Some time ago I wrote a post on relational database access in Scala since I was looking for a library and there were many of them available, making it hard to make a choice. It turns out that the situation is similar if not worse when it comes to JSON libraries in Scala.
There are just plenty of them. You have no idea. (me neither until I wrote this post)
The following is an attempt to provide a quick overview at how a subset of the libraries I found does a few of the most common things one would likely need to do in regard to JSON:
parsing it from a raw string
browsing the AST
building an AST
mapping to a case class
There are of course plenty more valid use-cases but this post is not going to cover those.
Let’s get started!
Scala JSON libraries
play-json
The Play Framework comes with a JSON library that covers most of the common use-cases one would encounter when building web applications:
This is quite useful, it removes the boilerplate formats hanging around
json4s
json4s is a bit like slf4j in the sense that it tries to unite all kind of rogue libraries serving the same purpose by providing a common interface. But not every library uses it, which means that chances are high that your project will contain json4s in addition to another (few) Scala JSON libraries. Hopefully it will one day succeed with its slogan “One AST to rule them all”.
json4s has its roots in lift-json so this will look familiar:
The sphere-json library focuses on providing an easy way to get de/serializers for entire families of case classes. This is really useful when working with any kind of protocol-related system. I am using it in a CQRS system where commands and events are travelling back and forth between the server and a Javascript UI. Instead of having to define a codec for each one of my case classes, I simply have them extend a common abstract class and let the library take care of the rest. Watch for yourselves:
rapture’sjson library is the ultimate Scala JSON library. It doesn’t really do anything with JSON itself, instead, it abstracts over the following JSON libraries (which it calls backends):
Robert J. Macomber built the rojoma-json library which I only discovered now and am too tired to cover (sorry)
Great, now which one to pick?
All of them
Honestly I don’t have any good advice here. These days I am sticking to play-json and sphere-json, but that’s for no particular reason other than these libraries being there and doing what I need to do (parse and write JSON, traverse and some rare times transform the AST, bind to case classes, make it possible to support custom Java types). If play-json had support for hierarchies out of the box I would probably not have even looked for anything else.
Because for all the joy there seems to be in implementing JSON libraries in Scala, one thing has to be said: JSON de/serialization is boring. It’s this annoying thing that you have to do in order to get your application to talk to another computerized system, period.
I have never met a developer who told me how much pleasure they derived from turning classes into JSON strings and back. That’s just not a thing.
I have, however, met more than one developer that has run into trouble getting library X to cover one of the simple use-cases outlined above. Believe me, there is nothing more frustrating than having to spend time on the annoying task of setting up JSON de/serialization in order to do the boring thing of tossing strings back and forth the network. That’s time you will never get back.