How do you make Jackson use Kotlin default params for missing values?

I've a Kotlin data class that looks like this,

data class SomeData(
private val notInJson: String = "some default value",
private val inJson: String
)

and a json string, I wish to deserialize into an instance of this class -

{
"inJson" : "value"
}

When I try to deserialize this data, I get an error that the parameter ```notInJson``` cannot be null. Seems like Jackson is passing a null value for it since it's missing in the json string.

Is there a way to have Jackson not pass in any value for it, so that the default specified in the class definition could be used?

--EDIT--

I do register a KotlinModule() with the mapper. Using version 2.7.8 or all jackson packages involved.

The following code -

@Test fun jackson_kotlin_module() {
val mapper = ObjectMapper().registerModule(KotlinModule())
println(
mapper.readValue("""{ "inJson" : "some value" }""", SomeData::class.java))
}

gives me -

com.fasterxml.jackson.databind.JsonMappingException: Instantiation of [simple
type, class SomeData] value failed (java.lang.IllegalArgumentException):
Parameter specified as non-null is null: method SomeData.<init>, parameter
notInJson
at [Source: { "inJson" : "some value" }; line: 1, column: 27]

Default params is a Kotlin feature. Jackson does not know about it hence we need to inform it somehow to be able to support it. Thankfully there's the excellent [jackson-module-kotlin][1] which let's you do:

val mapper: ObjectMapper = ObjectMapper()
.registerModule(KotlinModule()) //let Jackson know about Kotlin
...
data class SomeData(
private val notInJson: String = "some default value",
private val inJson: String
)
...
val data = mapper.readValue<SomeData>("""{"inJson": "sample value"}""")
println(data) // -> SomeData(notInJson=some default value, inJson=sample value)


For instructions how to use it take a look at [jackson-module-kotlin Readme][1] and **pay special attention to versions** of the module and Jackson itself. In order for the default parameters to work you need [at least version `2.8.4`](

[To see links please register here]

) of [jackson-module-kotlin][1]


[1]:

[To see links please register here]

"jackson-module-kotlin"