scala - 如何编写处理集合的 json4s CustomSerializer

Question

我有一个类，我正在尝试使用 json4s CustomSerializer 功能反序列化。由于 inability of json4s to deserialize mutable collections 我需要这样做.

这是我要反序列化的类的基本结构(不用担心为什么类是这样的结构):

case class FeatureValue(timestamp:Double)

object FeatureValue{
  implicit def ordering[F <: FeatureValue] = new Ordering[F] {
    override def compare(a: F, b: F): Int = {
      a.timestamp.compareTo(b.timestamp)
    }
  }
}

class Point {
  val features = new HashMap[String, SortedSet[FeatureValue]]

  def add(name:String, value:FeatureValue):Unit = {
    val oldValue:SortedSet[FeatureValue] = features.getOrElseUpdate(name, SortedSet[FeatureValue]())
    oldValue += value
  }
}

Json4s 序列化这个很好。序列化实例可能如下所示:

{"features":
  {
   "CODE0":[{"timestamp":4.8828914447482E8}],
   "CODE1":[{"timestamp":4.8828914541333E8}],
   "CODE2":[{"timestamp":4.8828915127325E8},{"timestamp":4.8828910097466E8}]
  }
}

我尝试编写自定义反序列化器，但我不知道如何处理列表尾部。在普通匹配器中，您可以递归调用自己的函数，但在这种情况下，该函数是匿名的，并且通过 json4s API 调用。我找不到任何涉及此问题的示例，也无法弄清楚。

目前，我只能匹配单个哈希键及其值中的单个 FeatureValue 实例。这是 CustomSerializer 的现状:

import org.json4s.{FieldSerializer, DefaultFormats, Extraction, CustomSerializer}
import org.json4s.JsonAST._

class PointSerializer extends CustomSerializer[Point](format => (
  {
    case JObject(JField("features", JObject(Nil)) :: Nil) => new Point
    case JObject(List(("features", JObject(List(
      (feature:String, JArray(List(JObject(List(("timestamp",JDouble(ts)))))))))
    ))) => {
      val point = new Point
      point.add(feature, FeatureValue(ts))
      point
    }
  },
  {
    // don't need to customize this, it works fine
    case x: Point => Extraction.decompose(x)(DefaultFormats + FieldSerializer[Point]())
  }
  ))

如果我尝试更改为使用::分隔列表格式，到目前为止我已经收到编译器错误。即使我没有遇到编译器错误，我也不确定我会用它们做什么。

Answer 1

您可以获取模式匹配中的 json 功能列表，然后映射此列表以获取功能及其代码。

class PointSerializer extends CustomSerializer[Point](format => (
  {
    case JObject(List(("features", JObject(featuresJson)))) => 
      val features = featuresJson.flatMap { 
        case (code:String, JArray(timestamps)) =>
          timestamps.map { case JObject(List(("timestamp",JDouble(ts)))) =>
            code -> FeatureValue(ts)
          }
      }

      val point = new Point
      features.foreach((point.add _).tupled)
      point
  }, {
    case x: Point => Extraction.decompose(x)(DefaultFormats + FieldSerializer[Point]())
  }
))

反序列化你的 json 如下:

import org.json4s.native.Serialization.{read, write}
implicit val formats = Serialization.formats(NoTypeHints) + new PointSerializer

val json = """
{"features":
  {
   "CODE0":[{"timestamp":4.8828914447482E8}],
   "CODE1":[{"timestamp":4.8828914541333E8}],
   "CODE2":[{"timestamp":4.8828915127325E8},{"timestamp":4.8828910097466E8}]
  }
}
"""

val point0 = read[Point]("""{"features": {}}""")
val point1 = read[Point](json)

point0.features // Map()
point1.features 
// Map(
//   CODE0 -> TreeSet(FeatureValue(4.8828914447482E8)), 
//   CODE2 -> TreeSet(FeatureValue(4.8828910097466E8), FeatureValue(4.8828915127325E8)), 
//   CODE1 -> TreeSet(FeatureValue(4.8828914541333E8))
// )