Kotlin, how to retrieve field value via reflection

Question

So I have hundreds of fields in a couple of classes and I'd like to write some methods on them where they automatically println each field and its corresponding value

At the moment I have this:

inner class Version(val profile: Profile) {

    @JvmField val MINOR_VERSION = glGetInteger(GL_MINOR_VERSION)

    fun write(file: File? = null) {
        //file.printWriter().use { out -> out.pri }
        this::class.java.fields.forEach {
            println(it.isAccessible)
            println(it.getInt(it)) }
    }
}

But this is what I get:

false
Exception in thread "main" java.lang.IllegalArgumentException: Can not set final int field uno.caps.Caps$Version.MINOR_VERSION to java.lang.reflect.Field
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:167)
    at sun.reflect.UnsafeFieldAccessorImpl.throwSetIllegalArgumentException(UnsafeFieldAccessorImpl.java:171)
    at sun.reflect.UnsafeFieldAccessorImpl.ensureObj(UnsafeFieldAccessorImpl.java:58)
    at sun.reflect.UnsafeQualifiedIntegerFieldAccessorImpl.getInt(UnsafeQualifiedIntegerFieldAccessorImpl.java:58)

Any idea?

hey @elect check my answer: https://stackoverflow.com/a/56115232/2939453 — Adam Kis, May 13 '19 at 15:23

score 45 · Accepted Answer · answered Jul 10 '17 at 18:00

45

Instead of using Java fields and Java reflection code, you can also use Kotlin properties and Kotlin reflection classes:

class Reflector {
    val Foo = 1;

    fun printFields() {
        this::class.memberProperties.forEach {
            if (it.visibility == KVisibility.PUBLIC) {
                println(it.name)
                println(it.getter.call(this))
            }
        }
    }
}

answered Jul 10 '17 at 18:00

BladeCoder

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I guess this is more appropriate – elect Jul 11 '17 at 07:21
How about java class? – quangkid Mar 17 '18 at 03:09
hey @elect Please check my solution: https://stackoverflow.com/a/56115232/2939453 – Adam Kis May 13 '19 at 15:24
Is this still correct? I don't see `memberProperties` but only `members`. Kotlin version 1.5.20. – Damn Vegetables Jun 24 '21 at 21:15
Yes it's still there: https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.reflect.full/member-properties.html – BladeCoder Jun 29 '21 at 14:18
@DamnVegetables needs `implementation "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"` – DA_123 Aug 25 '21 at 17:27

score 1 · Answer 2 · answered Jul 10 '17 at 15:50

It seems that you are passing the Field variable it as a parameter getInt whereas the parameter should be the object the field belongs to this:

From the Javadoc for Field.getInt(Object obj):

obj - the object to extract the int value from

Perhaps this is what you meant to do:

class Reflector {
    @JvmField val Foo = 1;

    fun printFields() {
        this.javaClass.fields.forEach {
            println(it.isAccessible)
            println(it.getInt(this))
        }
    }
}

fun main(args : Array<String>) {
    Reflector().printFields()
}

Kotlin, how to retrieve field value via reflection

2 Answers2