kotlin - 如何在运行时获取函数文字的注解

Question

package _z_additional

import org.junit.jupiter.api.Test
import org.junit.jupiter.api.TestInstance
import kotlin.reflect.KFunction
import kotlin.reflect.full.declaredMemberFunctions
import kotlin.reflect.jvm.reflect

/*https://stackoverflow.com/questions/63439469/how-to-obtain-an-annotation-of-a-function-literal-at-runtime*/
@TestInstance(TestInstance.Lifecycle.PER_CLASS)
object _13_Reflection_ObtainingAnnotationOfAFunctionLiteral_Test {

    @Target(AnnotationTarget.FUNCTION)
    private annotation class A

    //@formatter:off
    @A private fun f1() {}
    private val f2: () -> Unit = @A {}
    private val f3: () -> Unit = @A fun() {}
    //@formatter:on

    @Test
    fun demonstrate_usingReflect() {
        listOf(::f1, f2.reflect(), f3.reflect()).filterNotNull().withIndex().forEach { (index, f) ->
            println("[$index] Function name = ${f.name}")
            println("[$index] Annotations = ${f.annotations.joinToString()}")
            println()
        }
    }

    @Test
    fun demonstrate_usingClass() {
        listOf(f2::class, f3::class).withIndex().forEach { (index, cls) ->
            val invokeFunction: KFunction<*> = cls.declaredMemberFunctions.first {it.name == "invoke" && it.isOperator}
            println("[$index] Function name = ${invokeFunction.name}")
            println("[$index] Annotations = ${invokeFunction.annotations.joinToString()}")
            println()
        }
    }

}

demonstrate_usingReflect输出

[0] 函数名 = f1 [0] 注释 = @_z_additional._13_Reflection_ObtainingAnnotationOfAFunctionLiteral_Test$A()

[1] 函数名 = [1] 注释 =

[2] 函数名 = [2] 注释 =

demonstrate_usingClass错误输出

java.lang.UnsupportedOperationException：这个类是Kotlin编译器生成的内部合成类，比如lambda的匿名类、SAM包装器、可调用引用等。它不是Kotlin类或接口，所以反射库不知道它有什么声明。请使用 Java 反射检查此类：class _z_additional._13_Reflection_ObtainingAnnotationOfAFunctionLiteral_Test$f2$1

标签： kotlinreflectionannotationsfunction-literalkotlin-function-type