Gson Deserialization with Kotlin, Initializer block not called

Gson is intended for usage with pure Java and doesn’t interpret Kotlin primary constructors properly.

If there is a default no-argument constructor, it is called (in your case, there is one: the primary constructor with default value "" for someString), otherwise Gson calls no constructor at all.

Then Gson sets the property values (also, it bypasses actual setters of Kotlin properties and sets the fields directly, which may sometimes lead to unexpected behavior).

---

As an alternative, you can use jackson-module-kotlin, it should work good in your case (it understands Kotlin primary constructors, uses Kotlin setters and supports default parameter values as well since 2.8.x).

This example compares jackson-module-kotlin and Kotson behavior:

class SomeClass(val id: Int = -1) {
    init { println("init $id") }
}

class SomeClassNoDefault(val id: Int) {
    init { println("init $id") }
}

fun main(args: Array<String>) {
    val mapper = jacksonObjectMapper()
    val gson = Gson()

    val idDefault = "{}"
    val id123 = "{\"id\": 123 }"

    println("jackson-module-kotlin, { }:")
    val o1 = mapper.readValue<SomeClass>(idDefault)
    println("after construction: ${o1.id}\n")

    println("jackson-module-kotlin, { \"id\" = 123 }:")
    val o2 = mapper.readValue<SomeClass>(id123)
    println("after construction: ${o2.id}\n")

    println("kotson, { }:")
    val o3 = gson.fromJson<SomeClass>(idDefault)
    println("after construction: ${o3.id}\n")

    println("kotson, { \"id\" = 123 }:")
    val o4 = gson.fromJson<SomeClass>(id123)
    println("after construction: ${o4.id}\n")

    println("---\n")

    println("jackson-module-kotlin, no default value, { \"id\" = 123 }:")
    val o5 = mapper.readValue<SomeClassNoDefault>(id123)
    println("after construction: ${o5.id}\n")

    println("kotson, no default value, { \"id\" = 123 }:")
    val o6 = gson.fromJson<SomeClassNoDefault>(id123)
    println("after construction: ${o6.id}\n")
}

The output is

jackson-module-kotlin, { }:
init -1
after construction: -1

jackson-module-kotlin, { "id" = 123 }:
init 123
after construction: 123

kotson, { }:
init -1
after construction: -1

kotson, { "id" = 123 }:
init -1
after construction: 123

---

jackson-module-kotlin, no default value, { "id" = 123 }:
init 123
after construction: 123

kotson, no default value, { "id" = 123 }:
after construction: 123