jodersky / scalapb-ujson   0.3.2

Apache License 2.0 GitHub

ScalaPB to JSON conversions and vice versa

Scala versions: 3.x
Scala.js versions: 1.x
Scala Native versions: 0.4


stability: firm

Read and write ScalaPB messages to and from JSON.

This project is a library that strives to implement the Canonical Protobuf to JSON Mapping, for ScalaPB-generated messages, using the ujson library.



syntax = "proto3";

message Request {
  string name = 1;
  int32 number = 2;

  enum State {
    UNKNOWN = 0;
    OK = 1;
    ERROR = 2;
  State state = 3;

  message Attr {
    string a = 1;
    string b = 2;
  map<string, Attr> attributes = 4;


@main def main() =
  import protos.Request

  // the ScalaPB message
  val req = Request(
    name = "john smith",
    number = 42,
    state = Request.State.OK,
    attributes = Map(
        "attr1" -> Request.Attr("hello", "world"),
        "attr2" -> Request.Attr("foo", "bar")

  // convert to JSON string
  val json: String = scalapb.ujson.JsonFormat.write(req, 2)

  val input: String =
       |  "name": "john smith",
       |  "number": 42,
       |  "state": "OK",
       |  "attributes": {
       |    "attr1": {
       |      "a": "hello",
       |      "b": "world"
       |    },
       |    "attr2": {
       |      "a": "foo",
       |      "b": "bar"
       |    }
       |  }

  // parse JSON string as a ScalaPB message
  val message: Request =[Request](input)

The full example source code is contained in example/. You can run it with ./mill example.


The central element of scalapb-ujson is the scalapb.usjon.JsonFormat object.

It has several write and read methods, which allow you to convert between ScalaPB messages and JSON.

Have a look at the API docs.

The conversion methods support writing and reading the resulting JSON to/from various outputs/inputs. For example, the simplest write creates a string, however there are others which create ujson.Value objects and others which stream bytes directly without materializing the whole JSON structure in memory first.


Some aspects of ScalaPB to JSON conversions can be customized as per the specification. These customizations are given as parameters to the JsonFormat companion class. Hence, if you would like to customize the output, you can use a new instance of JsonFormat rather than the default object.

For example, if you would like to hide default values in the JSON output:

val fmt = scalapb.ujson.JsonFormat(includeDefaultValueFields = false)


Note that readers are not customizable. They are flexible and support reading JSON from any customized output.


This project tries to minimize dependencies, however, as a bridge between libraries, some dependencies are required.

Dependency Reason
ujson 3.x One end of the conversion. Note that only the ujson and the core library is needed. There is NO dependency on the upickle macro library.
scalapb-runtime 0.11.x The other end of the conversion.
Scala Java Time 2.x A Scala implementation of the java.time package. The time package is needed for serializing some well-known protocol buffers to JSON strings, and the Scala-implementation is needed to support cross-building for JS and native.

Scala Versions

This project works with Scala 3 on the JVM, JS and native. We try to be conservative with the minimal required versions. As of this writing, the following minimums are required:

  • Scala: 3.2.2
  • Scala JS: 1.13.0
  • Scala Native: 0.4.10

Maven Coordinates

The project is published to Maven Central:



Most of the Canonical Protobuf to JSON Mapping is implemented, including the writer options. Some advanced features are still lacking however, and will be implemented on an as-is-needed basis by the authors. Contributions are welcome however!

Missing features:

  • special serialization of google.protobuf.Any
  • special serialization of ListValue, Value, Struct and NullValue

Only Protocol Buffers version 3 has been tested.


Protocol buffers are a binary format which makes them very efficient. However, being binary also means that they are cumbersome to work-with in an ad-hoc manner such as when building a quick prototype or during live debug interactions. In these situations, which are crucial for developer productivity, text-based formats always beat binary formats.

Therefore, by implementing a mapping between protobuf and JSON, the best of both worlds can be achieved:

  • you can use a language-agnostic IDL to keep an overview of your application's API
  • you get the benefit of protobuf's efficiency
  • you retain the ability to simply interact with your application in an ad-hoc way via JSON

scalapb-ujson strives to be very efficient and light on memory by fully embracing ujson's visitor design. This means that JSON can be read and written from various forms and doesn't need to go through an intermediate representation.


  • scalapb-json4s is a direct inspiration for this project. It provides similar features, however it also diverges with this project in a couple of important points:

    • it uses json4s rather than ujson (which is the recommended JSON library from the Scala Toolkit)
    • it only works on the JVM
  • gRPC Gateway. scalapb-ujson integrates directly with ScalaPB and is not designed to be a converter of protobuf on the wire. In case you have a service architecture then gRPC gateway is a viable alternative. scalapb-ujson is only designed for standalone applications (and doesn't support gRPC service mapping).


  • compile all modules: ./mill __.compile
  • run tests: ./mill __.test
  • run tests on only the main JVM project ./mill scalapb-ujson.jvm.test
  • run a specific test on all targets: ./mill __.test RwTests.empty
  • run tests important tests before publishing ci/test
  • publish to maven central ci/publish


Some helpers have been copied and adapted from scalapb-json4s, released under the Apache 2.0 license. An appropriate notice is included in the relevant files.