Defaults for missing properties in play 2 JSON formats

Play 2.6+

As per @CanardMoussant's answer, starting with Play 2.6 the play-json macro has been improved and proposes multiple new features including using the default values as placeholders when deserializing :

implicit def jsonFormat = Json.using[Json.WithDefaultValues].format[Foo]

For play below 2.6 the best option remains using one of the options below :

play-json-extra

I found out about a much better solution to most of the shortcomings I had with play-json including the one in the question:

play-json-extra which uses [play-json-extensions] internally to solve the particular issue in this question.

It includes a macro which will automatically include the missing defaults in the serializer/deserializer, making refactors much less error prone !

import play.json.extra.Jsonx
implicit def jsonFormat = Jsonx.formatCaseClass[Foo]

there is more to the library you may want to check: play-json-extra

Json transformers

My current solution is to create a JSON Transformer and combine it with the Reads generated by the macro. The transformer is generated by the following method:

object JsonExtensions{
  def withDefault[A](key:String, default:A)(implicit writes:Writes[A]) = __.json.update((__ \ key).json.copyFrom((__ \ key).json.pick orElse Reads.pure(Json.toJson(default))))
}

The format definition then becomes :

implicit val fooformats: Format[Foo] = new Format[Foo]{
  import JsonExtensions._
  val base = Json.format[Foo]
  def reads(json: JsValue): JsResult[Foo] = base.compose(withDefault("status","bidon")).reads(json)
  def writes(o: Foo): JsValue = base.writes(o)
}

and

Json.parse("""{"id":"1", "value":"foo"}""").validate[Foo]

will indeed generate an instance of Foo with the default value applied.

This has 2 major flaws in my opinion:

• The defaulter key name is in a string and won't get picked up by a refactoring
• The value of the default is duplicated and if changed at one place will need to be changed manually at the other

The cleanest approach that I've found is to use "or pure", e.g.,

...      
((JsPath \ "notes").read[String] or Reads.pure("")) and
((JsPath \ "title").read[String] or Reads.pure("")) and
...

This can be used in the normal implicit way when the default is a constant. When it's dynamic, then you need to write a method to create the Reads, and then introduce it in-scope, a la

implicit val packageReader = makeJsonReads(jobId, url)