用于接口/实现的序列化程序

Question

假设有一个接口，和2个(或更多(实现：

interface IRunnable {
fun run()
}
class Horse : IRunnable {
override fun run() {    println("horse running")  }
}
class Dog : IRunnable {
override fun run() {    println("dog running")  }
}

我想为网络传输实现最简洁的 JSON，即{"runnable":"H"}用于马，{"runnable":"D"}用于狗

如果我不能修改接口和实现，我想将接口/实现序列化为JSON，根据Kotlin的文档，我必须编写一个自定义序列化程序，并使用SerializersModule来实现目标。

假设只有Horse和Dog实现IRunnable，这就是我所做的：

class RunnableSerializer : KSerializer<IRunnable> {
override val descriptor: SerialDescriptor
get() = StringDescriptor.withName("runnable")

override fun serialize(encoder: Encoder, obj: IRunnable) {
val stringValue = when (obj) {
is Horse -> { "H" }
is Dog -> { "D" }
else -> { null }
}
stringValue?.also {
encoder.encodeString(it)
}
}
override fun deserialize(decoder: Decoder): IRunnable {
return decoder.decodeString().let { value ->
when(value) {
"H" -> Horse()
"D" -> Dog()
else -> throw RuntimeException("invalid $value")
}
}
}
}

但是当我尝试将Horse转换为 JSON 时......

class RunnableTest {
private val logger = KotlinLogging.logger { }
@ImplicitReflectionSerializer
@Test
fun testJson() {
val module1 = serializersModuleOf(IRunnable::class, RunnableSerializer())
Json(context = module1).stringify(IRunnable::class.serializer(), Horse()).also {
logger.info("json = {}", it)
}
}
}

它输出错误：

kotlinx.serialization.SerializationException: 
Can't locate argument-less serializer for class IRunnable. For generic classes, such as lists, please provide serializer explicitly.

如何实现类似 在这种情况下{"runnable":"H"}或{"runnable":"D"}？

谢谢。

环境：

<kotlin.version>1.3.60</kotlin.version>
<serialization.version>0.14.0</serialization.version>

已更新，完整的错误消息：

kotlinx.serialization.SerializationException: Can't locate argument-less serializer for class IRunnable. For generic classes, such as lists, please provide serializer explicitly.
at kotlinx.serialization.PlatformUtilsKt.serializer(PlatformUtils.kt:12)
at RunnableTest.testJson(RunnableTest.kt:17)
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:50)
at org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
at org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:47)
at org.junit.internal.runners.statements.InvokeMethod.evaluate(InvokeMethod.java:17)
at org.junit.runners.ParentRunner.runLeaf(ParentRunner.java:325)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:78)
at org.junit.runners.BlockJUnit4ClassRunner.runChild(BlockJUnit4ClassRunner.java:57)
at org.junit.runners.ParentRunner$3.run(ParentRunner.java:290)
at org.junit.runners.ParentRunner$1.schedule(ParentRunner.java:71)
at org.junit.runners.ParentRunner.runChildren(ParentRunner.java:288)
at org.junit.runners.ParentRunner.access$000(ParentRunner.java:58)
at org.junit.runners.ParentRunner$2.evaluate(ParentRunner.java:268)
at org.junit.runners.ParentRunner.run(ParentRunner.java:363)
at org.junit.runner.JUnitCore.run(JUnitCore.java:137)
at com.intellij.junit4.JUnit4IdeaTestRunner.startRunnerWithArgs(JUnit4IdeaTestRunner.java:68)
at com.intellij.rt.junit.IdeaTestRunner$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)

Answer 1

您尝试实现的称为多态序列化，kotlinx.serialization 支持多态序列化。简而言之：它要求您将所有派生类型的IRunnable注册为SerialModule中的多态。对于您的代码，它应该如下所示：

val serialModule = SerializersModule {
polymorphic(IRunnable::class) {
Horse::class with HorseSerializer
Dog::class with DogSerializer
}
}

目前，您只注册IRunnable，正如异常正确指示的那样，无法初始化。您将如何初始化接口？你不能。相反，您想知道要初始化哪个特定的派生类型。这需要在 JSON 数据中嵌入类型信息，这就是多态序列化的作用。

此外，您不一定必须实现自定义序列化程序。根据您链接到的文档，您可以为库类创建外部序列化程序。以下内容可能就足够了，因为 kotlinx.serialization 编译器插件将尝试为您生成合适的序列化程序，类似于直接在类上添加@Serializable：

@Serializer(forClass = Horse::class)
object HorseSerializer {}
@Serializer(forClass = Dog::class)
object DogSerializer {}