How to change a kotlin private val using reflection? - kotlin

Question

I can access a private val value using reflection as below
fun main() {
val mainClass = MainClass()
val f = MainClass::class.memberProperties.find { it.name == "info" }
f?.let {
it.isAccessible = true
val w = it.get(mainClass) as String
println(w)
}
}
class MainClass {
private val info: String = "Hello"
}
But if I want to change info, how could I do it using reflection?

Answer 1

Answer
In short, you have to use Java reflection APIs in this case, and here is how to do it:
fun main() {
val mainClass = MainClass()
val f = MainClass::class.java.getDeclaredField("info")
f.isAccessible = true
f.set(mainClass, "set from reflection")
mainClass.printInfo() // Prints "set from reflection"
}
class MainClass {
private val info: String = "Hello"
fun printInfo() = println(info)
}
Reason for using Java reflection APIs
It is not possible to do with Kotlin reflection APIs since no setter code is generated for a read-only (val) property. So to change it, we need to use Java reflection APIs which is more low-level. First, we use Tools -> Kotlin -> Show Kotlin Bytecode to see what the generated bytecode looks like. Then we see this:
// ================MainClass.class =================
// class version 50.0 (50)
// access flags 0x31
public final class MainClass {
// access flags 0x12
private final Ljava/lang/String; info = "Hello"
// ...
i.e that the info fields in the MainClass Kotlin class causes the compiler to emit JVM code for a regular MainClass Java class with a final String info field. So to change it, we can use Java reflection APIs, as in the code above.
Kotlin reflection API attempt
If the field would have been private var you would be able to Use Kotlin reflection APIs like this:
f?.let {
val mutableProp = it as KMutableProperty<*>
it.isAccessible = true
mutableProp.setter.call(mainClass, "set from Kotlin reflection")
val w = it.get(mainClass) as String
println(w)
}
but if you try this with private val you will get the below exception
Exception in thread "main" java.lang.ClassCastException: class kotlin.reflect.jvm.internal.KProperty1Impl cannot be cast to class kotlin.reflect.KMutableProperty (kotlin.reflect.jvm.internal.KProperty1Impl and kotlin.reflect.KMutableProperty are in unnamed module of loader 'app')
at MainKt.main(main.kt:107)
at MainKt.main(main.kt)
since no setter code is generated for val fields, and thus the info property will have a Kotlin Reflection API type of KProperty and not KMutableProperty.

Answer 2

This is working solution
import kotlin.reflect.KMutableProperty
import kotlin.reflect.full.memberProperties
class MySolution {
var name = ""
var email = ""
}
#Suppress("UNCHECKED_CAST")
fun main() {
//Dummy Result Set
val rs = mapOf<String,String>("name" to "My Name", "email" to "My Email")
val mySol = MySolution();
val xyzMp = MySolution::class.memberProperties;
xyzMp.forEach { mp ->
val prop = mp as KMutableProperty<String>
prop.setter.call(mySol, rs[mp.name])
}
println(mySol.name)
println(mySol.email)
println("*****Enjoy*******")
output
My Name
My Email
**** *Enjoy*******