Kotlin extension function access Java private field

Question

I'd like to access Java's private field when using Kotlin extension function.

Suppose I have a Java class ABC. ABC has only one private field mPrivateField. I'd like to write an extension function in Kotlin which uses that field for whatever reason.

public class ABC {     private int mPrivateField;  } 

the Kotlin function would be:

private fun ABC.testExtFunc() {     val canIAccess = this.mPrivateField; } 

the error I'm getting is:

Cannot access 'mPrivateField': It is private in 'ABC'

Any way of getting around that limitation?

Answer 1

First, you need to obtain a Field and enable it can be accessible in Kotlin, for example:

val field = ABC::class.java.getDeclaredField("mPrivateField")  field.isAccessible = true 

Then, you can read the field value as Int by Field#getInt from the instance of the declaring class, for example:

val it: ABC = TODO()  val value = field.getInt(it) 

Last, your extension method is looks like as below:

private inline fun ABC.testExtFunc():Int {     return javaClass.getDeclaredField("mPrivateField").let {         it.isAccessible = true         val value = it.getInt(this)         //todo         return@let value;     } } 

Answer 2

That is not possible by design. Extension functions essentially resolve to static functions with the receiver as its first parameter. Thus, an extension function

fun String.foo() {   println(this) } 

compiles to something like:

public static void foo(String $receiver) {   System.out.println($receiver); } 

Now it's clear to see you cannot access private member of $receiver, since they're, well, private.

If you really want to access that member, you could do so using reflection, but you'll lose all guarantees.