json - Play json 写入子类给出不明确的隐含值错误

Question

我正在使用 play 框架，并有一个抽象类:

 abstract class Base{...}

它在伴随对象中有自己的隐式 JSON 编写器

object Base {
   implicit val baseWrites: Writes[Base] = (...)(unlift(Base.unapply))
}

我继承了这个抽象类:

case class SubClass{...}

它的伴生对象中也有自己的隐式 JSON 编写器

object SubClass {
   implicit val subClassWrites: Writes[SubClass] = (...)(unlift(SubClass.unapply))
}

当我尝试使用 Json.toJson(SubClass) 序列化子类对象时，出现错误:

[error]  both value subClassWrites in object SubClass of type => play.api.libs.json.
Writes[models.SubClass]
[error]  and value baseWrites in object Base of type =>        
play.api.libs.json.Writes[models.Base]
[error]  match expected type play.api.libs.json.Writes[models.SubClass]
[error]  Ok(Json.toJson(SubClass.find(id)))

有什么办法可以消除歧义吗？

Answer 1

您正在发生碰撞，因为 Writes有一个逆变类型参数 A :

trait Writes[-A] extends AnyRef

这意味着 Writes[Base]是 Writes[SubClass] 的子类- 您可以使用 Writes[Base]哪里 Writes[SubClass]是必须的。

问题在这里:

val base: Base = new SubClass(...)
val jsBase = Json.toJson(base)

所以 Writes[Base]应该能够序列化 SubClass 的实例.您可以使用 ADT在这种情况下:

sealed trait Base
object Base {
  implicit val baseWrites: Writes[Base] = 
    new Writes[Base]{
      def writes(o: Base): JsValue = o match {
        case s: SubClass => SubClass.writes.writes(s)
        case s: SubClass2 => SubClass2.writes.writes(s)
      }
    }
}

case class SubClass(...) extends Base
object SubClass {
  val writes: Writes[SubClass] = (...)(unlift(SubClass.unapply))
}

case class SubClass2(...) extends Base
object SubClass2 {
  val writes: Writes[SubClass2] = (...)(unlift(SubClass2.unapply))
}

与 sealed关键字你会收到警告 match并不详尽。