How Scala Macros Simplify Protobuf‑Java ↔ Scala Case Class Conversions

Background

In GrowingIO backend development we use gRPC for microservice communication, defining Protobuf messages for each service and generating Java code with

protoc

. Scala services use case classes, so converting between protobuf‑java objects and Scala case classes becomes repetitive and error‑prone, especially with nested types.

Typical mappings (e.g.,

java.util.List

↔

Seq

,

Timestamp

↔

ZonedDateTime

,

Option[String]

↔

StringValue

) exist, but a generic, type‑safe solution that minimizes boilerplate was needed.

Inspired by Play‑JSON Reader/Writer and Chimney macro‑based conversion, we created

scala-protobuf-java

, a DSL that combines Scala macros with implicit‑parameter design.

Solution Effect

Defining a case class

User

and a protobuf message

UserPB

, manual conversion requires many lines of code. With the DSL a single call

Protoable[User,UserPB].toProto(user)

performs the conversion safely and concisely.

DSL Design

The core traits are

Protoable[-S,+P]

and

Scalable[+S,-P]

, representing conversion directions.

<code>trait Protoable[-S, +P] {
  def toProto(entity: S): P
}
trait Scalable[+S, -P] {
  def toScala(proto: P): S
}</code>

Companion objects provide implicit converters for primitive and common types, such as Java boxed types,

StringValue

, and

Timestamp

↔

ZonedDateTime

.

Macro‑Generated Code

Using the DSL we write concrete converters for

User

and

UserPB

. The macro expands the boilerplate, handling optional fields, collections, and nested types automatically.

<code>new Protoable[User, UserPB] {
  override def toProto(entity: User): UserPB = {
    val builder = UserPB.newBuilder()
    builder.setId(entity.id)
    builder.setName(entity.name)
    if (entity.phoneNumber.isDefined) {
      builder.setPhoneNumber(implicitly[Protoable[String,StringValue]].toProto(entity.phoneNumber))
    }
    builder.addAllHobbies(implicitly[Protoable[Seq[String], java.util.List[String]]].toProto(entity.hobbies))
    builder.build
  }
}

new Scalable[User, UserPB] {
  override def toScala(proto: UserPB): User = {
    User(
      id = proto.getId,
      name = proto.getName,
      phoneNumber = if (proto.hasPhoneNumber) Some(implicitly[Scalable[String,StringValue]].toScala(proto.getPhoneNumber)) else None,
      hobbies = implicitly[Scalable[Seq[String], java.util.List[String]]].toScala(proto.getHobbiesList)
    )
  }
}</code>

Builder Customisation

For field‑level rules we provide

ScalableBuilder

and

ProtoableBuilder

that let users inject lambdas, e.g., ensuring an

id

is non‑negative.

<code>val scalable = ScalableBuilder[User,UserPB]
  .setField(_.id, pb => if (pb.getId < 0) 0 else pb.getId)
  .build
scalable.toScala(...)</code>

Scala 3 Support

Scala 3 introduces cleaner implicit definitions and a new inline/Quotes macro system, which can simplify the DSL further, although some issues remain, such as protoc‑generated Java files not compiling.