Custom deserializer via @JsonDeserialize should be invoked before NULL-checking

[MartinHaeusler]

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Describe the bug

When specifying a custom deserializer via @JsonDeserialize, the deserializer should be invoked before the value is checked for null. It is entirely possible that the custom deserializer produces a non-null object out of a JSON null value. Currently this doesn't seem to be possible.

To Reproduce

class MyClass(
    @JsonDeserialize(using = CustomListDeserializer::class)
    val myList: List<String>
)

// the following is a minimal implementation to demonstrate the point;
// for the sake of argument, assume that the deserializer contains further complex custom logic.
class CustomListDeserializer : JsonDeserializer<List<String>>() {

    override fun deserialize(p: JsonParser, ctxt: DeserializationContext): List<Any> {

        val jsonToken = p.currentToken

        return when (jsonToken) {
            JsonToken.VALUE_NULL -> {
                emptyList()
            }
            JsonToken.VALUE_STRING -> {
                listOf(p.valueAsString)
            }
            else -> {
                throw JsonParseException(p, "Failed to parse value!")
            }
        }
    }

}

... and try to deserialize the following JSON:

{
    "myList": null
}

This will result in a MissingKotlinParameterException, claiming that: value failed for JSON property myList due to missing (therefore NULL) value for creator parameter myList which is a non-nullable type

Expected behavior

I would expect that the JSON is deserialized by invoking the CustomListDeserializer on the JSON null value, producing an empty list. This empty list should then be handed to the primary constructor of MyClass.

Versions

Kotlin: 1.9.20
Jackson-module-kotlin: 2.15.3
Jackson-databind: 2.15.3

Additional context

No response

[cowtowncoder]

I don't think this is Kotlin-specific, so will move to jackson-databind; and then explain why this is not a bug but "works-as-implemented".

[cowtowncoder]

(note: moved from jackson-module-kotlin, hence Kotlin reproduction)

So, no this is not quite correct: in Jackson deserializers are NOT asked to deserialize from null (JsonToken.VALUE_NULL) -- original intent being to avoid every deserializer from having to do null-handling. Whether this is a good or bad decision at this point is another question but it is fairly fundamental limitation.

Deserializers are, however, asked to produce null value (later addition to try to support ability for deserializers to influence handling of nulls) by calling this method:

   public T getNullValue(DeserializationContext ctxt) throws JsonMappingException;

so it should be possible to implement this method and produce appropriate null replacement by custom deserializer. One downside is that no contextual information is passed regarding parsing state (JsonParser not passed). It is also possible that this method is only called once and not every time null is encountered (I don't remember if caching is done or not).

But it should be sufficient for this use case. I would expect Kotlin module to call this method as well.

[cowtowncoder]

As per my comment, JsonDeserializers' deserialization methods are not called for null tokens; there are separate accessors (getNullValue() and getNullAccessPattern()). Closing.