将多态案例类转换为json并返回

Question

我试图在scala中使用spray-json来识别转换为Json和返回时Ec2Provider和OpenstackProvider之间的选择.我希望能够在"提供者"中做出选择,如果这些选择不符合可用的选择,那么它就不应该验证.

我在这方面的尝试可以在以下代码中看到:

import spray.json._
import DefaultJsonProtocol._ 

case class Credentials(username: String, password: String)
abstract class Provider
case class Ec2Provider(endpoint: String,credentials: Credentials) extends Provider
case class OpenstackProvider(credentials: Credentials) extends Provider
case class Infrastructure(name: String, provider: Provider, availableInstanceTypes: List[String])
case class InfrastructuresList(infrastructures: List[Infrastructure])

object Infrastructures extends App with DefaultJsonProtocol {
   implicit val credFormat = jsonFormat2(Credentials)
   implicit val ec2Provider = jsonFormat2(Ec2Provider)
   implicit val novaProvider = jsonFormat1(OpenstackProvider)
   implicit val infraFormat = jsonFormat3(Infrastructure)
   implicit val infrasFormat = jsonFormat1(InfrastructuresList)

  println(
    InfrastructuresList(
      List(
        Infrastructure("test", Ec2Provider("nova", Credentials("user","pass")), List("1", "2")) 
      )
    ).toJson
  )
}

不幸的是,它失败了,因为它找不到Provider抽象类的格式化程序.

test.scala:19: could not find implicit value for evidence parameter of type Infrastructures.JF[Provider]

有人有任何解决方案吗？

Answer 1

您想要做的事情不是开箱即用的(即通过类型提示允许解串器知道要实例化的具体类),但它确实可以通过一些小腿工作.首先,使用您在上面发布的代码的简化版本的示例:

case class Credentials(user:String, password:String)
abstract class Provider
case class Ec2Provider(endpoint:String, creds:Credentials) extends Provider
case class OpenstackProvider(creds:Credentials) extends Provider
case class Infrastructure(name:String, provider:Provider)

object MyJsonProtocol extends DefaultJsonProtocol{
  implicit object ProviderJsonFormat extends RootJsonFormat[Provider]{
    def write(p:Provider) = p match{
      case ec2:Ec2Provider => ec2.toJson
      case os:OpenstackProvider => os.toJson
    }

    def read(value:JsValue) = value match{
      case obj:JsObject if (obj.fields.size == 2) => value.convertTo[Ec2Provider]
      case obj:JsObject => value.convertTo[OpenstackProvider]
    }
  }

  implicit val credFmt = jsonFormat2(Credentials)
  implicit val ec2Fmt = jsonFormat2(Ec2Provider)
  implicit val openStackFmt = jsonFormat1(OpenstackProvider)
  implicit val infraFmt = jsonFormat2(Infrastructure)
}

object PolyTest {
  import MyJsonProtocol._

  def main(args: Array[String]) {
    val infra = List(
      Infrastructure("ec2", Ec2Provider("foo", Credentials("me", "pass"))),
      Infrastructure("openstack", OpenstackProvider(Credentials("me2", "pass2")))
    )
    val json = infra.toJson.toString
    val infra2 = JsonParser(json).convertTo[List[Infrastructure]]
    println(infra == infra2)
  }
}

为了能够序列化/反序列化抽象类的实例Provider,我创建了一个自定义格式化程序,我在其中提供读写Provider实例的操作.我在这些函数中所做的只是检查一个简单的条件(这里只有2个impl Provider)来查看它是什么类型,然后委托逻辑来处理该类型.

对于写作,我只需要知道哪个实例类型很容易.阅读虽然有点棘手.为了阅读,我正在检查对象有多少属性,因为两个impls有不同数量的道具,我可以区分哪个是这样的.我在这里做的检查非常简陋,但它表明,如果你可以查看Json AST并区分类型,那么你可以选择要反序列化的那个.您的实际检查可以像您希望的那样简单或复杂,只要它在区分类型方面具有确定性.