kotlinx-serialization：找不到多态序列化器，因为缺少类鉴别器（'null'）

Posted 2023-04-15

【中文标题】kotlinx-serialization：找不到多态序列化器，因为缺少类鉴别器（\'null\'）

【英文标题】：kotlinx-serialization: Polymorphic serializer was not found for missing class discriminator ('null')kotlinx-serialization：找不到多态序列化器，因为缺少类鉴别器（'null'）

【发布时间】：2021-06-15 19:57:19

【问题描述】：

我正在尝试序列化一个 json，但它会抛出 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documentation，并且按照要求执行了相同的操作，但仍然无法按预期工作。任何帮助将不胜感激！

PS：我正在使用的依赖：

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： https://github.com/Kotlin/kotlinx.serialization/issues/1382

【问题讨论】：

为什么您的 serializersModule 引用 InspectionModule？它不在您的代码 sn-p 中的其他位置。

@Pawel 哎呀。对不起这是我的错。我更新了问题。

【参考方案1】：

我们可以使用JsonContentPolymorphicSerializer。

根据文档：

允许选择多态的自定义序列化程序的基类 在内容上没有专用类鉴别器的序列化器 基础。

这就是我的成就：

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")
        
    

【讨论】：

在这种情况下，classDiscriminator = "#class" 没有被使用，应该被省略。