How can I reflect on a field annotation (Java) in a Scala program?

Question

I'm using Scala 2.13 and I know there's been a lot deprecated since older versions.

I've got this annotation:

@Inherited
@Target({ElementType.PARAMETER, ElementType.METHOD, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
public @interface Foo {
    int index() default 0;
}

(I know... I've got lots of ElementTypes there, but I'm struggling to see where this pops up in reflection so wanted to maximize my chances of a hit!)

Used like this:

case class Person(name: String, @Foo(index = 3) age: Int)
val p = Person("Fred", 29)

How can I reflect on this to get the my Java Annotation (Foo), so I can 1) know whether @Foo exists on a given field, and 2) get the index value. Note I have a declared default value for Foo.index that may be overridden at runtime, so this is a runtime-scoped annotation.

Answer 1

Here is solution using Java reflection.

For @(Foo @field)(index = 3) age: Int do

println(classOf[Person].getDeclaredFields.map(_.getDeclaredAnnotationsByType(classOf[Foo]).map(_.index)).deep)
//Array(Array(), Array(3))

For @(Foo @getter)(index = 3) age: Int do

println(classOf[Person].getDeclaredMethods.map(_.getDeclaredAnnotationsByType(classOf[Foo]).map(_.index)).deep)
//Array(Array(), Array(), Array(3), Array(), Array(), Array(), Array(), Array(), Array(), Array(), Array(), Array(), Array())

For @(Foo @param)(index = 3) age: Int or just @Foo(index = 3) age: Int do

println(classOf[Person].getDeclaredConstructors.map(_.getParameters.map(_.getDeclaredAnnotationsByType(classOf[Foo]).map(_.index))).deep)
//Array(Array(Array(), Array(3)))

You can combine meta-annotations @field, @getter, @param.

I guess similar thing can be done with Scala reflection.