ApiGatewayResponse.scala

package hello

import scala.beans.BeanProperty

case class ApiGatewayResponse(@BeanProperty statusCode: Integer, @BeanProperty body: String,
    @BeanProperty headers: java.util.Map[String, Object], @BeanProperty base64Encoded: Boolean = false)

Handler.scala

package hello

import com.amazonaws.services.lambda.runtime.{Context, RequestHandler}
import scala.collection.JavaConverters

class Handler extends RequestHandler[Request, Response] {

	def handleRequest(input: Request, context: Context): Response = {
		Response("Go Serverless v1.0! Your function executed successfully!", input)
	}
}

class ApiGatewayHandler extends RequestHandler[Request, ApiGatewayResponse] {

  def handleRequest(input: Request, context: Context): ApiGatewayResponse = {
    val headers = Map("x-custom-response-header" -> "my custom response header value")
    ApiGatewayResponse(200, "Go Serverless v1.0! Your function executed successfully!",
      JavaConverters.mapAsJavaMap[String, Object](headers),
      true)
  }
}

Request.scala

package hello

import scala.beans.BeanProperty

class Request(@BeanProperty var key1: String, @BeanProperty var key2: String, @BeanProperty var key3: String) {
  def this() = this("", "", "")
}

Response.scala

package hello

import scala.beans.BeanProperty

case class Response(@BeanProperty message: String, @BeanProperty request: Request)

serverless.yml

package:
  artifact: target/scala-2.12/hello.jar

functions:
  hello:
    handler: hello.Handler