Dynamic Proxies with Kotlin for accessing private classes.

DynamicProxies.kt

/*
 * Code by @vmironov on Kotlinlang slack.
 *
 * This code uses dynamic proxies in Java to make it easier to access inaccessible classes via an accessible representation.
 */

inline fun <reified T : Any> createMirror(value: Any) = createMirror(value, T::class.java)

fun <T> createMirror(value: Any, clazz: Class<T>): T {
    val loader = clazz.classLoader
    val interfaces = arrayOf(clazz)

    return clazz.cast(Proxy.newProxyInstance(loader, interfaces) { proxy, method, args ->
        val field = value.javaClass.getDeclaredField(method.name)
        val accessible = field.isAccessible
        field.isAccessible = true
        field.get(value)
        field.isAccessible = accessible
    })
}

Example.kt

fun main(args: Array<String>) {
    val object: Any = Secret("Hello", "Secret")
    val mirror: SecretMirror = createMirror<SecretMirror>(object)
    
    // Above `object` is a given object accessed by other means with the type of Any (or Object in Java).
    // That object is then passed to createMirror and told to create a representation of `object` via
    // the SecretMirror class.
}

data class Secret(
    private val foo: String,
    private val bar: String
)

class SecretMirror {
    fun foo(): String
    fun bar(): String
}

The code does not work because SecretMirror is not interface