接口/实现序列化程序

Question

假设有一个接口和2个(或多个)实现：

interface IRunnable {
  fun run()
}

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

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

我想要为网络传输实现最简洁的JSON，即用于Horse的{"runnable":"H"}和用于Dog的{"runnable":"D"}

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来实现这个目标。

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")
      }
    }
  }
}

但当我试图把Horse转换成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.

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

谢谢。

环境：

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

更新的全错误消息：

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

您正在尝试实现的称为多态序列化( kotlinx.serialization支持哪个 )。简而言之:它要求您将IRunnable的所有派生类型注册为SerialModule中的多态类型。对于您的代码，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，正如异常正确指出的那样，不能初始化它。如何初始化接口？您不能。相反，您想知道要初始化哪种特定的派生类型。这需要在JSON数据中嵌入类型信息，多态序列化就是这样做的。

此外，您不必实现自定义序列化程序。根据链接到的文档，可以创建库类的外部序列化器。以下可能就足够了，因为kotlinx.serialization编译器插件将尝试为您生成一个合适的序列化程序，类似于您要直接在类中添加@Serializable：

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

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