I've come across an issue where some Java code is ...

# getting-started

[Paul N]

I've come across an issue where some Java code is using org.apache.commons.beanutils.PropertyUtils.setProperty to try and set a property on a Kotlin class. I've annotated the field with @JvmField to get past the first issue I had, and now I'm getting Unknown property 'someField' on class 'class rubbish.SomeKotlinClass'

Is there a way of making Kotlin POJO type classes 100% compatible with code like PropertyUtils that relies on Java reflection ?

// <https://mvnrepository.com/artifact/commons-beanutils/commons-beanutils>
implementation("commons-beanutils:commons-beanutils:1.9.3")

package rubbish

import io.kotest.matchers.nulls.shouldNotBeNull
import io.kotest.matchers.shouldBe
import org.apache.commons.beanutils.PropertyUtils
import org.junit.jupiter.api.Test

data class SomeKotlinClass(
    @JvmField
    var someField: String? = null,
)

class SetTest {

    @Test
    fun kotlinTestData() {

        val x = SomeKotlinClass::class.java.getDeclaredConstructor().newInstance()

        x.shouldNotBeNull()

        val fields = SomeKotlinClass::class.java.declaredFields
        fields.forEach {
            println("Field is $it")
            println("trying to get field")
            SomeKotlinClass::class.java.getField(it.name)
            println("got field")
        }

        PropertyUtils.setProperty(x,"someField","hello")

        x.someField shouldBe "hello"
    }
}

[ephemient]

non-@JvmField properties should be compatible with Java Bean conventions. why did you need it?

[Paul N]

because if you don't have it the same code fails with a NoSuchFieldException. Maybe that's the issue I should focus on.

The test case posted is basically what the Java code that I have in production is doing (it gets a field and later on tries to set it; I can't change the Java code (it's in a company jar that I can't change).

If I convert SomeKotlinClass back to java it works fine.

[ephemient]

data class SomeKotlinClass(
    var someField: String? = null,
)

fun main(vararg args: String) {
    val x = SomeKotlinClass::class.java.getDeclaredConstructor().newInstance()
    PropertyUtils.setProperty(x,"someField","hello")
    println(x)
}

this works fine for me, prints SomeKotlinClass(someField=hello)

[Paul N]

Yes that works, but the code I'm using does a getField before the set property.

So :

SomeKotlinClass::class.java.getField("someField")

fails, but works if SomeKotlinClass is in Java

[ephemient]

that code assumes that the backing field is public, which is a pretty terrible idea

[ephemient]

you could express the same as what I assume you were doing in Java,

class SomeKotlinClass {
    @JvmField
    var someField: String? = null
    fun getSomeField(): String? = someField
    fun setSomeField(someField: String?) {
        this.someField = someField
    }
}

but Kotlin's not going to assist you with that

[Paul N]

Thanks,I'm going to do some more digging, with a better example.

[Paul N]

It turns out that the only reason the Java porous work in our production code is indeed because they have been defined as being public, so I could manually add getters and a JVM field, but I might has well leave them in Java. Thanks again.