ennru / play-json

The Play JSON library

Play JSON

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.

We've provided some documentation here on how to use Play JSON in your app (without Play). For more information on how to use Play JSON in Play, please refer to the Play documentation.

Getting Started

To get started, you can add play-json as a dependency in your project:

• sbt

  libraryDependencies += "com.typesafe.play" %% "play-json" % "2.8.1"

• Gradle

  compile group: 'com.typesafe.play', name: 'play-json_2.13', version: '2.8.1'
  

• Maven

  <dependency>
    <groupId>com.typesafe.play</groupId>
    <artifactId>play-json_2.13</artifactId>
    <version>2.8.1</version>
  </dependency>

Play JSON supports Scala 2.12 and 2.13. Choosing the right JAR is automatically managed in sbt. If you're using Gradle or Maven then you need to use the correct version in the artifactId.

JSON AST

The base type in Play JSON is play.api.libs.json.JsValue, and has several subtypes representing different JSON types:

• JsObject: a JSON object, represented as a Map. Can be constructed from an ordered Seq or any kind of Map using JsObject.apply
• JsArray: a JSON array, consisting of a Seq[JsValue]
• JsNumber: a JSON number, represented as a BigDecimal.
• JsString: a JSON string.
• JsBoolean: a JSON boolean, either JsTrue or JsFalse.
• JsNull: the JSON null value.

The play.api.libs.json package includes several features for constructing JSON from scratch, as well as for converting to and from case classes.

Basic reading and writing

The play.api.libs.json.Json object has several methods for reading and writing:

Json.parse parses a JSON string or InputStream into a JSON tree:

val json: JsValue = Json.parse("""
{
  "name" : "Watership Down",
  "location" : {
    "lat" : 51.235685,
    "long" : -1.309197
  },
  "residents" : [ {
    "name" : "Fiver",
    "age" : 4,
    "role" : null
  }, {
    "name" : "Bigwig",
    "age" : 6,
    "role" : "Owsla"
  } ]
}
""")

and Json.stringify is used to convert a JsValue to a String of JSON:

val jsonString = Json.stringify(json)
// {"name":"Watership Down","location":{"lat":51.235685,"long":-1.309197},"residents":[{"name":"Fiver","age":4,"role":null},{"name":"Bigwig","age":6,"role":"Owsla"}]}

Traversing a JsValue

Play JSON provides a traversal DSL that lets you query fields in the JSON:

Simple path \

Applying the \ operator will return the property corresponding to the field argument, supposing this is a JsObject.

val lat = (json \ "location" \ "lat").get
// returns JsNumber(51.235685)

The (json \ "location" \ "lat") returns a JsLookupResult which may or may not contain a value. Note that the get operation is not always safe; it throws an exception if the path doesn't exist.

You can also use \ to look up indices within a JsArray:

val bigwig = (json \ "residents" \ 1).get
// returns {"name":"Bigwig","age":6,"role":"Owsla"}

Recursive path \

Applying the \\ operator will do a lookup for the field in the current object and all descendants.

val names = json \\ "name"
// returns Seq(JsString("Watership Down"), JsString("Fiver"), JsString("Bigwig"))

Index lookup

You can retrieve a value in a JsObject or JsArray using an apply operator with the index number or key.

val name = json("name")
// returns JsString("Watership Down")

val bigwig = json("residents")(1)
// returns {"name":"Bigwig","age":6,"role":"Owsla"}

Like get, this will throw an exception if the index doesn't exist. Use the Simple Path \ operator and validate or asOpt (described below) if you expect that they key may not be present.

Reading and writing objects

To convert a Scala object to and from JSON, we use Json.toJson[T: Writes] and Json.fromJson[T: Reads] respectively. Play JSON provides the Reads and Writes typeclasses to define how to read or write specific types. You can get these either by using Play's automatic JSON macros, or by manually defining them.

You can also read JSON from a JsValue using validate, as and asOpt methods. Generally it's preferable to use validate since it returns a JsResult which may contain an error if the JSON is malformed.

