scala - Scala类型参数似乎被“卡住”

我正在使用lift-json将JSON字符串反序列化为Scala案例类。我正在解析的JSON具有一个通用结构：数据，成功和错误字段，其中数据包含有趣的位。我创建了一个APIResponse类来说明此结构，并创建了一个简单的parse方法，该方法采用类型参数作为'data'中包含的对象类型的参数：

import net.liftweb.json.Serialization.read
import net.liftweb.json.DefaultFormats

object JSONParseTest extends App {
  implicit val formats = DefaultFormats

  def parse[T: Manifest](json: String) = {
    read[APIResponse[T]](json)
  }

  val resultA = parse[TypeA](""" { "data": { "foo": "string" }, "success": true } """)
  println(resultA)
  val resultB = parse[TypeB](""" { "data": { "bar": "string" }, "success": true } """)
  println(resultB)
}

case class TypeA(foo: String)
case class TypeB(bar: String)

case class APIResponse[D](data: D, success: Boolean, error: Option[String]) {
  override def toString: String =
    if(success) {
      "SUCCESSFUL: " + data.toString
    } else {
      "ERROR: " + error.get
    }
}

一切正常……对于第一个解析的对象。但是，由于某种原因，解析的第二个对象似乎“卡住了”，执行起来就像是将第一个类型参数而不是第二个类型参数传递给了它。从下面的输出中可以看到，lift-json正在JSON字符串中寻找一个“ foo”字段，但找不到它。 'foo'存在于TypeA上，但不存在于TypeB上。我在parse方法中检查了清单，并在那里找到了正确的类型。如果我注释掉第一个分析/打印，则第二个开始解析/打印。关于为什么这不符合预期的任何指示？这有点令我困惑。

SUCCESSFUL: TypeA(string)

Exception in thread "main" net.liftweb.json.MappingException: No usable value for data
No usable value for foo
Did not find value which can be converted into java.lang.String
    at net.liftweb.json.Meta$.fail(Meta.scala:191)
    at net.liftweb.json.Extraction$.mkValue$1(Extraction.scala:357)
    at net.liftweb.json.Extraction$.build$1(Extraction.scala:317)
    at net.liftweb.json.Extraction$$anonfun$12.apply(Extraction.scala:253)
    at net.liftweb.json.Extraction$$anonfun$12.apply(Extraction.scala:253)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:233)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:233)
    at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59)
    at scala.collection.immutable.List.foreach(List.scala:76)
    at scala.collection.TraversableLike$class.map(TraversableLike.scala:233)
    at scala.collection.immutable.List.map(List.scala:76)
    at net.liftweb.json.Extraction$.instantiate$1(Extraction.scala:253)
    at net.liftweb.json.Extraction$.newInstance$1(Extraction.scala:286)
    at net.liftweb.json.Extraction$.build$1(Extraction.scala:315)
    at net.liftweb.json.Extraction$.net$liftweb$json$Extraction$$extract0(Extraction.scala:366)
    at net.liftweb.json.Extraction$.net$liftweb$json$Extraction$$extract0(Extraction.scala:199)
    at net.liftweb.json.Extraction$.extract(Extraction.scala:43)
    at net.liftweb.json.JsonAST$JValue.extract(JsonAST.scala:300)
    at net.liftweb.json.Serialization$.read(Serialization.scala:58)
    at jmullin.api.Test$.parse(API.scala:11)
    at jmullin.api.Test$delayedInit$body.apply(API.scala:16)
    at scala.Function0$class.apply$mcV$sp(Function0.scala:34)
    at scala.runtime.AbstractFunction0.apply$mcV$sp(AbstractFunction0.scala:12)
    at scala.App$$anonfun$main$1.apply(App.scala:60)
    at scala.App$$anonfun$main$1.apply(App.scala:60)
    at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59)
    at scala.collection.immutable.List.foreach(List.scala:76)
    at scala.collection.generic.TraversableForwarder$class.foreach(TraversableForwarder.scala:30)
    at scala.App$class.main(App.scala:60)
    at jmullin.api.Test$.main(API.scala:7)
    at jmullin.api.Test.main(API.scala)
Caused by: net.liftweb.json.MappingException: No usable value for foo
Did not find value which can be converted into java.lang.String
    at net.liftweb.json.Meta$.fail(Meta.scala:191)
    at net.liftweb.json.Extraction$.mkValue$1(Extraction.scala:357)
    at net.liftweb.json.Extraction$.build$1(Extraction.scala:317)
    at net.liftweb.json.Extraction$$anonfun$12.apply(Extraction.scala:253)
    at net.liftweb.json.Extraction$$anonfun$12.apply(Extraction.scala:253)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:233)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:233)
    at scala.collection.LinearSeqOptimized$class.foreach(LinearSeqOptimized.scala:59)
    at scala.collection.immutable.List.foreach(List.scala:76)
    at scala.collection.TraversableLike$class.map(TraversableLike.scala:233)
    at scala.collection.immutable.List.map(List.scala:76)
    at net.liftweb.json.Extraction$.instantiate$1(Extraction.scala:253)
    at net.liftweb.json.Extraction$.newInstance$1(Extraction.scala:286)
    at net.liftweb.json.Extraction$.build$1(Extraction.scala:315)
    at net.liftweb.json.Extraction$.mkValue$1(Extraction.scala:351)
    ... 29 more
Caused by: net.liftweb.json.MappingException: Did not find value which can be converted into java.lang.String
    at net.liftweb.json.Meta$.fail(Meta.scala:191)
    at net.liftweb.json.Extraction$.convert(Extraction.scala:403)
    at net.liftweb.json.Extraction$.build$1(Extraction.scala:314)
    at net.liftweb.json.Extraction$.mkValue$1(Extraction.scala:351)
    ... 42 more

最佳答案

事实证明，lift-json在后台进行类到构造函数映射的类记忆，而忽略了类型args。当对具有不同类型参数的同一类执行查找时，将返回旧的构造函数，从而引起混乱。谜底解决了（尽管问题没有解决）。