json - 单个字段类的 Scala json 解析

Question

我有以下类(class)和 Reads Writes 用于 json 解析/写入

import play.api.libs.json._
import play.api.libs.functional.syntax._

import scala.collection.immutable.{Seq => ISeq, Set => ISet}

  case class FileExcInc( isExclude: Boolean, isFile: Boolean, pattern: String) {  }

  implicit val fEIReads: Reads[FileExcInc] = (
    (__ \ "isExclude").read[Boolean] and
    (__ \ "isFile").read[Boolean] and
    (__ \ "pattern").read[String]
  )(FileExcInc.apply _)

  implicit val fEIWrites: Writes[FileExcInc] = (
    (__ \ "isExclude").write[Boolean] and
    (__ \ "isFile").write[Boolean] and
    (__ \ "pattern").write[String]
  )(unlift(FileExcInc.unapply))

  case class DirConf(sourceDir: String, csdataFile: String, fileExcIncSeq: ISeq[FileExcInc]) { }

  implicit val dCReads: Reads[DirConf] = (
    (__ \ "sourceDir").read[String] and
    (__ \ "csdataFile").read[String] and
    (__ \ "fileExcIncSeq").read[ISeq[FileExcInc]]
  )(DirConf.apply _)

  implicit val dCWrites: Writes[DirConf] = (
    (__ \ "sourceDir").write[String] and
    (__ \ "csdataFile").write[String] and
    (__ \ "fileExcIncSeq").write[ISeq[FileExcInc]]
  )(unlift(DirConf.unapply))

这可以编译，但下面不能:

  case class Conf(dirConfSeq: ISeq[DirConf]) {  }


  implicit val cReads: Reads[Conf] = (
    (__ \ "dirConfSeq").read[ISeq[DirConf]]
  )(Conf.apply _)

  implicit val cWrites: Writes[Conf] = (
    (__ \ "dirConfSeq").write[ISeq[DirConf]]
  )(unlift(Conf.unapply))

它产生以下错误:

[info] Compiling 17 Scala sources and 1 Java source to /home/jk/workspace/Gen/target/scala-2.12/classes...
[error] /home/jk/workspace/Gen/src/main/scala/com/example/CTool.scala:341: overloaded method value read with alternatives:
[error]   (t: scala.collection.immutable.Seq[com.example.CTool.DirConf])play.api.libs.json.Reads[scala.collection.immutable.Seq[com.example.CTool.DirConf]] <and>
[error]   (implicit r: play.api.libs.json.Reads[scala.collection.immutable.Seq[com.example.CTool.DirConf]])play.api.libs.json.Reads[scala.collection.immutable.Seq[com.example.CTool.DirConf]]
[error]  cannot be applied to (scala.collection.immutable.Seq[com.example.CTool.DirConf] => com.example.CTool.Conf)
[error]     (__ \ "dirConfSeq").read[ISeq[DirConf]]
[error]                             ^
[error] /home/jk/workspace/Gen/src/main/scala/com/example/CTool.scala:345: overloaded method value write with alternatives:
[error]   (t: scala.collection.immutable.Seq[com.example.CTool.DirConf])(implicit w: play.api.libs.json.Writes[scala.collection.immutable.Seq[com.example.CTool.DirConf]])play.api.libs.json.OWrites[play.api.libs.json.JsValue] <and>
[error]   (implicit w: play.api.libs.json.Writes[scala.collection.immutable.Seq[com.example.CTool.DirConf]])play.api.libs.json.OWrites[scala.collection.immutable.Seq[com.example.CTool.DirConf]]
[error]  cannot be applied to (com.example.CTool.Conf => scala.collection.immutable.Seq[com.example.CTool.DirConf])
[error]     (__ \ "dirConfSeq").write[ISeq[DirConf]]

最近的读取、写入有什么问题？如何为只有一个字段的类创建读取、写入？

Answer 1

要为只有一个字段的案例类创建Reads/Writes，请使用map 和contramap将字段与案例类相互转换。

implicit val cReads: Reads[Conf] =
  (__ \ "dirConfSeq").read[ISeq[DirConf]].map(Conf)

implicit val cWrites: Writes[Conf] =
  (__ \ "dirConfSeq").write[ISeq[DirConf]].contramap(_.dirConfSeq)

更简单的方法是使用 Json.format 宏来生成所有的 Reads/Writes 样板文件。它处理具有任意数量字段的案例类，前提是字段名称与 JSON 匹配。 Format[T] 既是 Reads[T] 又是 Writes[T]。

implicit val fEIFormat: Format[FileExcInc] = Json.format[FileExcInc]
implicit val dCFormat: Format[DirConf] = Json.format[DirConf]
implicit val cFormat: Format[Conf] = Json.format[Conf]

参见:https://www.playframework.com/documentation/2.6.x/ScalaJsonAutomated