Using Kotlin Default Parameter Values when JSON value is null and Kotlin parameter type is Non-Nullable

I’ve got the following simplified JSON example, which I’m trying to decode into the simplified Kotlin Data Class below.

{ 
   "boolField": null,
    "stringField": null
}

data class TestObject(
        val boolField: Boolean = true,
        val stringField: String = "default"
)

The key thing here is that the Kotlin properties are not nullable, but there is a known default value for them. However, the JSON sometimes contains null for those fields.

I am trying to get the example JSON to decode using the default values in place of the nulls since the type is non-nullable. However, this doesn’t work out of the box, instead throwing a MissingKotlinParameterException.

I had a look at modifying the code with a feature flag to behave the way I wanted. This was easy enough to do with some minor alterations to createFromObjectWith() in KotlinValueInstantiator for the String case. However, for the Boolean case it does not work, as in Java, that non-optional Boolean becomes a boolean primitive type, which cannot take null and thus Jackson Data Binding sets it with the default value of false.

So, assuming I haven’t missed the point completely with this, I’m wondering if there’s a way in the KotlinValueInstantiator to know that the primitive types were set with their default values by Jackson Data Binding in order to make this work for primitive types too?

[or-dvir]

There are conflicts with various settings and picking one model.

Kotlin itself does not treat null and as the same. Neither does Jackson by default which would throw an exception for null being set into a primitive depending on the setting of the FAIL_ON_NULL_FOR_PRIMITIVES feature switch.

For a non nullable type should we treat null as when there is a default value available in the method signature? what about a nullable type? how do we know when null is intentional versus meaning ?

expected behavior (at least for me):

• trying to set null to a non-nullable variable (e.g. String) if variable has default value, use it. otherwise throw exception
• trying to set null to a nullable variable (e,g, ‘String?’) set to null regardless if there is a default value or not (as the developer has explicitly declared that null is acceptable).

Your workaround for now is to remove the null values from your JSON completely to mean “absent” instead of using null

not a good solution as many developers are using 3rd party APIs which they have no control over

[apatrida]

Branch 2.10 now allows null to be treated as “use the default” value, but requires that you set a flag when creating the Kotlin module, otherwise the behavior stays as it was as the default behavior.

ObjectMapper().registerModule(KotlinModule(nullisSameAsDefault = true)