json - 如何在Play Framework 2.x中实现嵌入式对象的隐式Json Writes

有Foo和Bar两类。 Foo包含一个Bar字段。问题是，如何为类Writes实现隐式json Foo？

这是代码:

package models

import play.api.libs.json._

case class Foo(id: String, bar: Bar)

object Foo {
  implicit val implicitFooWrites = new Writes[Foo] {
    def writes(foo: Foo): JsValue = {
      Json.obj(
        "id" -> foo.id,
        "bar" -> foo.bar
      )
    }
  }
}

case class Bar(x: String, y: Int)

object Bar {
  implicit val implicitBarWrites = new Writes[Bar] {
    def writes(bar: Bar): JsValue = {
      Json.obj(
        "x" -> bar.x,
        "y" -> bar.y
      )
    }
  }
}

当我尝试编译时，出现以下错误:



我不了解此编译器错误，因为我为models.Bar类实现了隐式Writes。这里有什么问题？

最佳答案

这是可见性的问题，当您声明隐式Writes [Foo]时，您不会在其中看到隐式Writes [Bar]:

scala> :paste
// Entering paste mode (ctrl-D to finish)

import play.api.libs.json._

case class Bar(x: String, y: Int)

object Bar {
  implicit val implicitBarWrites = new Writes[Bar] {
    def writes(bar: Bar): JsValue = {
      Json.obj(
        "x" -> bar.x,
        "y" -> bar.y
      )
    }
  }
}

case class Foo(id: String, bar: Bar)

object Foo {

  import Bar._

  implicit val implicitFooWrites = new Writes[Foo] {
    def writes(foo: Foo): JsValue = {
      Json.obj(
        "id" -> foo.id,
        "bar" -> foo.bar
      )
    }
  }
}

// Exiting paste mode, now interpreting.

import play.api.libs.json._
defined class Bar
defined module Bar
defined class Foo
defined module Foo

scala> Json.prettyPrint(Json.toJson(Foo("23", Bar("x", 1))))
res0: String =
{
  "id" : "23",
  "bar" : {
    "x" : "x",
    "y" : 1
  }
}

另外，如果您使用的是Play 2.1+，请确保检查2.10宏的全新用法:http://www.playframework.com/documentation/2.1.0/ScalaJsonInception

如果您对案例类的使用感到满意，并且将val / vars名称用作json输出中的键(例如在案例BTW中)，则可以使用两个单行代码:

implicit val barFormat = Json.writes[Bar]
implicit val fooFormat = Json.writes[Foo]

这将为您提供完全相同的效果:

scala> import play.api.libs.json._
import play.api.libs.json._

scala> case class Bar(x: String, y: Int)
defined class Bar

scala> case class Foo(id: String, bar: Bar)
defined class Foo

scala> implicit val barWrites = Json.writes[Bar]
barWrites: play.api.libs.json.OWrites[Bar] = play.api.libs.json.OWrites$$anon$2@257cae95

scala> implicit val fooWrites = Json.writes[Foo]
fooWrites: play.api.libs.json.OWrites[Foo] = play.api.libs.json.OWrites$$anon$2@48f97e2a

scala> Json.prettyPrint(Json.toJson(Foo("23", Bar("x", 1))))
res0: String =
{
  "id" : "23",
  "bar" : {
    "x" : "x",
    "y" : 1
  }
}