Question: Deserialize json to subtypes with complex constructor

[arturk9]

I am using kotlin and jackson and I am looking for a way to deserialize json with the following structure, that can vary between two cases:

Case 1:

{
    "parent1": "parentvalue1",
    "parent2": "parentvalue2",
    "child1": "childvalue1",
    "child2": "childvalue2"
}

Case 2:

{
    "parent1": "parentvalue1",
    "parent2": "parentvalue2",
    "child3": "childvalue3",
    "child4": "childvalue4"
}

My Model classes look as follows:

Parent:

open class Parent(
    val parent1: String,
    val parent2: String
) {
    constructor (parent: Parent) : this(
        parent1 = parent.parent1,
        parent2 = parent.parent2
    )
}

Children:

class Child1(
    parent: Parent,
    val child1: String,
    val child2: String
) : Parent(parent)

class Child2(
    parent: Parent,
    val child3: String,
    val child4: String
) : Parent(parent)

Is it doable to deserialize such structure using Jackson?

val parent = ObjectMapper().readValue(myJson, Parent::class.java)

[dinomite]

Since you don't actually have a parent/child relationship represented in the JSON, I would model this differently:

data class Widget(
    val parent1: String,
    val parent2: String,
    val child1: String?,
    val child2: String?,
    val child3: String?,
    val child4: String?
)

You can add methods to that class to transform it into the appropriate one your application uses internally based upon which child fields are present.

Alternatively, you might be able to use SubTypes with deduction, but the whole parent & children at the same level is still a bit weird.

[arturk9]

Hello @dinomite, Thank you for your answer!

Real classes are a bit more complex - parent class has 10 properties and we have 20 different children with 10 different properties each.

In this case, adding 200 fields to parent class would make it really complex.

What we could do is some factory method, for instance:

@JsonCreator constructor(@JsonProperty child1: String, @JsonProperty child2: String, @Parentobject parent: Parent).

Please note that if we skip @SubType annotations, if configured correctly, object mapper would return Parent object and ignore fields unique for the children, so maybe there is some way to access original object in JsonCreator?

[dinomite]

Hmm, I'm still a bit confused. With everything at the same level in the JSON, you don't really have a parent-child relationship, just a bunch of optional fields, though it sounds like those optional fields come exclusively in groups (i.e. you'll always have the same subsets together).

I hear your concern about way too many fields in the object, but I don't know another way to approach this. As this isn't Kotlin specific, the jackson-user mailing list might be of help.

[arturk9]

I think you are right, from json perspective it is more a bunch of optional fields, havent thought about it this way.

Thank you for help, I will ask there!

[dinomite]

Great, happy to revisit if there are Kotlin questions!