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uJson: fast, flexible and intuitive JSON for Scala

Posted 2018-03-31
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uJson is a new JSON library for the Scala programming language. It serves as the back-end for the uPickle serializaiton library, but can be used standalone to manipulate JSON in a way that is fast, flexible and intuitive, far more than the existing JSON libraries in the Scala library ecosystem. This post will go over what makes uJson an improvement over other JSON libraries that are available, and why you might consider using uJson and uPickle in your next big project.

About the Author: Haoyi is a software engineer, and the author of many open-source Scala tools such as the Ammonite REPL and the Mill Build Tool. If you enjoyed the contents on this blog, you may also enjoy Haoyi's book Hands-on Scala Programming


There are a lot of existing JSON libraries in Scala: Circe, Argonaut, Play-Json, and many others. However, none of them achieve the heights of intuitiveness that someone coming from Python, Ruby or Javascript might expect working with JSON data types.

Consider a simple JSON data structure:

  "id": "c730433b-082c-4984-9d66-855c243266f0",
  "name": "Foo",
  "counts": [1, 2, 3],
  "values": {
    "bar": true,
    "baz": 100.001,
    "qux": ["a", "b"]

Trying to update the name field, e.g. to reverse the string, would seem like a trivial operation that should have a trivial solution. In most languages, such as Python, Ruby or Javascript, it is trivial:


import json
data = json.loads("...")
data["name"] = reversed(data["name"])
updated = json.dumps(data)  


require "json"
data = JSON.parse("...")
data["name"] = data["name"].reverse
updated = JSON.generate(data)


data = JSON.parse("...")
data["name"] = data["name"].split("").reverse().join("")
updated = JSON.stringify(data)

Each of the above examples is trivial; even a beginner learning Python, Ruby or Javascript would have no problem understanding what each one is doing! A trivial code snippet is a fitting way of implementing a trivial task.


With uJson, manipulating JSON should be just as simple and familiar regardless of what your programming background is:

val data = ujson.read(input)
data("name") = data("name").str.reverse
val updated = data.render()

Apart from the minor syntactic differences from Python/Ruby/Javascript (e.g. using ("name") instead of ["name"] for dictionary lookup), the only additional syntax here is the .str method call. This is simply a property of working in a statically-typed language: it's effectively a cast to say "this JSON value is a String" to allow you to work with it as a String. There are similar methods .num .arr .obj for casting to other common JSON types.

Constructing JSON values using uJson is also simple, using the Js.* constructors. Here's an example constructing a JSON dictionary (Js.Obj) from the Ammonite codebase:

import ujson._
val dict = Js.Obj(
  "tag_name" -> ammoniteVersion,
  "name" -> ammoniteVersion,
  "body" -> s"http://www.lihaoyi.com/Ammonite/#$ammoniteVersion"

Nested structures are constructed similarly:

val nested = Js.Arr(
  Js.Obj("myFieldA" -> 1, "myFieldB" -> "g"),
  Js.Obj("myFieldA" -> 2, "myFieldB" -> "k")

And trivial structures are trivial to construct:

val nums = Js.Arr(1, 2, 3)

Due to uJson's mutable data model, you probably already know how to modify any of the structures above, including modifying values nested in arrays and dictionaries:

nested(0)("myFieldA") = 123

With uJson, you have a JSON library that does what you want to do, rather than forcing you to learn how to library wants to do things. uJson makes it both simple and easy to work with JSON, so you can spend less time thinking about JSON and more time on things which are actually important to you: your business logic, application, and product.

Scala Alternatives

Compare the above uPickle examples to the complexity of manipulating JSON in the various Scala libraries:


import scalaz._, Scalaz._
import argonaut._, Argonaut._

val data = "...".parseOption.get
val data2 = data.withObject(o =>
    o.fields.map{f =>
      f -> (if (f == "name") o(f).get.withString(_.reverse) else o(f).get)
val updated = data2.toString 


import io.circe._, io.circe.parser._
val data = parse("...").right.get

val data2 = data.hcursor.downField("name").withFocus(_.mapString(_.reverse)).top.get

val updated = data2.toString


import play.api.libs.json._
val data = Json.parse(input)
val jsonTransformer = (__ \ "name").json.update(
    __.read[JsString].map(x => JsString(x.value.reverse))
val data2 = data.transform(jsonTransformer)
val updated = data2.toString

None of these examples using Scala libraries are as simple, or easy, as the Python, Ruby, or Javascript examples, or the uJson example shown above. Some of the complexity is incidental: e.g. Circe's syntax for doing it is much less verbose than Argonaut's or Play-Json's. However, some of the complexity is fundamental: these libraries have a JSON data type that aims to be immutable. Thus, users can no longer us their familiar mutation operations (data["name"] = reversed(data["name"])) to manipulate the JSON, and are forced to use less-common tools like Lenses or Cursors or Zippers to transform it.