For example:

val unsafeName = (json \ "name").as[String]
// "Watership Down"

val unsafeBogusName = (json \ "bogus").as[String]
// throws exception

val nameOption = (json \ "name").asOpt[String]
// Some("Watership Down")

val bogusOption = (json \ "bogus").asOpt[String]
// None

val nameResult = (json \ "name").validate[String]
// JsSuccess("Watership Down")

val bogusResult = (json \ "bogus").validate[String]
// JsError

val unsafeName2 = json("name").as[String]
// "Watership Down"

val unsafeBogusName2 = json("bogus").as[String]
// throws exception

Automatic conversion

Usually you don't need to traverse JSON AST directly. Play JSON comes equipped with some convenient macros to convert to and from case classes.

For example, suppose I have the following class:

case class Resident(name: String, age: Int, role: Option[String])

I can define a Reads (JSON parser), Writes (JSON writer) using convenient macros:

implicit val residentReads = Json.reads[Resident]
implicit val residentWrites = Json.writes[Resident]

I can also define a Format that does both:

implicit val residentFormat = Json.format[Resident]

With the Reads and/or Writes in scope, I can then easily convert my class using toJson and fromJson

Constructing Reads and Writes

Play JSON provides a convenient functional DSL for constructing Reads and Writes. For example, assume I have the following classes:

case class Location(lat: Double, long: Double)
case class Resident(name: String, age: Int, role: Option[String])
case class Place(name: String, location: Location, residents: Seq[Resident])

Then I could construct Reads for them as follows:

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

implicit val locationReads: Reads[Location] = (
  (JsPath \ "lat").read[Double](min(-90.0) keepAnd max(90.0)) and
  (JsPath \ "long").read[Double](min(-180.0) keepAnd max(180.0))
)(Location.apply _)

implicit val residentReads: Reads[Resident] = (
  (JsPath \ "name").read[String](minLength[String](2)) and
  (JsPath \ "age").read[Int](min(0) keepAnd max(150)) and
  (JsPath \ "role").readNullable[String]
)(Resident.apply _)

implicit val placeReads: Reads[Place] = (
  (JsPath \ "name").read[String](minLength[String](2)) and
  (JsPath \ "location").read[Location] and
  (JsPath \ "residents").read[Seq[Resident]]
)(Place.apply _)


val json = { ... }

json.validate[Place] match {
  case s: JsSuccess[Place] => {
    val place: Place = s.get
    // do something with place
  }
  case e: JsError => {
    // error handling flow
  }
}

Similarly, I could construct Writes like this:

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

implicit val locationWrites: Writes[Location] = (
  (JsPath \ "lat").write[Double] and
  (JsPath \ "long").write[Double]
)(unlift(Location.unapply))

implicit val residentWrites: Writes[Resident] = (
  (JsPath \ "name").write[String] and
  (JsPath \ "age").write[Int] and
  (JsPath \ "role").writeNullable[String]
)(unlift(Resident.unapply))

implicit val placeWrites: Writes[Place] = (
  (JsPath \ "name").write[String] and
  (JsPath \ "location").write[Location] and
  (JsPath \ "residents").write[Seq[Resident]]
)(unlift(Place.unapply))


val place = Place(
  "Watership Down",
  Location(51.235685, -1.309197),
  Seq(
    Resident("Fiver", 4, None),
    Resident("Bigwig", 6, Some("Owsla"))
  )
)

val json = Json.toJson(place)

It is also possible to implement custom logic by implementing the Reads, Writes and/or Format traits manually, but we recommend using the automatic conversion macros or the functional DSL if possible.

Manual JSON construction

JSON can also be manually constructed using a DSL:

val json: JsValue = Json.obj(
  "name" -> "Watership Down",
  "location" -> Json.obj("lat" -> 51.235685, "long" -> -1.309197),
  "residents" -> Json.arr(
    Json.obj(
      "name" -> "Fiver",
      "age" -> 4,
      "role" -> JsNull
    ),
    Json.obj(
      "name" -> "Bigwig",
      "age" -> 6,
      "role" -> "Owsla"
    )
  )
)

License

Play JSON is licensed under the Apache license, version 2. See the LICENSE file for more information.