Hi, I am trying to maintain some backward compatibility to serialized JSON: for example ``` { "a": {...

# serialization

[jakiej]

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"?

[sandwwraith]

I haven't understood question completely, are you implying class Wrapper(val a: Map<String, Any> = linkedMapOf("b" to "c"))?

[jakiej]

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"] }}.

[sandwwraith]

That's because you have Any as type parameter. Can you replace it with String?

[jakiej]

Thanks. Yes, I understand that. Unfortunately no, it's indeed Any. could be a String, another Map or a List.

[jakiej]

Another finding is that @Serializable class Wrapper(val a: Map<String, CharSequence> = mapOf()) will serialize to { "a": { "b": "c" }} too, but deserialization does not work either.

[sandwwraith]

Since you have no types known in advance, you can probably resort to using JsonTreeMapper to parse untyped structure to a generic json tree

[jakiej]

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?

[jakiej]

Also, is the latter case by design? By default, kotlinx.serialization can only serialize certain object to JSON, but can't not deserialize the JSON produced by itself to the same object?

[sandwwraith]

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.

[jakiej]

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.

[jakiej]

@sandwwraith sorry for the @, but what's the suggested solutions? for a serializable class with fields like this a: Map<String, java.io.Serializable>

[sandwwraith]

I don't see how java.io.Serializable related

[jakiej]

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.