build.sbt

import com.trueaccord.scalapb.compiler.Version.protobufVersion

PB.targets in Compile := Seq(
  PB.gens.java(protobufVersion) -> ((sourceManaged in Compile).value / "protobuf-java"),
  scalapb.gen(javaConversions=true) -> ((sourceManaged in Compile).value / "protobuf-scala")
)

libraryDependencies += "com.google.protobuf" % "protobuf-java-util" % "3.4.0"

Main.scala

object Main extends App {
  import com.google.protobuf.util.JsonFormat

  import example.user.User

  val user = User(id = 1, name = "foo")
  println(user)

  val userJson = JsonFormat.printer.print(User.toJavaProto(user))
  println(userJson)


  import com.google.protobuf.MessageOrBuilder
  import com.google.protobuf.util.JsonFormat
  import com.trueaccord.scalapb.JavaProtoSupport

  implicit class GeneratedMessageOps[A](val self: A) extends AnyVal {
    def toJsonString[B <: MessageOrBuilder](implicit A: JavaProtoSupport[A, B]): String =
      JsonFormat.printer.print(A.toJavaProto(self))
  }

  println(User(id = 2, name = "bar").toJsonString)


  def jsonStringToUser(json: String): User = {
    val parser = JsonFormat.parser()
    val builder = example.UserOuterClass.User.newBuilder()

    // val registry = JsonFormat.TypeRegistry.newBuilder().add(User.descriptor).build()
    // val parser = JsonFormat.parser().usingTypeRegistry(registry)

    parser.merge(json, builder)
    User.fromJavaProto(builder.build())
  }

  val user2 = jsonStringToUser(userJson)
  println("user2")
  println(user2)
}

project_plugins.sbt

addSbtPlugin("com.thesamet" % "sbt-protoc" % "0.99.12")
libraryDependencies += "com.trueaccord.scalapb" %% "compilerplugin" % "0.6.6"

src_main_protobuf_user.proto

syntax = "proto3";

package example;

message User {
  int64 id = 1;
  string name = 2;
}