Kotlin reflection - How to get class reference from KParameter

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I have a class reference and a set of data for initializing the class, I need to create instance of that class dynamically. I get the constructor from the class reference, I loop through its parameters and do a match between that set of data. Then call the constructor.callby(param). All is fine, until one of the property is a custom class rather than a kotlin type like kotlin.Int. Let’s see how to solve it.

Some setup

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data class Student(
    val name:String,
    val age:Int,
    val lessons:List<Lesson>
)

val classRef= Student::class
val cons = classRef.primaryConstructor!!
cons.parameters.forEach{
    // do something with each parameter
}

The above code is simple, we retrieve a class and its constructor, then we loop through its parameters.
Now you can do the match up thing to restore the class.

Get the class reference from KParameter

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fun generateClass(param:KParameter):Any? {
    var result:Any? = null
    val paramName= param.type.toString()

    if (!paramName.startsWith("kotlin")) {
        result = param.type.classifier as KClass<*>
    }

    return result
}

In the above forEach, when we found that the parameter is a custom class, we need to do something about it. First, we make sure it’s not a kotlin type by checking the paramName not startsWith("kotlin"), all kotlin types are started from kotlin even for Collection type.

Then we return param.type.classifier as KClass<*>, then you can repeat the code in Some setup to restore this nested property.

Get the class reference from KParameter which is a List<*>

But what happen if the property is not a custom class, but also a List<CustomClass>. Like the lessons property in the following Student class.

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data class Lesson(
    val title:String,
    val length:Int
)
data class Student(
    val name:String,
    val age:Int,
    val lessons:List<Lesson>
)

We need to refactor our generateClass function.

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fun generateClass(param:KParameter):Any? {
    var result:Any? = null
    val paramName= param.type.toString()

    if (paramName.startsWith("kotlin.collections.List")){
        val itemTypeName = paramName.substring(24).removeSuffix(">")
        if (!itemTypeName.startsWith("kotlin")){
            result = param.type.arguments[0].type?.classifier as KClass<*>
        }
    } else if (!paramName.startsWith("kotlin")) {
        result = param.type.classifier as KClass<*>
    }

    return result
}

Here things get a little bit interesting. First, we check it’s a List type, then we type to retrieve the item type of that list. Let’s say it’s a kotlin.collections.List<kotlin.Int>, then it’s not a list of custom class. But if the itemTypeName is not startsWith("kotlin"), then it is a custom class.

Then we retrieve the item type by using param.type.arguments[0].type?.classifier as KClass<*>

End

That’s all, hope it helps. :)

Thanks for reading!

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