【发布时间】：2016-02-15 06:02:42
【问题描述】：
我正在尝试获取具有共同特征的项目列表，并将它们与 Json 进行转换。我在这里展示的例子是一列带有引擎和汽车的火车。创建火车分为三个类：Engines、Passenger Cars 和 Freight Cars。 （我认为一个简单的基于现实的例子最容易理解，它也没有我试图解决的问题那么复杂。）
火车各部分定义如下：
package models
sealed trait Vehicle {
val kind: String
val maxSpeed: Int = 0
def load: Int
}
case class Engine(override val maxSpeed: Int, val kind: String,
val power: Float, val range: Int) extends Vehicle {
override val load: Int = 0
}
case class FreightCar(override val maxSpeed: Int, val kind: String,
val load: Int) extends Vehicle {}
case class PassengerCar(override val maxSpeed: Int, val kind: String,
val passengerCount: Int) extends Vehicle {
override def load: Int = passengerCount * 80
}
（文件 Vehicle.scala）
火车定义为：
package models
import scala.collection.mutable
import play.api.Logger
import play.api.libs.json._
case class Train(val name: String, val cars: List[Vehicle]) {
def totalLoad: Int = cars.map(_.load).sum
def maxSpeed: Int = cars.map(_.maxSpeed).min
}
object Train {
def save(train: Train) {
Logger.info("Train saved ~ Name: " + train.name)
}
}
（文件 Train.scala）
如您所见，通过将“汽车”添加到存储在 Train 中的列表来创建火车。
将我的火车转换为 Json 或尝试从 Json 读取它时，我的问题出现了。这是我目前执行此操作的代码：
package controllers
import play.api.mvc._
import play.api.libs.json._
import play.api.libs.json.Reads._
import play.api.data.validation.ValidationError
import play.api.libs.functional.syntax._
import models.Vehicle
import models.Engine
import models.FreightCar
import models.PassengerCar
import models.Train
class Trains extends Controller {
implicit val JsPathWrites = Writes[JsPath](p => JsString(p.toString))
implicit val ValidationErrorWrites =
Writes[ValidationError](e => JsString(e.message))
implicit val jsonValidateErrorWrites = (
(JsPath \ "path").write[JsPath] and
(JsPath \ "errors").write[Seq[ValidationError]]
tupled
)
implicit object engineLoadWrites extends Writes[Engine] {
def writes(e: Engine) = Json.obj(
"maxSpeed" -> Json.toJson(e.maxSpeed),
"kind" -> Json.toJson(e.kind),
"power" -> Json.toJson(e.power),
"range" -> Json.toJson(e.range)
)
}
implicit object freightCarLoadWrites extends Writes[FreightCar] {
def writes(fc: FreightCar) = Json.obj(
"maxSpeed" -> Json.toJson(fc.maxSpeed),
"kind" -> Json.toJson(fc.kind),
"load" -> Json.toJson(fc.load)
)
}
implicit object passengerCarLoadWrites extends Writes[PassengerCar] {
def writes(pc: PassengerCar) = Json.obj(
"maxSpeed" -> Json.toJson(pc.maxSpeed),
"kind" -> Json.toJson(pc.kind),
"passengerCount" -> Json.toJson(pc.passengerCount)
)
}
implicit object trainWrites extends Writes[Train] {
def writes(t: Train) = Json.obj(
"name" -> Json.toJson(t.name),
"cars" -> Json.toJson(t.cars) // Definitely not correct!
)
}
/* --- Writes above, Reads below --- */
implicit val engineReads: Reads[Engine] = (
(JsPath \ "maxSpeed").read[Int] and
(JsPath \ "kind").read[String] and
(JsPath \ "power").read[Float] and
(JsPath \ "range").read[Int]
)(Engine.apply _)
implicit val freightCarReads: Reads[FreightCar] = (
(JsPath \ "maxSpeed").read[Int] and
(JsPath \ "kind").read[String] and
(JsPath \ "load").read[Int]
)(FreightCar.apply _)
implicit val passengerCarReads: Reads[PassengerCar] = (
(JsPath \ "maxSpeed").read[Int] and
(JsPath \ "kind").read[String] and
(JsPath \ "passengerCount").read[Int]
)(PassengerCar.apply _)
implicit val joistReads: Reads[Train] = (
(JsPath \ "name").read[String](minLength[String](2)) and
(JsPath \ "cars").read[List[Cars]] // Definitely not correct!
)(Train.apply _)
/**
* Validates a JSON representation of a Train.
*/
def save = Action(parse.json) { implicit request =>
val json = request.body
json.validate[Train].fold(
valid = { train =>
Train.save(train)
Ok("Saved")
},
invalid = {
errors => BadRequest(Json.toJson(errors))
}
)
}
}
（文件 Trains.scala）
所有为 FreightCars、Engines 列表创建和使用 Json 的代码都可以工作，但我无法创建 Json 来同时处理所有三种类型，例如：
implicit object trainWrites extends Writes[Train] {
def writes(t: Train) = Json.obj(
"name" -> Json.toJson(t.name),
"cars" -> Json.toJson(t.cars)
)
}
列表的 Json.toJson 根本不起作用；它的阅读对应物也没有。当我用类 Engines 或我的任何单个具体类替换上面代码中的 t.cars 时，一切正常。
我该如何优雅地解决这个问题以使我的 Json 阅读器和编写器正常工作？或者，如果 Scala Play 的 Json 编码器解码器不是此类任务的好选择，是否有更合适的 Json 库？
【问题讨论】：
标签： json scala playframework