Play2 does not find my implicit Reads or Format fo

问题:

This is my Search Object:

package models.helper
import play.api.libs.json.Format
import play.api.libs.json.JsValue
import play.api.libs.json.JsObject
import play.api.libs.json.JsString

case class Search (name: String, `type`:String){

  implicit object SearchFormat extends Format[Search] {
    def reads(json: JsValue): Search = Search(
      (json \ "name").as[String],
      (json \ "type").as[String]
    )

    def writes(s: Search): JsValue = JsObject(Seq(
        "name" -> JsString(s.name),
        "type" -> JsString(s.`type`)
    ))  
  }
}

I'm trying ot use this class when calling a webservice using WS:

val search = response.json.as[Search]

But the scala compiler keeps complaining on this line:

No Json deserializer found for type models.helper.Search. Try to implement an implicit Reads or Format for this type.

Could anybody tell me what I'm doing wrong?

• got the example from https://sites.google.com/site/play20zh/scala-developers/working-with-json
• this thread discusses the same issue but gives no solution, what example on what site? https://groups.google.com/forum/?fromgroups#!topic/play-framework/WTZrmQi5XxY

回答1:

Indeed the example is wrong. You need your implicit Format[Search] value to be available in the implicit scope.

In your case the Format[Search] is defined as a nested value of the class Search, so you can reach it only from an instance of Search.

So, what you want to do is to define it in another place, where it could be referenced without having to create an instance of Search, e.g. in a Formats object:

object Formats {
  implicit SearchFormat extends Format[Search] {
    …
  }
}

Then you can use it as follows:

import Formats.SearchFormat
val search = response.json.as[Search]

You can also get rid of the import tax by defining the Format[Search] value in the companion object of the Search class. Indeed the Scala compiler automatically looks in companion objects of type parameters when it needs an implicit value of a given type:

case class Search(name: String, `type`: String)

object Search {
  implicit object SearchFormat extends Format[Search] {
    …
  }
}

Then you can use it without having to import it:

val search = response.json.as[Search]