Failed to build annotation processor in Kotlin

Question

I am currently trying to write an annotation processor for Android in Kotlin. The project structure is as follows:

/annotation
  /src/main/kotlin/<package>
    Annotation.kt
    AnnotationProcessor.kt
/sample

project/build.gradle

buildscript {
  dependencies {
    classpath 'com.android.tools.build:gradle:3.1.1'
    classpath "org.jetbrains.kotlin:kotlin-gradle-plugin:1.2.31"
  }
}

annotation/build.gradle

apply plugin: 'kotlin'

sourceCompatibility = 1.7
targetCompatibility = 1.7

dependencies {
  implementation "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlinVersion"
}

sample/build.gradle

apply plugin: 'com.android.application'
apply plugin: 'kotlin-android'

android {
  ...
  compileOptions {
    sourceCompatibility JavaVersion.VERSION_1_7
    targetCompatibility JavaVersion.VERSION_1_7
  }

dependencies {
  implementation "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlinVersion"
  implementation project(':annotation')
  kapt project(':annotation')
}

Annotation.kt

@Target(AnnotationTarget.CLASS, AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.SOURCE)
annotation class Annotation(val comment: String = "")

AnnotationProcessor.kt

class AnnotationProcessor : AbstractProcessor() {
  override fun process(annotations: MutableSet<out TypeElement>?, roundEnvironment: RoundEnvironment?): Boolean = true
}

With kapt the build breaks on sample:javaPreCompileDebug with the following message:

org.gradle.api.tasks.TaskExecutionException: Execution failed for task ':sample:javaPreCompileDebug'
Caused by: java.lang.RuntimeException: Annotation processors must be explicitly declared now.  The following dependencies on the compile classpath are found to contain annotation processor.  Please add them to the annotationProcessor configuration.
- annotation.jar (project :annotation)
Alternatively, set android.defaultConfig.javaCompileOptions.annotationProcessorOptions.includeCompileClasspath = true to continue with previous behavior.  Note that this option is deprecated and will be removed in the future.
See https://developer.android.com/r/tools/annotation-processor-error-message.html for more details.

When I replace kapt with annotationProcessor, the build breaks on sample:transformClassesWithInstantRunForDebug with the following message:

Caused by: java.io.IOException: Failed to find byte code for javax/annotation/processing/AbstractProcessor
at com.android.build.gradle.internal.incremental.AsmUtils.lambda$static$0(AsmUtils.java:89)
at com.android.build.gradle.internal.incremental.AsmUtils.loadClass(AsmUtils.java:307)
at com.android.build.gradle.internal.incremental.AsmUtils.readClassAndInterfaces(AsmUtils.java:165)
at com.android.build.gradle.internal.incremental.AsmUtils.loadClass(AsmUtils.java:278)
at com.android.build.gradle.internal.incremental.IncrementalVisitor.instrumentClass(IncrementalVisitor.java:342)
at com.android.build.gradle.internal.transforms.InstantRunTransform.transformToClasses2Format(InstantRunTransform.java:406)
at com.android.build.gradle.internal.transforms.InstantRunTransform.lambda$doTransform$3(InstantRunTransform.java:268)
at com.android.build.gradle.internal.transforms.InstantRunTransform.lambda$null$4(InstantRunTransform.java:297)
at java.util.concurrent.ForkJoinTask$AdaptedCallable.exec(ForkJoinTask.java:1424)
... 4 more

When I disable Instant Run, everything works fine.

My question is now, where did I went wrong with my configuration? I followed sample project like this and the only big difference I see is that the annotation module is split into two (a runtime and a compiler). The error messages though indicates problems with either Kotlin or Instant Run.

Answer 1

You have to split your "annotation" Project into two Parts.

Example:

implementation project(':annotation-lib') // here you put Annotation.kt
kapt project(':annotation-compiler') // AnnotationProcessor.kt

don't forget to add the processor to your META-INF inside the annotation-compiler project see What is the default annotation processors discovery process?