kapt: Support Kotlin code generation

libraries/examples/annotation-processor-example/src/main/kotlin/example/ExampleAnnotationProcessor.kt

@@ -1,5 +1,6 @@
 package example
 
+import java.io.File
 import javax.annotation.processing.AbstractProcessor
 import javax.annotation.processing.RoundEnvironment
 import javax.lang.model.SourceVersion
@@ -12,6 +13,8 @@ public class ExampleAnnotationProcessor : AbstractProcessor() {
     private companion object {
         val ANNOTATION_FQ_NAME = javaClass<ExampleAnnotation>().getCanonicalName()
         val SUFFIX_OPTION = "suffix"
+        val GENERATE_KOTLIN_CODE_OPTION = "generate.kotlin.code"
+        val KAPT_KOTLIN_GENERATED_OPTION = "kapt.kotlin.generated"
     }
 
     override fun process(annotations: MutableSet<out TypeElement>?, roundEnv: RoundEnvironment): Boolean {
@@ -20,17 +23,27 @@ public class ExampleAnnotationProcessor : AbstractProcessor() {
         val elementUtils = processingEnv.getElementUtils()
         val filer = processingEnv.getFiler()
 
-        val generatedFileSuffix = processingEnv.getOptions().get(SUFFIX_OPTION) ?: "Generated"
+        val options = processingEnv.getOptions()
+        val generatedFileSuffix = options[SUFFIX_OPTION] ?: "Generated"
+        val generateKotlinCode = "true" == options[GENERATE_KOTLIN_CODE_OPTION]
+        val kotlinGenerated = options[KAPT_KOTLIN_GENERATED_OPTION]
 
         for (element in elements) {
             val packageName = elementUtils.getPackageOf(element).getQualifiedName().toString()
-            val className = element.getSimpleName().toString().capitalize() + generatedFileSuffix
+            val simpleName = element.getSimpleName()
+            val className = simpleName.toString().capitalize() + generatedFileSuffix
 
             filer.createSourceFile(className).openWriter().use { with(it) {
                 appendln("package $packageName;")
-                appendln()
                 appendln("public final class $className {}")
             }}
+
+            if (generateKotlinCode && kotlinGenerated != null && element.getKind() == ElementKind.CLASS) {
+                File(kotlinGenerated, "$simpleName.kt").writer().buffered().use {
+                    it.appendln("package $packageName")
+                    it.appendln("fun $simpleName.customToString() = \"$simpleName: \" + toString()")
+                }
+            }
         }
 
         return true;