How to get actual type arguments of a reified gene

问题:

Using reified type parameters, one can write an inline function which works with the type parameter through reflection at runtime:

inline fun <reified T: Any> f() {
    val clazz = T::class
    // ...
}

But when f is called with a parameter which is itself a generic class, there seems to be no way to obtain its actual type arguments through T::class:

f<List<Integer>>() // T::class is just kotlin.collections.List

Is there a way to get actual type arguments of a reified generic through reflection?

回答1:

Due to type erasure, actual generic arguments cannot be obtained through T::class token of a generic class. Different objects of a class must have the same class token, that's why it cannot contain actual generic arguments.

But there is a techinque called super type tokens which can give actual type arguments in case when the type is known at compile time (it is true for reified generics in Kotlin because of inlining).

The trick is that the compiler retains actual type arguments for a non-generic class derived from a generic class (all its instances will have the same arguments, good explanation here). They are accessible through clazz.genericSuperClass.actualTypeArguments of a Class<*> instance.

Given all that, you can write a util class like this:

abstract class TypeReference<T> : Comparable<TypeReference<T>> {
    val type: Type = 
        (javaClass.genericSuperclass as ParameterizedType).actualTypeArguments[0]

    override fun compareTo(other: TypeReference<T>) = 0
}

^{Explained in Jackson TypeReference which uses the same approach. Jackson Kotlin module uses it on reified generics.}

After that, in an inline function with a reified generic, TypeReference needs to be subclassed (an object expression will go), and then its type can be used.

Example:

inline fun <reified T: Any> printGenerics() {
    val type = object : TypeReference<T>() {}.type
    if (type is ParameterizedType)
        type.actualTypeArguments.forEach { println(it.typeName) }
}

printGenerics<HashMap<Int, List<String>>>():

java.lang.Integer
java.util.List<? extends java.lang.String>