Fundamentally, nobody wants to become an expert at the JSON library they're using. They want to become experts on AI, financial software, UI-development, distributed systems, and other things. They need to become experts in the system they maintain, the proprietary data integrations or algorithms that give their system value, or the needs & workflows of their users. While there is a place for sophisticated libraries that require learning & in-depth knowledge to use well, I think processing JSON is not one of those places.

What of Immutability?

A lot of the complexity in Argonaut, Circe and Play-Json arise from the issue of immutability.

Without the ability to mutate values at runtime, transforming JSON blobs becomes much more cumbersome. Immutability forces you to re-construct the JSON structure from the ground up, with the changes you want, rather than simply mutating the original JSON structure. While this is a potential performance issue (reconstructing trees with changed bits tends to be slower than point-mutations) the most obvious difference is the amount of code you need to write: reconstructing-tree-with-changes has a lot more going on than mutating a single variable!

There are tools that help you reduce the boilerplate of immutability's reconstruct-tree-with-changes approach to JSON manipulation: Lenses like Play-Json's __ \ "name" or __.read save you some boilerplate of writing the recursive-transformation manually, and fluent APIs like Circe's data.hcursor.downField("name").withFocus saves further boilerplate. Nevertheless, these are only a partial workaround: even with tools such as these, Circe's fluent lens-based API:

val data2 = data.hcursor.downField("name").withFocus(_.mapString(_.reverse)).top.get

Still has a lot more boilerplate & new concepts to learn than uJson's mutation based API:

data("name") = data("name").str.reverse

One question you may ask is what is immutability good for? Obviously it helps avoid bugs where you accidentally mutate something that isn't meant to be mutated: re-initializing an already-initialized object, mutating your function's parameters in a way that accidentally affects other places that parameter is passed, etc..

However, in my experience these are simply not common failure modes when mangling JSON data: JSON structures tend to be transient - they are parsed, manipulated and serialized - without the long lifetimes to make them susceptable to accidental mutation. While complicated functions mutating a JSON structure tend to be confusing, complicated sequences of lenses/cursors/zippers tend to be confusing as well.

For the 99% use case, uJson exposes a dead-simple - but mutable - JSON data structure that makes it easy for anyone to step up and start manipulating JSON without any prior experience. However, you may bump into cases where the JSON processing you are doing is complex enough that the benefits of immutability outweigh the cost of learning how to deal with cursors/lenses/zippers. uJson has an answer for that too, by supporting third-party JSON libraries in all it's JSON processing operations

JSON Library Interoperability

uJson comes with a rich collection of basic operations to manipulate JSON:

package object ujson{
  def read(s: Transformable): Js.Value
  def copy(t: Js.Value): Js.Value
  def write(t: Js.Value, indent: Int = -1): String
  def writeTo(t: Js.Value, out: java.io.Writer, indent: Int = -1): Unit
  def validate(s: Transformable): Unit
  def reformat(s: Transformable, indent: Int = -1): String
  def reformatTo(s: Transformable, out: java.io.Writer, indent: Int = -1): Unit
  def transform[T](t: Transformable, v: Visitor[_, T]): T

The uPickle library that uses uJson builds on top of these, exposing similar operations that work on any type T with a provided Reader or Writer:

trait Api{
  def read[T: Reader](s: Transformable): T
  def readJs[T: Reader](s: Js.Value): T
  def reader[T: Reader]: Reader[T]
  def write[T: Writer](t: T, indent: Int = -1): String
  def writeJs[T: Writer](t: T): Js.Value
  def writeTo[T: Writer](t: T, out: java.io.Writer, indent: Int = -1): Unit
  def writer[T: Writer]: Writer[T]
  def writable[T: Writer](t: T): Transformable
  def readwriter[T: ReadWriter]: ReadWriter[T]
  case class transform[T: Writer](t: T) extends Transformable{
    def to[V](f: ujson.Visitor[_, V]): V
    def to[V](implicit f: Reader[V]): V

Apart from the "obvious" operations like ujson.read or Api#write, the most interesting APIs in uJson and uPickle are ujson.transform and Api#transform#to. These can be used to process any Transformable by any Visitor, or to process any type T with a Writer by either a Visitor or by a Reader.

What is interesting about uJson and uPickle is that the generic Transformable and Visitor types can be defined for any JSON structure, not just the "default" one provided by uJson! uJson comes with default integrations with Argonaut, Circe, Json4s and Play-Json: JSON structure from any of those libraries can take part in any uJson/uPickle operations. This means, taking Argonaut as an example:

import ujson.argonaut.ArgonautJson
val argJson: argonaut.Json = ArgonautJson(
  """["hello", "world"]"""
val items: Seq[String] = ArgonautJson.transform(
val rewritten = upickle.default.transform(items).to(ArgonautJson)
val stringified = ArgonautJson.transform(rewritten, StringRenderer()).toString

While the above examples are for Argonaut, uJson can perform all the same operations on the JSON data type of any of the other libraries. uJson can even convert the JSON data type from one library to another, directly & without overhead.

