kotlinx 序列化：找不到缺少类鉴别器('null')的多态序列化器

Question

我试图序列化一个json，但它抛出了JsonDecodingException。检查代码：

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
)

错误日志：

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)

我查看了文档，并按照要求进行了同样的操作，但仍然没有按预期工作。任何帮助都将不胜感激！

PS:我使用的依赖项：

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

kotlin版本是1.4.31

看起来这是一个问题，我在这里报告了这个问题：https://github.com/Kotlin/kotlinx.serialization/issues/1382

Answer 1

我们可以使用JsonContentPolymorphicSerializer。

根据文件：

允许选择多态的自定义序列化程序的基类没有专用类鉴别器的序列化程序原因

这就是我实现的方式

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")
}
}
}

Answer 2

对于那些希望在不使用JsonContentPolymorphicSerializer的情况下实现此功能的人，您可以在每个类上使用@SerialName注释来指定正确的discriminator值，并让kotlinx.serialization进行处理。

class SerializationTestCase {
private val json = Json {
ignoreUnknownKeys = true
isLenient = true
prettyPrint = true
allowStructuredMapKeys = true
encodeDefaults = true
classDiscriminator = "type" // default is "type", so you can just omit it
}
@Test
fun test() {
val inputJson = """
[
{
"type": "team",
"data": {
"readOnly": true
}
}
]
""".trimIndent()
val moduleList: List<Module> = json.decodeFromString(inputJson)
println(moduleList)
}
}

@Serializable
sealed class Module {
abstract val type: String
}
@Serializable
@SerialName("team") // <-- this is the class discriminator. It should match the value of the `type` property in the JSON.
data class TeamModule(override val type: String, val data: TeamData) : Module()
@Serializable
@SerialName("org") // <-- this is the class discriminator. It should match the value of the `type` property in the JSON.
data class OrgModule(override val type: String, val data: TeamData) : Module()
@Serializable
data class TeamData(
val readOnly: Boolean = false
)