Question

我花了最后一天搜索和阅读各种网站和文章，试图找到这个问题的答案，我没有发现任何有用的东西。我甚至不确定这是否可行。我的问题是我试图使用lift-json解析并提取Json响应。响应由4部分组成，其中前3个部分对于我所做的每种类型的请求的每个响应总是相同的。最后一部分取决于请求的类型，但它总是一个类型的列表。我希望能做到这样的事情：

abstract class MyObjects
case class Apple(id: Int, name: String, color: String) extends MyObjects
case class Orange(id: Long, name: String, state: String) extends MyObjects

abstract class MyResponse
case class Fruits[T <: MyObjects](aisle: Int, bin: Int, hasWhat: Option[List[T]])

如果我想知道所有的苹果是什么，我会提出要求，并回复一个苹果列表的回复。当我尝试提取此示例时：

myJson.extract[Fruits[Apple]]

我收到此错误：

net.liftweb.json.MappingException: do not know how to get type parameter from T
    at net.liftweb.json.Meta$.fail(Meta.scala:128)
    at net.liftweb.json.Meta$Reflection$.term$1(Meta.scala:206)
    at net.liftweb.json.Meta$Reflection$.typeParameters(Meta.scala:220)
    at net.liftweb.json.Meta$.mkContainer$1(Meta.scala:91)
    at net.liftweb.json.Meta$.fieldMapping$1(Meta.scala:101)
    at net.liftweb.json.Meta$.mkContainer$1(Meta.scala:90)
    at net.liftweb.json.Meta$.fieldMapping$1(Meta.scala:107)
    at net.liftweb.json.Meta$.toArg$1(Meta.scala:117)
    at net.liftweb.json.Meta$$anonfun$constructors$1$1$$anonfun$apply$1.apply(Meta.scala:83)
    at net.liftweb.json.Meta$$anonfun$constructors$1$1$$anonfun$apply$1.apply(Meta.scala:82)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:...

我正在使用lift-json 2.1＆amp;斯卡拉2.8。我确实有办法解决它，通过专门为每种类型的响应创建一个案例类，但我认为我想要做的更干净。只是想知道是否 a）这有可能吗？ b）如果是的话，我做错了什么？

编辑...示例应用程序：

val apples = """{ "aisle" : 1, "bin" : 1,
            "hasWhat" : [{ "id" : 4, "name" : "Granny", "color" : "green"}, 
                         { "id" : 4, "name" : "Fuji", "color" : "red"}] }"""

val oranges = """ { "aisle" : 3, "bin" : 2, 
             "hasWhat" : [{ "id" : 2, "name" : "Navel", "state" : "FL" }, 
                         { "id" : 2, "name" : "Clementine", "state" : "Spain" }]}"""

scala> val aJson = parse(apples)
aJson: net.liftweb.json.JsonAST.JValue = JObject(List(JField(aisle,JInt(1)), JField(bin,JInt(1)), JField(hasWhat,JArray(List(JObject(List(JField(id,JInt(4)), JField(name,JString(Granny)), JField(color,JString(green)))), JObject(List(JField(id,JInt(4)), JField(name,JString(Fuji)), JField(color,JString(red)))))))))

scala> val oJson = parse(oranges)
oJson: net.liftweb.json.JsonAST.JValue = JObject(List(JField(aisle,JInt(3)), JField(bin,JInt(2)), JField(hasWhat,JArray(List(JObject(List(JField(id,JInt(2)), JField(name,JString(Navel)), JField(state,JString(FL)))))))))

scala> val doesntWork = aJson.extract[Fruits]
doesntWork: org.spin.node.gogrid.objects.Fruits = Fruits(1,1,None)

scala> val works = aJson.extract[AFruit]
works: org.spin.node.gogrid.objects.AFruit = AFruit(1,1,Some(List(Apple(4,Granny,green), Apple(4,Fuji,red))))

我希望dontWork能像作品一样，在哪里：

case class AFruit(aisle: Int, bin: Int, hasWhat: Option[List[Apple]])

谢谢！ -newbie

Answer 1

尚不支持提取参数化案例类。一种解决方法（不确定这是否适用于您的情况）是使Fruits成为具体类型并将类型信息添加到JSON中。

import net.liftweb.json._
import net.liftweb.json.Extraction._
import net.liftweb.json.JsonAST._
import net.liftweb.json.Printer._

abstract class MyObjects
case class Apple(id: Int, name: String, color: String) extends MyObjects
case class Orange(id: Long, name: String, state: String) extends MyObjects

case class Fruits(aisle: Int, bin: Int, hasWhat: Option[List[MyObjects]])

object Test extends Application {
  // This configuration adds an extra field for MyObjects to JSON
  // which tells the real type of a MyObject.
  implicit val formats = Serialization.formats(FullTypeHints(List(classOf[MyObjects])))

  val fs = Fruits(0, 0, Some(List(
    Apple(1, "Granny Smith", "green"), 
    Apple(2, "Grenade", "red"))))
  val json = decompose(fs)
  println(pretty(render(json)))

  assert (json.extract[Fruits] == fs)
}

打印：

{
  "aisle":0,
  "bin":0,
  "hasWhat":[{
    "jsonClass":"Apple",
    "id":1,
    "name":"Granny Smith",
    "color":"green"
  },{
    "jsonClass":"Apple",
    "id":2,
    "name":"Grenade",
    "color":"red"
  }]
}

将lift-json提取到具有上限的case类中

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