Akka EventSourced Actor序列化报错：无法实例化内部类State

在尝试持久化事件并更新Actor状态时，抛出如下异常：

Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `module.MyActor$State`: non-static inner classes like this can only by instantiated using default, no-argument constructor

相关Actor代码如下：

object MyActor {

  final case class Settings(
      name: String = "",
      accessToken: AccessToken = AccessToken()      

     ) extends CborSerializable
     .
     .
     .
}

class MyActor()(implicit system: ActorSystem[_]) {
  import MyActor._

  private implicit val executionContext: ExecutionContextExecutor = system.executionContext
  private val sharding = ClusterSharding(system)

  // State
  final case class State(
      settings: Settings = Settings(),
      pollTime: Long = 0L
  ) extends CborSerializable
  . 
  .
  private def onEvent(state: State, event: Event): State = {
event match {
  case _ => {            
       state.copy(pollTime = state.pollTime.updated(timestampToStringDate(System.currentTimeMillis())))
   }
  }
 .
 .     
}

这是一个Typed EventSourced Actor，事件/命令处理程序因与序列化问题无关已省略。

使用Jackson序列化器，application.conf配置如下：

akka.actor.serialization-bindings {
  "module.CborSerializable" = jackson-cbor
}

模块中已定义trait CborSerializable，需排查操作错误点。

问题核心：State是MyActor类的非静态内部类，Jackson反序列化时无法正确构造这类实例——非静态内部类必须依赖外部MyActor实例才能创建，但反序列化场景下没有可用的外部实例，即便提供默认无参构造器也无法解决。

修复方法二选一：

• 将State移到MyActor的伴生对象object MyActor内部，此时它会成为静态嵌套类（Scala中伴生对象内的类不依赖外部类实例）
• 直接把State定义为顶层类（放在当前文件最外层或单独文件中）

调整后的代码示例（将State移至伴生对象）：

object MyActor {

  final case class Settings(
      name: String = "",
      accessToken: AccessToken = AccessToken()      

     ) extends CborSerializable

  // 把State移到伴生对象内，转为静态嵌套类
  final case class State(
      settings: Settings = Settings(),
      pollTime: Long = 0L
  ) extends CborSerializable
     .
     .
     .
}

class MyActor()(implicit system: ActorSystem[_]) {
  import MyActor._

  private implicit val executionContext: ExecutionContextExecutor = system.executionContext
  private val sharding = ClusterSharding(system)

  . 
  .
  private def onEvent(state: State, event: Event): State = {
event match {
  case _ => {            
       state.copy(pollTime = state.pollTime.updated(timestampToStringDate(System.currentTimeMillis())))
   }
  }
 .
 .     
}

调整后State成为独立的静态类，Jackson可通过无参构造器正常完成序列化与反序列化操作。

内容的提问来源于stack exchange，提问作者Arpit Sharma