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社区首页 >问答首页 >JSON4S类型提示不起作用

问JSON4S类型提示不起作用
EN

Stack Overflow用户
提问于 2017-10-19 15:55:44
回答 2查看 1K关注 0票数 6

下面的测试片段

代码语言：javascript
复制
implicit val formats = DefaultFormats + FullTypeHints(Contacts.classList)

val serialized = Serialization.write(List(Mail(field = "random@mail.com", note = "Random note.")))
println(serialized)

Serialization.read[List[Contact[_]]](serialized).isInstanceOf[List[Mail]] should be (true)

失败与

代码语言：javascript
复制
Can't find constructor for Contact[Object]
org.json4s.package$MappingException: Can't find constructor for Contact[Object]
    at org.json4s.reflect.package$.fail(package.scala:95)
    at org.json4s.reflect.ScalaSigReader$$anonfun$5.apply(ScalaSigReader.scala:21)
    at org.json4s.reflect.ScalaSigReader$$anonfun$5.apply(ScalaSigReader.scala:21)
    at scala.Option.getOrElse(Option.scala:121)
    at org.json4s.reflect.ScalaSigReader$.readConstructor(ScalaSigReader.scala:21)
    at org.json4s.reflect.Reflector$ClassDescriptorBuilder.ctorParamType(Reflector.scala:93)
    at org.json4s.reflect.Reflector$ClassDescriptorBuilder$$anonfun$createConstructorDescriptors$3$$anonfun$15.apply(Reflector.scala:156)
    at org.json4s.reflect.Reflector$ClassDescriptorBuilder$$anonfun$createConstructorDescriptors$3$$anonfun$15.apply(Reflector.scala:142)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
    at scala.collection.mutable.ResizableArray$class.foreach(ResizableArray.scala:59)
    at scala.collection.mutable.ArrayBuffer.foreach(ArrayBuffer.scala:48)
    at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
    at scala.collection.AbstractTraversable.map(Traversable.scala:104)
    at org.json4s.reflect.Reflector$ClassDescriptorBuilder$$anonfun$createConstructorDescriptors$3.apply(Reflector.scala:142)
    at org.json4s.reflect.Reflector$ClassDescriptorBuilder$$anonfun$createConstructorDescriptors$3.apply(Reflector.scala:136)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
    at scala.collection.TraversableLike$$anonfun$map$1.apply(TraversableLike.scala:234)
    at scala.collection.mutable.ArraySeq.foreach(ArraySeq.scala:74)
    at scala.collection.TraversableLike$class.map(TraversableLike.scala:234)
    at scala.collection.AbstractTraversable.map(Traversable.scala:104)
    at org.json4s.reflect.Reflector$ClassDescriptorBuilder.createConstructorDescriptors(Reflector.scala:136)
    at org.json4s.reflect.Reflector$ClassDescriptorBuilder.constructorsAndCompanion(Reflector.scala:121)
    at org.json4s.reflect.Reflector$ClassDescriptorBuilder.result(Reflector.scala:183)
    at org.json4s.reflect.Reflector$.createDescriptor(Reflector.scala:53)
    at org.json4s.reflect.Reflector$$anonfun$describe$1.apply(Reflector.scala:48)
    at org.json4s.reflect.Reflector$$anonfun$describe$1.apply(Reflector.scala:48)
    at org.json4s.reflect.package$Memo.apply(package.scala:36)
    at org.json4s.reflect.Reflector$.describe(Reflector.scala:48)
    at org.json4s.Extraction$$anonfun$extract$6.apply(Extraction.scala:393)
    at org.json4s.Extraction$$anonfun$extract$6.apply(Extraction.scala:392)
    at org.json4s.Extraction$.customOrElse(Extraction.scala:606)
    at org.json4s.Extraction$.extract(Extraction.scala:392)
    at org.json4s.Extraction$CollectionBuilder$$anonfun$7.apply(Extraction.scala:410)
    at org.json4s.Extraction$CollectionBuilder$$anonfun$7.apply(Extraction.scala:410)
    at scala.collection.immutable.List.map(List.scala:284)
    at org.json4s.Extraction$CollectionBuilder.mkCollection(Extraction.scala:410)
    at org.json4s.Extraction$CollectionBuilder.result(Extraction.scala:430)
    at org.json4s.Extraction$$anonfun$extract$5.apply(Extraction.scala:382)
    at org.json4s.Extraction$$anonfun$extract$5.apply(Extraction.scala:382)
    at org.json4s.Extraction$.customOrElse(Extraction.scala:606)
    at org.json4s.Extraction$.extract(Extraction.scala:382)
    at org.json4s.Extraction$.extract(Extraction.scala:39)
    at org.json4s.ExtractableJsonAstNode.extract(ExtractableJsonAstNode.scala:21)
    at org.json4s.jackson.Serialization$.read(Serialization.scala:50)
    at org.json4s.Serialization$class.read(Serialization.scala:30)
    at org.json4s.jackson.Serialization$.read(Serialization.scala:17)

