Mutator inference during deserialization doesn't work for Kotlin (data) objects

[hudson155]

Search before asking

• I searched in the issues and found nothing similar.
• I searched in the issues of databind and other modules used and found nothing similar.
• I have confirmed that the problem only occurs when using Kotlin.

Describe the bug

If I have a data class with a property defined in the body, deserializing works fine.

If I have a data object with a property defined in the body, deserializing throws an exception when deserializing due to an unrecognized field.

Exception in thread "main" com.fasterxml.jackson.databind.exc.UnrecognizedPropertyException: Unrecognized field "int" (class Second), not marked as ignorable (0 known properties: ])

I think this works fine for classes due to MapperFeature.ALLOW_FINAL_FIELDS_AS_MUTATORS and MapperFeature.INFER_PROPERTY_MUTATORS (both default to true), but with Kotlin objects it doesn't work.

To Reproduce

data class First(
  val foo: String,
) {
  val int: Int = 1
}

data object Second {
  val int: Int = 2
}

fun main() {
  val mapper = jacksonObjectMapper()
  println(mapper.writeValueAsString(First("bar"))) // {"foo":"bar","int":1}
  println(mapper.writeValueAsString(Second)) // {"int":2}
  println(mapper.readValue<First>("{\"foo\":\"bar\",\"int\":1}")) // First(foo=bar)
  println(mapper.readValue<Second>("{\"int\":2}")) // THROWS!
}

Expected behavior

I expect no exception to be thrown in the above example.

Versions

Kotlin: 2.0.0
Jackson-module-kotlin: 2.17.2
Jackson-databind: 2.17.2

Additional context

No response

[hudson155]

I just verified that this issue also exists when using Kotlin 1.9.25.

[k163377]

@hudson155
Sorry for the delay in replying.

I have checked and this appears to be normal behavior for Jackson.
When deserializing a class, if an unrequested property exists on JSON, it fails by default.
Enabling FAIL_ON_UNKNOWN_PROPERTIES will suppress this problem.
https://javadoc.io/doc/com.fasterxml.jackson.core/jackson-databind/latest/com/fasterxml/jackson/databind/DeserializationFeature.html#FAIL_ON_UNKNOWN_PROPERTIES

package com.fasterxml.jackson.module.kotlin

import com.fasterxml.jackson.databind.DeserializationFeature

data object Second {
    val int: Int = 2
}

fun main() {
    val mapper = jacksonObjectMapper().disable(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES)
    println(mapper.readValue<Second>("{\"int\":2}")) // -> Second
}