play-json 中使用默认值

Play JSON is a powerful Scala JSON library, originally developed by the Play team for use with Play Framework. It uses Jackson for JSON parsing and has no Play dependencies.

play-json 是我在开发中经常使用的 Scala Json 库，在将 json 字符串转换成 Scala 对象时可能会遇到 json 字符串中缺失了 Scala 对象需要的成员，在这种情况下可以这么处理：

case class Person1(name: String, age: Int, other: Option[String])
case class Person2(name: String, age: Int, other: String)

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

implicit val person1Format = (
  (JsPath \ "name").format[String] and
    (JsPath \ "age").format[Int] and
    (JsPath \ "other").formatNullable[String].inmap[Option[String]](x => Some(x.getOrElse("default")), y => y)
  ) (Person1.apply, unlift(Person1.unapply))

val str1 =
  """
    |{"name":"x", "age":18}
  """.stripMargin

val str2 =
  """
    |{"name":"x", "age":18, "other":"test"}
  """.stripMargin

println(Json.parse(str1).as[Person1])
println(Json.parse(str2).as[Person1])

implicit val person2Format = (
  (JsPath \ "name").format[String] and
    (JsPath \ "age").format[Int] and
    (JsPath \ "other").formatNullable[String].inmap[String](_.getOrElse("default"), Some(_))
  ) (Person2.apply, unlift(Person2.unapply))

println(Json.parse(str1).as[Person2])
println(Json.parse(str2).as[Person2])

上面的代码会输出：

Person1(x,18,Some(default))
Person1(x,18,Some(test))
Person2(x,18,default)
Person2(x,18,test)

通过这种方式，从 json 字符串转换到 Scala 对象时，可以给缺失的字段添加默认值。