json-serialize.md

title	type	description	num	previous-page	next-page
How to serialize an object to JSON?	section	How to write JSON with Scala Toolkit.	20	json-deserialize	json-files

{% include markdown.html path="_markdown/install-upickle.md" %}

Serializing a Map to JSON

uPickle can serialize your Scala objects to JSON, so that you can save them in files or send them over the network.

By default it can serialize primitive types such as Int or String, as well as standard collections such as Map and List.

{% tabs 'array' %} {% tab 'Scala 2 and 3' %}

valmap:Map[String, Int] =Map("Toolkitty"->3, "Scaniel"->5) valjsonString:String= upickle.default.write(map) println(jsonString) // prints: {"Toolkitty":3,"Scaniel":5}

{% endtab %} {% endtabs %}

Serializing a custom object to JSON

In Scala, you can use a case class to define your own data type. For example, to represent a pet owner with the name of its pets, you can

caseclassPetOwner(name: String, pets: List[String])

To be able to write a PetOwner to JSON we need to provide a ReadWriter instance for the case class PetOwner. Luckily, upickle is able to fully automate that:

{% tabs 'given' class=tabs-scala-version %} {% tab 'Scala 2' %}

importupickle.default._implicitvalownerRw:ReadWriter[PetOwner] = macroRW[PetOwner]

Some explanations:

• An implicit val is a value that can be automatically provided as an argument to a method or function call, without having to explicitly pass it.
• macroRW is a method provided by uPickle that can generate a instances of ReadWriter for case classes, using the information about its fields. {% endtab %} {% tab 'Scala 3' %}

importupickle.default.*caseclassPetOwner(name: String, pets: List[String]) derivesReadWriter

The derives keyword is used to automatically generate given instances. Using the compiler's knowledge of the fields in PetOwner, it generates a ReadWriter[PetOwner]. {% endtab %} {% endtabs %}

This means that you can now write (and read) PetOwner objects to JSON with upickle.default.write(petOwner).

Notice that you do not need to pass the instance of ReadWriter[PetOwner] explicitly to the write method. But it does, nevertheless, get it from the context, as "given" value. You may find more information about contextual abstractions in the Scala 3 Book.

Putting everything together you should get:

{% tabs 'full' class=tabs-scala-version %} {% tab 'Scala 2' %}

importupickle.default._caseclassPetOwner(name: String, pets: List[String]) implicitvalownerRw:ReadWriter[PetOwner] = macroRW valpetOwner=PetOwner("Peter", List("Toolkitty", "Scaniel")) valjson:String= write(petOwner) println(json) // prints: {"name":"Peter","pets":["Toolkitty","Scaniel"]}"

{% endtab %} {% tab 'Scala 3' %}

importupickle.default._caseclassPetOwner(name: String, pets: List[String]) derivesReadWritervalpetOwner=PetOwner("Peter", List("Toolkitty", "Scaniel")) valjson:String= write(petOwner) println(json) // prints: {"name":"Peter","pets":["Toolkitty","Scaniel"]}"

{% endtab %} {% endtabs %}