Is it possible to globally override the built-in s...

# serialization

[Mehmet]

Is it possible to globally override the built-in serializer for a primitive type, such as Boolean, without requiring explicit @Contextual annotations on every field?

[Mehmet]

More details:

I’m migrating from Gson and need to handle diverse Boolean representations (e.g., `true`/`false`, `"true"`/`"false"`, `1`/`0`, `"1"`/`"0"`, `1.0`/`0.0`) due to legacy backend inconsistencies.

I’ve created a custom KSerializer for Boolean and registered it as a contextual serializer in my Json instance:

private val json: Json = Json {
    serializersModule = SerializersModule {
        contextual(Boolean::class, MyCustomBooleanSerializer())
    }
}

However, Boolean fields are still deserialized using the default built-in serializer unless I explicitly annotate them with @Contextual. It appears that contextual serializers are only resolved when a built-in serializer is not found (as seen in SerializersModule.reflectiveOrContextual).

Is there a way to prioritize my custom serializer over the built-in one for all instances of Boolean, without requiring per-field annotations? Or, is there any other approach to achieve global overriding of built-in serializers for primitive types?

[Mehmet]

Given this data class and Json instance :

class MyCustomBooleanSerializer : KSerializer<Boolean> {
    // ... custom deserialization logic to handle "1", "0", "true", "false", etc.
}

val json = Json {serializersModule = SerializersModule { contextual( MyCustomBooleanSerializer()) }

@Serializable data class MyData(val value: Boolean)

I expect this deserialization to work:

json.decodeFromString<MyData>("""{"value": "1"}""")

however, it fails with

kotlinx.serialization.json.internal.JsonDecodingException: Unexpected JSON token at offset 11: Expected valid boolean literal prefix, but had '1' at path: $.value
JSON input: {"value": 1}

if I annotate the value with @Contextual it works as expected:

@Serializable data class MyData( @Contextual val value: Boolean)

[ephemient]

I don't think there's a way beyond modifying the serial format itself (e.g. an extension of JsonConfiguration.isLenient mode)

[rnett]

Whenever I've needed to do something like this I've had to create custom encoders that delegate everything but the type I care about to the base encoder I want to use. This is not the first time this has come up in this channel, but I'm not aware of a current issue for it

[Mehmet]

I’ve had to create custom encoders that delegate everything but the type I care about to the base encoder I want to use.

@rnett that will work for me too. how can I write my custom encoder/decoder? I couldn’t see anything about that in docs

[rnett]

Look at the docs for custom formats

[Mehmet]

thanks, found it 👍