-metadata/jvm • Value-based API for kotlinx-metadata: youtrack.jetbrains.com/issue/KT-26602 • Gson TypeAdapters codegen: github.com/colriot/GsonKotgen • KotlinPoet: github.com/square/kotlinpoet • Annotation Processing in a Kotlin World by Zac Sweers: youtu.be/_yaaCtWF8aE?t=18968 • Annotation Processing Boilerplate Destruction by Jake Wharton: youtu.be/dOcs-NKK-RA • Multiple KAPT rounds demo: github.com/Takhion/generate-kotlin-multiple-rounds

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Kotlin-friendly Annotation Processing Sergey Ryabov @colriot