How Can I Return Protocol Buffers Directly in Play! 2.0 Framework?

Question

Play lets you return a number of different types directly in your controllers, such as JsValue or XML along with plain text. I would like to extend this to accept protocol buffers, so I can write:

def page = Action {
    val protobuf = //...
    Ok(protobuf)
}

Answer 1

Protocol buffers in Java all inherit from a single com.google.protobuf.Message class.

Add the following implicit conversions within the scope of your application controller:

implicit def contentTypeOf_Protobuf: ContentTypeOf[Message] = {
  ContentTypeOf[Message](Some("application/x-protobuf"))
}
implicit def writeableOf_Protobuf: Writeable[Message] = {
  Writeable[Message](message => message.toByteArray())
}

These will allow Play to serialize buffers directly in a response given by a status such as Ok(protobuf)

Update:

I have posted a working example of the reverse situation, where the incoming request can be parsed and a protobuf can be extracted automatically.

• https://gist.github.com/3455432

The parser takes the form of an action in this example, tho you could also code a body parser:

object Put extends Controller {
  def index = DecodeProtobuf(classOf[MyProtobuf]) { stack :MyProtobuf =>
    Action {
      // do something with stack
    }
  }
}

The client sending the request should serialize the buffer as a byte array, and pass it directly in the body of the request.