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ScalaPB

Protocol Buffer Compiler for Scala

ScalaPB and JSON

ScalaPB can convert protocol buffers to and from JSON, using json4s.

Setting up your project

Make sure that you are using ScalaPB 0.5.x or later.

In build.sbt add a dependency on scalapb-json4s:

// For ScalaPB 0.7.x:
libraryDependencies += "com.thesamet.scalapb" %% "scalapb-json4s" % "0.7.0"

// For ScalaPB 0.6.x (note the different groupId):
libraryDependencies += "com.trueaccord.scalapb" %% "scalapb-json4s" % "0.3.2"

// For ScalaPB 0.5.x (note the different groupId):
libraryDependencies += "com.trueaccord.scalapb" %% "scalapb-json4s" % "0.1.6"

In your code, you can now convert to JSON:

import scalapb.json4s.JsonFormat

val r: String = JsonFormat.toJsonString(myProto)

Parse JSON back to a protocol buffer:

import scalapb.json4s.JsonFormat

val proto: MyProto = JsonFormat.fromJsonString[MyProto](
    """{"x": "17"}""")

There are lower-level functions toJson() and fromJson() that convert from protos to json4s’s JValue:

def toJson(m: GeneratedMessage): JObject

def fromJson[Proto](value: JValue): Proto

Finally, in JsonFormat there are two implicit methods that instantiate Reader[Proto] and Writer[Proto].

More printing and parsing options

There are a few more options available to customize the format used to print and parse JSON. To take advantage of that, instantiate Printer and Parser and call toJson() / fromJson() as usual.

For example:

new scalapb.json4s.Printer(
  includingDefaultValueFields = true,
  preservingProtoFieldNames = true,
  formattingLongAsNumber = true
).toJson(myProto)

Options:

  • includingDefaultValueFields (default: false): should fields that are set to their default value be included in the output.
  • preservingProtoFileNames (default: false): by default, field names are mapped to lowerCamelCase and become JSON object keys. Setting this option to true would make the parser and the printer use the original field names as specified in the proto file (normally, in snake_case)
  • formatLongAsNumber (default: false): by default, longs are serialized as strings. To use the numeric representation, set this option to true. Note that due to the way Javascript represents numbers, there is a possibility to lose precision (more details here).

See the list of constructor paramerters here