Access Kotlin companion objects through JUnit and Java reflection

KotlinTestClass.kt

import org.junit.jupiter.api.BeforeAll
import org.junit.jupiter.api.Test
import org.junit.jupiter.api.io.TempDir

@Target(AnnotationTarget.FIELD)
annotation class KotlinFieldAnnotation

@Target(AnnotationTarget.PROPERTY)
annotation class KotlinPropertyAnnotation

@Target(AnnotationTarget.FUNCTION)
annotation class KotlinFunctionAnnotation

class KotlinTestClass {
  companion object {
    @KotlinFieldAnnotation
    @KotlinPropertyAnnotation
    @TempDir // Java field annotation
    val companionProperty = false
    
    @KotlinFunctionAnnotation
    @BeforeAll // Java method annotation
    fun companionMethod() = companionProperty
  }
  
  @Test
  fun myTest() {}
}

ReflectionTest.kt

import org.assertj.core.api.Assertions.assertThat
import org.junit.jupiter.api.BeforeAll
import org.junit.jupiter.api.Test
import org.junit.jupiter.api.io.TempDir
import org.junit.platform.commons.support.HierarchyTraversalMode
import org.junit.platform.commons.support.ModifierSupport
import org.junit.platform.commons.support.ReflectionSupport

// https://kotlinlang.org/docs/annotations.html#annotation-use-site-targets
// https://kotlinlang.org/docs/annotations.html#java-annotations
class ReflectionTest {
  @Test
  fun `reflect on companion property`() {
    val testClassFields = ReflectionSupport.findFields(
      KotlinTestClass::class.java, 
      { true }, 
      HierarchyTraversalMode.TOP_DOWN)
    
    val companionProperty = testClassFields.find { 
      it.name == "companionProperty"
    }
    assertThat(companionProperty).isNotNull
    companionProperty!!
    assertThat(ModifierSupport.isPrivate(companionProperty)).isTrue()
    assertThat(ModifierSupport.isStatic(companionProperty)).isTrue()
    // because `val` instead of `var`
    assertThat(ModifierSupport.isFinal(companionProperty)).isTrue()
    assertThat(companionProperty.isSynthetic).isFalse()
    assertThat(companionProperty.isAnnotationPresent(KotlinFieldAnnotation::class.java)).isTrue()
    assertThat(companionProperty.isAnnotationPresent(KotlinPropertyAnnotation::class.java)).isFalse()
    assertThat(companionProperty.isAnnotationPresent(TempDir::class.java)).isTrue()
    assertThat(companionProperty.annotations).hasSize(2)
  }
  
  @Test
  fun `reflect on companion property with Java reflection`() {
    val methods = KotlinTestClass::class.java.declaredMethods
    val companionPropertyGetter = methods.find { it.name.startsWith("access$") }
    assertThat(companionPropertyGetter).isNotNull
    companionPropertyGetter!!
    assertThat(ModifierSupport.isPublic(companionPropertyGetter)).isTrue()
    assertThat(ModifierSupport.isStatic(companionPropertyGetter)).isTrue()
    assertThat(ModifierSupport.isFinal(companionPropertyGetter)).isTrue()
    // that's why this method not visible through ReflectionSupport
    assertThat(companionPropertyGetter.isSynthetic).isTrue()
    assertThat(companionPropertyGetter.isAnnotationPresent(KotlinFieldAnnotation::class.java)).isFalse()
    assertThat(companionPropertyGetter.isAnnotationPresent(KotlinPropertyAnnotation::class.java)).isFalse()
    assertThat(companionPropertyGetter.isAnnotationPresent(TempDir::class.java)).isFalse()
    assertThat(companionPropertyGetter.declaredAnnotations).isEmpty()
  }
  
  @Test
  fun `reflect on companion method`() {
    val companionClass = ReflectionSupport
      .findNestedClasses(KotlinTestClass::class.java) { 
        it.simpleName == "Companion"
      }.single()
    val companionMethod = ReflectionSupport
      .findMethod(companionClass, "companionMethod")
    assertThat(companionMethod).isPresent
    assertThat(ModifierSupport.isPublic(companionMethod.get())).isTrue()
    assertThat(ModifierSupport.isNotStatic(companionMethod.get())).isTrue()
    assertThat(ModifierSupport.isFinal(companionMethod.get())).isTrue()
    assertThat(companionMethod.get().isSynthetic).isFalse()
    assertThat(companionMethod.get().isAnnotationPresent(BeforeAll::class.java)).isTrue()
    assertThat(companionMethod.get().isAnnotationPresent(KotlinFunctionAnnotation::class.java)).isTrue()
    assertThat(companionMethod.get().annotations).hasSize(2)
  }

  @Test
  fun `reflect on companion object`() {
    val companionObjectField = ReflectionSupport.findFields(
      KotlinTestClass::class.java,
      { it.name == "Companion" },
      HierarchyTraversalMode.TOP_DOWN
    ).single()
    assertThat(ModifierSupport.isPublic(companionObjectField)).isTrue()
    assertThat(ModifierSupport.isStatic(companionObjectField)).isTrue()
    assertThat(ModifierSupport.isFinal(companionObjectField)).isTrue()
    assertThat(companionObjectField.isSynthetic).isFalse()
    val companionObject = ReflectionSupport.tryToReadFieldValue(companionObjectField, null)
    assertThat(companionObject.toOptional()).isPresent
  }
}