Use Play Framework as a component

build.sbt

libraryDependencies ++= Seq(
  "org.scala-lang" % "scala-swing" % "2.11.0-M7",
  "com.typesafe.play" %% "play" % "2.4.3",
  "com.typesafe.play" %% "play-netty-server" % "2.4.3",
  "org.scalaj" %% "scalaj-http" % "2.2.0"
)

LocalWebServer.scala

import java.io.File

import play.api.ApplicationLoader.Context
import play.api._
import play.api.routing.Router
import play.core.server.{ServerConfig, NettyServer}

abstract class LocalWebServer {
  val environment = new Environment(
    new File("."),
    getClass.getClassLoader,
    play.api.Mode.Dev
  )

  val context = ApplicationLoader.createContext(environment)

  def routes:Router.Routes
  if (routes == null) throw new Exception("Routes is null")
  val components = new BuiltInComponentsFromContext(context) {
    override def router: Router = Router.from(routes)
  }

  val applicationLoader = new ApplicationLoader {
    override def load(context: Context): Application = components.application
  }

  val application = applicationLoader.load(context)

  Play.start(application)

  private object CausedBy {
    def unapply(e: Throwable): Option[Throwable] = Option(e.getCause)
  }

  private def startFindPort(port:Int): (Int, NettyServer) = {
    try {
      (port, NettyServer.fromApplication(application, ServerConfig(port = Some(port))))
    } catch {
      case CausedBy(e : java.net.BindException) => {
        startFindPort(port + 1)
      }
    }
  }

  val (port, server) = startFindPort(21000)

  def stop() = server.stop()

}