如何通过反射访问 kotlin 中的类型使用注释？

Question

A library I'm using utilises type use annotations to add constraints to variables. For example, if you wanted a configurable opacity (in percent, represented by a float ), you'd annotate the float type:

public @Setting.Constrain.Range(min = 0, max = 1, step = 0.1) float opacity = 1f;

I could not get the same library to work on identical kotlin code.

To simplify the question, let's declare the annotation and usage ourselves:

@Target(ElementType.TYPE_USE)
@Retention(RetentionPolicy.RUNTIME)
public @interface JavaAnnot { }

public class TestJava {

    @JavaAnnot int a = 5;

    public static void main(String[] args) {
        System.out.println(TestJava.class.getDeclaredFields()[0].getAnnotatedType().isAnnotationPresent(JavaAnnot.class));
    }

}

Running TestJava#main yields true , indicating that the JavaAnnot annotation is present on the type.

If we introduce a new kotlin class TestKotlin with a similar a field and annotated type:

class TestKotlin {

    val a: @JavaAnnot Int = 5

}

The previous check, now on the TestKotlin class:

TestKotlin.class.getDeclaredField("a").getAnnotatedType().isAnnotationPresent(JavaAnnot.class)

yields false , indicating that the JavaAnnot annotation is NOT present on the Int type of a , even though we clearly did annotate it.

I've tried:

• Annotating a with @JvmField . This did not change the behaviour.
• Declaring an identical annotation in kotlin:

@Target(AnnotationTarget.TYPE)
@Retention(AnnotationRetention.RUNTIME)
annotation class KotlinAnnot

val a: @KotlinAnnot Int = 5

Did not fix the problem.

• Inspecting other metadata accessible through reflection: I did not find anything useful.

Is this a kotlin bug? Do type use annotations not exist in kotlin, or is the syntax different? What am I doing wrong?

Thanks.

Answer 1

Annotations of AnnotatedType of Kotlin fields are stored with Kotlin Metadata annotation and can be accessed only by Kotlin Reflection.

Update:

Since Kotlin 1.4.0, the compiler writes type annotations as it is expected.