Deserialization of Custom Wrapper Class Fails in Jackson 2.18.1 Without Nested Object Structure

[Shanzeee]

Describe your Issue

When upgrading from Jackson 2.17.2 to 2.18.1, I encountered an issue with deserializing a custom wrapper class (ValidatedList) directly from an array in JSON. Previously, in 2.17.2, the deserialization worked as expected.

Here is the code representation of my classes:

data class Profile(
    val name: String,
    val list: ValidatedList<Record>,
)

class ValidatedList<Record>(vararg list: Record) {
    var items: List<Record> = listOf()

    init {
        this.items = list.asList()
    }
}

data class Record(
    val nationality: String,
    val firstName: String,
)

In Jackson 2.17.2, the following JSON deserialized correctly:

{
  "name": "JohnDoe",
  "list": [
    {
      "nationality": "US",
      "firstName": "John"
    },
    {
      "nationality": "GB",
      "firstName": "Jane"
    }
  ]
}

But in 2.18.1 i have a problem with deserialization:
Error: Cannot deserialize value of type `example.ValidatedList<example.Record>` from Array value (token `JsonToken.START_ARRAY`) at [Source: REDACTED (`StreamReadFeature.INCLUDE_SOURCE_IN_LOCATION` disabled); line: 3, column: 11] (through reference chain: example.Profile["list"])

To make the deserialization work in 2.18.1, the JSON has to be structured with an additional items key like this:

{
 "name": "JohnDoe",
 "list": {
   "items": [
     {
       "nationality": "US",
       "firstName": "John"
     },
     {
       "nationality": "GB",
       "firstName": "Jane"
     }
   ]
 }
}

Question
Is there a way to configure Jackson 2.18.1 to accept the original JSON format (without the nested items key)? I would like to maintain compatibility with the original JSON format while using the newer version of Jackson.

Any advice or configuration suggestions to address this would be greatly appreciated

[pjfanning]

2.18.2 is out and has some fixes that could help

[Shanzeee]

I’ve just tested with Jackson 2.18.2, and the same issue persists.

[pjfanning]

If this is Kotlin, then you should report this to the jackson-module-kotlin project.

[cowtowncoder]

Moving to Kotlin module repo.

[cowtowncoder]

I am fairly certain that the issue is with un-annotated 1-arg constructor of ValidatedList -- formerly apparently being heuristically chosen to be Mode.DELEGATING (map value from JSON into type of argument, List here) vs 2.18.0 choosing Mode.PROPERTIES (single-element JSON Object with specified named property).

I am not 100% sure why heuristics details changed, aside from saying that the whole property introspection (including Creator method introspection, that is, constructor/factory method introspection) was completely rewritten to address problems with Records and mismatching of annotations.

But given inherent difficulty in correctly guessing intent of 1-argument constructors, the usual way is to mark Constructor with

@JsonCreator(mode = JsonCreator.Mode.DELEGATING)

(or use @JsonValue)

which will force use of specific mode.