The following example shows us parsing JSON into a Circe JSON structure, manipulating it, converting it to the Play-Json JSON structure, manipulating it more, and finally writing it out as a String:

import ujson.circe.CirceJson
val circeJson: io.circe.Json = CirceJson(
  """["hello", "world"]"""

val updatedCirceJson =
  circeJson.mapArray(_.map(x => x.mapString(_.toUpperCase)))

import ujson.play.PlayJson
import play.api.libs.json._

val playJson: play.api.libs.json.JsValue = CirceJson.transform(

val updatedPlayJson = JsArray(
  for(v <- playJson.as[JsArray].value)
  yield JsString(v.as[String].reverse)

val stringified = PlayJson.transform(updatedPlayJson, StringRenderer()).toString

stringified ==> """["OLLEH","DLROW"]"""

While uJson provides a simple mutable JSON data type that should be easy for anyone to pick up, using uJson doesn't bind you to it: any of uJson's or uPickle's operations can work just as easily with the JSON data type provided by any other library. uJson's high-performance direct-conversion protocol (the .transform methods you see above) mean it is both easy and efficient to convert your JSON data into whatever form you prefer manipulating.

uJson and uPickle are not just a convenient JSON manipulation library and serialization library: they effectively act as a common protocol shared by all popular JSON libraries in the Scala ecosystem.

uPickle builds on top of uJson and gains the ability to read/write to/from every JSON library in existance, with high performance & zero overhead. Other serialization libraries could also be written on top of uJson to receive the same read/write-anything capability. Similarly, any other JSON library that chooses to integrate with uJson (which is a tiny amount of work) will similarly receive a rich library of JSON operators, compatibility with uPickle & other serialization libraries, and fast/zero-overhead conversions to/from every other JSON library out there.

Zero Overhead Serialization

Most JSON serialization libraries out there follow a familiar pattern:

Sometimes you care about the JSON data type in between: you want to manipulate it, inspect it, modify it, etc.. But other times you do not, and just want to go from String to case class as quickly as possible. Why should we spend time building a JSON structure that we're just going to throw away?

Above, I described how uJson allows for zero-overhead conversions between JSON data types from different libraries. This converts from one JSON type to another JSON type without any intermediate structures. This zero-overhead-conversion capability also lets uJson perform zero-overhead conversions between anything that uJson knows how to read or write: uJson can read Strings, and uPickle can write case classes, and so uJson can do zero-overhead conversion from String to case class (and vice versa) without constructing an intermediate AST.

This direct serialization means uJson/uPickle have great performance. The following (somewhat ad-hoc) benchmark counts how many times uJson/uPickle can read/write a simple case class to JSON Strings in a fixed span of time (higher is better)

Library Reads Writes
Jackson Scala 2,038,770 11,324,495
Play Json 987,940 1,357,490
Circe 2,360,411 2,139,692
upickle.default 3,135,576 3,496,939

As you can see, uPickle has a significant speed advantage over Circe, which is impressive given that Circe is already more than twice as fast as Play-Json. uPickle has had much less effort put into micro-optimizations than Circe, and there are definitely still a lot of long hanging fruit to be picked just by spending a bit more time with a profiler.

The difference in speed is even larger in Scala.js:

Library Reads Writes
Play Json 117,181 194,582
Circe 132,519 441,906
upickle.default 613,727 1,041,798

Here, we see uPickle's reads are about 5 times as fast as Play-Json or Circe, and it's writes are 5x or 2x as fast as each of those libraries respectively.


uJson is a fast, flexible, and intuitive JSON library for Scala. I have shown how using uJson to manipulate raw JSON compares favorably to the most concise scripting languages out there, how uJson allows seamless integration of any library's JSON data type into all of it's core functions & operations, and how uJson's zero-overhead-conversion protocol can be leveraged to provide zero-overhead JSON serialization that parses a String directly into a case class (or vice versa) giving you significant performance wins.

I think JSON libraries are a big pain point for Scala: many of the existing libraries are designed for experts, exposing great power to those sophisticated enough to wield them. What is missing is a "JSON for dummies" library, that can handle the 99% of "boring" use cases with minimal required learning or onboarding.

Next time you find yourself looking for a simple, fast JSON library or serialization library to use in some application, it's probably worth giving uJson and uPickle a try!

About the Author: Haoyi is a software engineer, and the author of many open-source Scala tools such as the Ammonite REPL and the Mill Build Tool. If you enjoyed the contents on this blog, you may also enjoy Haoyi's book Hands-on Scala Programming

Mill: Better Scala BuildsHow To Drive Change as a Software Engineer