Contact在哪里

代码语言：javascript
复制
abstract class Contact[Field : Validable](
  val field: Field,
  val created: Long,
  val updated: Long,
  val note: String) { }

而Mail是

代码语言：javascript
复制
case class Mail(
  override val field: String,
  override val created: Long = System.currentTimeMillis(),
  override val updated: Long = System.currentTimeMillis(),
  override val note: String)
extends Contact[String](field, created, updated, note)(Mail)

case object Mail extends Validable[String] {
  override def valid(field: String): Boolean = {
    Validator.email(field)
  }
}

而测试输出是

代码语言：javascript
复制
[{"jsonClass":"whatever.core.entities.utility.contact.Mail","field":"random@mail.com","created":1508428385266,"updated":1508428385266,"note":"Random note."}]

深入研究JSON4S的代码基表明，根本不使用类型提示。

如何强制JSON4S使用类型提示？

干杯

serialization
json4s
hive-serde
json
scala
EN

回答 2

Stack Overflow用户

回答已采纳

发布于 2017-10-27 09:25:53

使用Contact实现trait，它将工作。

票数 0
EN

Stack Overflow用户

发布于 2017-10-25 22:13:57

这是一个如何使用FullTypeHints的例子。

您不能隐式地请求Validable[Field]，也不能添加多个显式参数列表(隐式参数也是如此，请参见下面的说明)，因为它们不受JSON4S支持。

注意:使用Validable[Field]语法隐式请求Contact[Field: Validable]等于使用implicit validable: Validable[Field]添加额外的参数列表。

相反，您可以向Contact构造函数中添加一个额外的参数Contact，或者(如下面的示例所示)一个应该被扩展Contact抽象类的具体类(如Mail)覆盖的validable字段。

代码语言：javascript
复制
trait Validable[T]{
  def valid(field: T): Boolean
}

abstract class Contact[Field](
  val field: Field, val created: Long, 
  val updated: Long, val note: String) {
  val validable: Validable[Field]
}

object Contacts{ val classList = List(classOf[Mail]) }

case class Mail(
  override val field: String,
  override val created: Long = System.currentTimeMillis(),
  override val updated: Long = System.currentTimeMillis(),
  override val note: String
) extends Contact[String](field, created, updated, note){
  override val validable: Validable[String] = Mail
}

case object Mail extends Validable[String] {
  override def valid(field: String): Boolean = true
}

implicit val formats = DefaultFormats + FullTypeHints(Contacts.classList)
代码语言：javascript
复制
val mail: List[Mail] = List(Mail(field = "random@mail.com", note = "Random note."))
val serialized = Serialization.write(mail)
val mailS = Serialization.read[List[Contact[_]]](serialized)

print(mail == mailS)

serialized JSON表示形式如下：

代码语言：javascript
复制
[
  {
    "jsonClass":"whatever.core.entities.utility.contact$Mail",
    "field":"random@mail.com",
    "created":1509098018776,
    "updated":1509098018776,
    "note":"Random note."
  }
]
票数 0
EN
页面原文内容由Stack Overflow提供。腾讯云小微IT领域专用引擎提供翻译支持
原文链接：

https://stackoverflow.com/questions/46834046

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