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: stackoverflow.com/a/56115232/2939453 — Adam Kis
– Adam Kis, Commented May 13, 2019 at 15:23

BladeCoder · Accepted Answer · 2017-07-10 18:00:12Z

52

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, 2017 at 18:00

BladeCoder

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5 Comments

quangkid Over a year ago

How about java class?

Adam Kis Over a year ago

hey @elect Please check my solution: stackoverflow.com/a/56115232/2939453

Damn Vegetables Over a year ago

Is this still correct? I don't see memberProperties but only members. Kotlin version 1.5.20.

BladeCoder Over a year ago

Yes it's still there: kotlinlang.org/api/latest/jvm/stdlib/kotlin.reflect.full/…

DA_123 Over a year ago

@DamnVegetables needs implementation "org.jetbrains.kotlin:kotlin-reflect:$kotlin_version"

Frank Wilson · Accepted Answer · 2017-07-10 15:50:47Z

1

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()
}

answered Jul 10, 2017 at 15:50

Frank Wilson

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Comments

Mahozad · Accepted Answer · 2023-09-23 08:38:03Z

1

Note that you should first add the following dependency:

dependencies {
    implementation(kotlin("reflect"))
    // ...
}

Also, to get fields of a static Java class or fields that are static, use staticProperties instead of memeberProperties:

// R.raw is a static class in Android and its fields are also static
for (property in R.raw::class.staticProperties) {
    println(property.name)
    println(property.get())
}

answered Sep 23, 2023 at 8:38

Mahozad

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