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Fix Polymorphic Deserialization Error with Subclass using JsonTransformingSerializer #3139

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Fix Polymorphic Deserialization Error with Subclass using JsonTransformingSerializer #3139

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e40d112
Fix: Allow `JsonTransformingSerializer` with polymorphic types
oungsi2000 Jan 14, 2026
55c02a3
feat: Add test for nested polymorphic deserialization
oungsi2000 Jan 14, 2026
56bf91e
Chore: Remove trailing newline
oungsi2000 Jan 16, 2026
28af339
feat(json): add PolymorphicJsonDecoder interface
oungsi2000 Jan 30, 2026
8a1f5d5
docs: add document for PolymorphicJsonDecoder
oungsi2000 Jan 30, 2026
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74 changes: 69 additions & 5 deletions ...ts/json-tests/commonTest/src/kotlinx/serialization/json/JsonTransformingSerializerTest.kt
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Original file line number Diff line number Diff line change
Expand Up @@ -95,12 +95,19 @@ class JsonTransformingSerializerTest : JsonTestBase() {
@Test
fun testDocumentationSample() = parametrizedTest { streaming ->
val correctExample = DocExample("str1")
assertEquals(correctExample, json.decodeFromString(DocExample.serializer(), """{"data":["str1"]}""", streaming))
assertEquals(correctExample, json.decodeFromString(DocExample.serializer(), """{"data":"str1"}""", streaming))
assertEquals(
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@sandwwraith sandwwraith Jan 20, 2026

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Please do not add irrelevant changes to the PR, especially if they're about formatting.

correctExample,
json.decodeFromString(DocExample.serializer(), """{"data":["str1"]}""", streaming)
)
assertEquals(
correctExample,
json.decodeFromString(DocExample.serializer(), """{"data":"str1"}""", streaming)
)
}

// Wraps/unwraps {"data":null} to just `null`, because StringData.data is not nullable
object NullableStringDataSerializer : JsonTransformingSerializer<StringData?>(StringData.serializer().nullable) {
object NullableStringDataSerializer :
JsonTransformingSerializer<StringData?>(StringData.serializer().nullable) {
override fun transformDeserialize(element: JsonElement): JsonElement {
val data = element.jsonObject["data"]?.jsonPrimitive
if (data?.contentOrNull.isNullOrBlank()) return JsonNull
Expand All @@ -120,7 +127,64 @@ class JsonTransformingSerializerTest : JsonTestBase() {
fun testNullableTransformingSerializer() {
val normalInput = """{"stringData":{"data":"str1"}}"""
val nullInput = """{"stringData":{"data":null}}"""
assertJsonFormAndRestored(NullableStringDataHolder.serializer(), NullableStringDataHolder(StringData("str1")), normalInput)
assertJsonFormAndRestored(NullableStringDataHolder.serializer(), NullableStringDataHolder(null), nullInput)
assertJsonFormAndRestored(
NullableStringDataHolder.serializer(),
NullableStringDataHolder(StringData("str1")),
normalInput
)
assertJsonFormAndRestored(
NullableStringDataHolder.serializer(),
NullableStringDataHolder(null),
nullInput
)
}

@Serializable
sealed class BaseExample

@Serializable(SubExample.PolymorphicSerializer::class)
@KeepGeneratedSerializer
@SerialName("Sub")
data class SubExample(
val data: String
) : BaseExample() {
object PolymorphicSerializer : JsonTransformingSerializer<SubExample>(generatedSerializer())
}

@Test
fun testPolymorphicExampleCanBeParsed() {
val baseExample: BaseExample = SubExample("str1")
val polymorphicInput = Json.encodeToString(baseExample)
assertEquals(baseExample, Json.decodeFromString(polymorphicInput))
}

@Serializable
sealed class NestedBaseExample

@Serializable
@SerialName("leaf")
data class LeafSubExample(val value: String) : NestedBaseExample()

@Serializable
@SerialName("composite")
data class CompositeSubExample(
val name: String,
val children: List<NestedBaseExample>,
val metadata: NestedBaseExample? = null
) : NestedBaseExample()

@Test
fun testNestedPolymorphicExampleCanBeParsed() {
val root: NestedBaseExample = CompositeSubExample(
name = "root",
children = listOf(
LeafSubExample("child1"),
CompositeSubExample("sub-root", listOf(LeafSubExample("grand-child")))
),
metadata = LeafSubExample("meta-info")
)

val polymorphicInput = Json.encodeToString(root)
assertEquals(root, Json.decodeFromString(polymorphicInput))
}
}
2 changes: 1 addition & 1 deletion formats/json/commonMain/src/kotlinx/serialization/json/internal/TreeJsonDecoder.kt
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Expand Up @@ -18,7 +18,7 @@ import kotlin.jvm.*
@JsonFriendModuleApi
public fun <T> readJson(json: Json, element: JsonElement, deserializer: DeserializationStrategy<T>): T {
val input = when (element) {
is JsonObject -> JsonTreeDecoder(json, element)
is JsonObject -> JsonTreeDecoder(json, element, deserializer.descriptor.classDiscriminator(json))
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@sandwwraith sandwwraith Jan 20, 2026

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Sadly, this change is too broad to fix the problem. By using classDiscriminator(json), you're adding a discriminator to every JsonTreeDecoder, even one that was not originally polymorphic. Consider this class:

@Serializable(SubExample2.PolymorphicSerializer::class)
@KeepGeneratedSerializer
@SerialName("Sub")
data class SubExample2(
    val data: String
) {
    object PolymorphicSerializer : JsonTransformingSerializer<SubExample2>(generatedSerializer())
}

It is almost identical to yours, but it does not have a superclass and therefore is not polymorphic. Therefore, we expect this test to fail:

@Test
fun testNonPolymorphicExampleShouldNotBeParsed() {
    val input = """{"type":"Sub","data":"str1"}"""
    val j = Json { ignoreUnknownKeys = false }
    println(j.decodeFromString<SubExample2>(input)) // should throw "Encountered an unknown key 'type'"
}

However, with your change, this test passes.

is JsonArray -> JsonTreeListDecoder(json, element)
is JsonLiteral, JsonNull -> JsonPrimitiveDecoder(json, element as JsonPrimitive)
}
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