Hi, I am trying to maintain some backward compatibility to serialized JSON: for example

{
  "a": {
    "b": "c"
  }
}

to

@Serializable
class Wrapper(val a: Map<String, Any> = mapOf())

what's the easiest way? (if there a way I can specify default (e.g,

LinkedHashMap

) for Map and default (

String

) for "c"?

I haven't understood question completely, are you implying

class Wrapper(val a: Map<String, Any> = linkedMapOf("b" to "c"))

?

Sorry for the confusion. The root question is how to deserialize

{ "a": { "b": "c" }}

to

Wrapper

with ease. The default (de)serializer assumes type information is also present:

{ "a": { "b": ["kotlin.String", "c"] }}

.

That's because you have

Any

as type parameter. Can you replace it with

String

?

Thanks. Yes, I understand that. Unfortunately no, it's indeed

Any

. could be a

String

, another

Map

or a

List

.

Since you have no types known in advance, you can probably resort to using

JsonTreeMapper

to parse untyped structure to a generic json tree

In the real case, the Wrapper class is much more complicated. I minimized it to only show the problematic part. So

JsonTreeMapper

is not really a choice, and I really need to deserialize the JSON into its binding. Is there a way to specify a default type for certain JSON node?

It can do it, but for it it should save objects with type information. By default, it will save your map like this:

{ "a": { "b": ["kotlin.String", "c"] }}

. So the problem here is only when you're integrating with other JSON forms of type recording. Deserializing values into

Any

without type information is indeed prohibited by design: kotlinx.serialization has pull model, where you first determine the type, then look into the input stream, and if the type mismatches, it's an error.

By default, I meant any

@Serializable class Wrapper(val a: Map<String, java.io.Serializable>)

can be serialized to a JSON string, but the same JSON string can NOT be deserialized.

I don't see how

java.io.Serializable

related

My original class has a field

a: Map<String, java.io.Serializable>

(or it can be relaxed to

a: Map<String, Any>

). with

java.io.Serializable

, this field can be serialized to

"a": { "b": "c" }

, but can't be deserialized. with

Any

, it can be deserialized, but the serialization has type info (`"a": { "b": [String, "c"] }) with is not backward compatible.