JsonDecodingException: Polymorphic serializer was not found for missing class discriminator ('null')

Describe the bug Unable to deserialize json string. Upon performing decodeFromString() throws Exception:

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)

To Reproduce I wrote a test case for demonstrate this:

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
)

Expected behavior It should deserialize.

Environment

• Kotlin version: 1.4.31
• Library version: 1.1.0
• Kotlin platforms: JVM
• Gradle version: 6.5
• IDE version: Android Studio 4.1.2
• Build #AI-201.8743.12.41.7042882, built on December 20, 2020
• Runtime version: 1.8.0_242-release-1644-b3-6222593 amd64
• VM: OpenJDK 64-Bit Server VM by JetBrains s.r.o
• Linux 5.8.0-45-generic
• GC: ParNew, ConcurrentMarkSweep
• Memory: 1981M
• Cores: 8
• Registry: ide.new.welcome.screen.force=true, external.system.auto.import.disabled=true
• Non-Bundled Plugins: com.thoughtworks.gauge, com.intellij.marketplace, org.jetbrains.kotlin, com.google.mad-scorecard
• Current Desktop: ubuntu:GNOME

[sandwwraith]

I think that JsonContentPolymorphicSerializer is a feature you’re looking for: https://github.com/Kotlin/kotlinx.serialization/blob/master/docs/json.md#content-based-polymorphic-deserialization

[iChintanSoni]

@sandwwraith Yes I achieved it using JsonContentPolymorphicSerializer. Here’s link to SO where I answered my own question this morning: https://stackoverflow.com/questions/66690712/kotlinx-serialization-polymorphic-serializer-was-not-found-for-missing-class-di