kotlinx-serialization: Polymorphic serializer was not found for missing class discriminator ('null')

Question

I am trying to serialize a json, but its throwing JsonDecodingException. Check the code:

SerializationTestCase.kt:

import kotlinx.serialization.SerialName
import kotlinx.serialization.Serializable
import kotlinx.serialization.decodeFromString
import kotlinx.serialization.json.Json
import kotlinx.serialization.modules.SerializersModule
import kotlinx.serialization.modules.polymorphic
import org.junit.Test

class SerializationTestCase {

    private val json = Json {
        ignoreUnknownKeys = true
        isLenient = true
        prettyPrint = true
        allowStructuredMapKeys = true
        encodeDefaults = true
        classDiscriminator = "#class"
        serializersModule = SerializersModule {
            polymorphic(Module::class) {
                subclass(TeamModule::class, TeamModule.serializer())
            }
        }
    }

    @Test
    fun test() {
        val inputJson = """
    [
        {
          "type": "team",
          "data": {
            "readOnly": true
          }
        }
    ]
""".trimIndent()

        val moduleList: List<Module> = json.decodeFromString(inputJson)
        println(moduleList)
    }
}


@Serializable
sealed class Module(val type: String)

@Serializable
@SerialName("TeamModule")
data class TeamModule(val data: TeamData) : Module("team")

@Serializable
data class TeamData(
    val readOnly: Boolean = false
)

Error Log:

kotlinx.serialization.json.internal.JsonDecodingException: Polymorphic serializer was not found for missing class discriminator ('null')
JSON input: {"type":"team","data":{"readOnly":true}}

    at kotlinx.serialization.json.internal.JsonExceptionsKt.JsonDecodingException(JsonExceptions.kt:24)
    at kotlinx.serialization.json.internal.JsonExceptionsKt.JsonDecodingException(JsonExceptions.kt:32)
    at kotlinx.serialization.json.internal.PolymorphicKt.throwSerializerNotFound(Polymorphic.kt:80)
    at kotlinx.serialization.json.internal.PolymorphicKt.decodeSerializableValuePolymorphic(Polymorphic.kt:70)
    at kotlinx.serialization.json.internal.StreamingJsonDecoder.decodeSerializableValue(StreamingJsonDecoder.kt:32)
    at kotlinx.serialization.encoding.AbstractDecoder.decodeSerializableValue(AbstractDecoder.kt:43)
    at kotlinx.serialization.encoding.AbstractDecoder.decodeSerializableElement(AbstractDecoder.kt:70)
    at kotlinx.serialization.encoding.CompositeDecoder$DefaultImpls.decodeSerializableElement$default(Decoding.kt:536)
    at kotlinx.serialization.internal.ListLikeSerializer.readElement(CollectionSerializers.kt:80)
    at kotlinx.serialization.internal.AbstractCollectionSerializer.readElement$default(CollectionSerializers.kt:51)
    at kotlinx.serialization.internal.AbstractCollectionSerializer.merge(CollectionSerializers.kt:36)
    at kotlinx.serialization.internal.AbstractCollectionSerializer.deserialize(CollectionSerializers.kt:43)
    at kotlinx.serialization.json.internal.PolymorphicKt.decodeSerializableValuePolymorphic(Polymorphic.kt:63)
    at kotlinx.serialization.json.internal.StreamingJsonDecoder.decodeSerializableValue(StreamingJsonDecoder.kt:32)
    at kotlinx.serialization.json.Json.decodeFromString(Json.kt:85)
    at com.arinspect.domain.SerializationTestCase.test(SerializationTestCase.kt:61)
    at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
    at java.lang.reflect.Method.invoke(Method.java:498)
    at org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
    at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
    at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
    at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
    at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
    at org.junit.runners.BlockJUnit4ClassRunner$1.evaluate(BlockJUnit4ClassRunner.java:100)
    at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:366)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:103)
    at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:63)
    at org.junit.runners.ParentRunner$4.run(ParentRunner.java:331)
    at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:79)
    at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:329)
    at org.junit.runners.ParentRunner.access$100(ParentRunner.java:66)
    at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:293)
    at org.junit.runners.ParentRunner$3.evaluate(ParentRunner.java:306)
    at org.junit.runners.ParentRunner.run(ParentRunner.java:413)
    at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
    at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
    at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
    at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:230)
    at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:58)

I went through the documentation and I am doing the same as asked, but still its not working as expected. Any help would be appreciated!

PS: Dependency I am using:

api "org.jetbrains.kotlinx:kotlinx-serialization-json:1.1.0"

and kotlin version is 1.4.31

Looks like this is an issue, I have reported this here: https://github.com/Kotlin/kotlinx.serialization/issues/1382

Answer 1

We can use JsonContentPolymorphicSerializer.

According to documentation:

Base class for custom serializers that allows selecting polymorphic serializer without a dedicated class discriminator, on a content basis.

this is how I achieved:

class SerializationTestCase {

    private val json = Json {
        ignoreUnknownKeys = true
        isLenient = true
        prettyPrint = true
        encodeDefaults = true
        classDiscriminator = "#class"
    }

    @Test
    fun test() {
        val inputJson = """
    [
    {"type": "team","data": {"readOnly": true}},
    {"type": "org","data": {"readOnly": true}}
    ]
""".trimIndent()

        val moduleList: List<Module> =
            json.decodeFromJsonElement(json.parseToJsonElement(inputJson))
        println(moduleList)
    }
}


@Serializable(with = ModuleSerializer::class)
sealed class Module {
    abstract val type: String
}

@Serializable
data class TeamModule(val data: TeamData, override val type: String = "team") : Module()

@Serializable
data class OrgModule(val data: TeamData, override val type: String = "org") : Module()

@Serializable
data class TeamData(
    val readOnly: Boolean = false
)

object ModuleSerializer : JsonContentPolymorphicSerializer<Module>(Module::class) {
    override fun selectDeserializer(element: JsonElement): DeserializationStrategy<out Module> {
        return when (element.jsonObject["type"]?.jsonPrimitive?.content) {
            "team" -> TeamModule.serializer()
            "org" -> OrgModule.serializer()
            else -> throw Exception("Unknown Module: key 'type' not found or does not matches any module type")
        }
    }
}