Kotlin. How to check if the field is nullable via reflection?

Question

I'm developing a code generator that takes the data from the classes during runtime. This generator is designed to work only with Kotlin. At the moment, I was faced with the problem, as I don't know how to check if the field is nullable.

So the main question is how to implement this check via reflection?

Answer 1

You can check nullability with isMarkedNullable. The following code:

class MyClass(val nullable: Long?, val notNullable: MyClass)
MyClass::class.declaredMemberProperties.forEach {
    println("Property $it isMarkedNullable=${it.returnType.isMarkedNullable}")
}

will print:

Property val MyClass.notNullable: stack.MyClass isMarkedNullable=false
Property val MyClass.nullable: kotlin.Long? isMarkedNullable=true

Excerpt from documentation (emphasis mine):

For Kotlin types, it means that null value is allowed to be represented by this type. In practice it means that the type was declared with a question mark at the end. For non-Kotlin types, it means the type or the symbol which was declared with this type is annotated with a runtime-retained nullability annotation such as javax.annotation.Nullable.

Note that even if isMarkedNullable is false, values of the type can still be null. This may happen if it is a type of the type parameter with a nullable upper bound:

fun <T> foo(t: T) {
    // isMarkedNullable == false for t's type, but t can be null here 
}