Play / Subcut dependency injection in modules

Environment.scala

// Helper for keeping track of the active project bindings 
// (because we don't want to hard-code those inside singleton objects)

object Environment {
  def currentBindings =
    ref.get match {
      case null =>
        throw new NullPointerException("No BindingModule currently registered (app not running?)")
      case value =>
        value
    }

  def registerCurrentBindings(update: => BindingModule) {
    if (ref.get != null || !ref.compareAndSet(null, update))
      throw new IllegalStateException("Cannot register new BindingModule (the current one must be deregistered)")
  }

  def unregisterBindings(current: BindingModule) {
    if (!ref.compareAndSet(current, null) && !ref.compareAndSet(null, null))
      throw new IllegalStateException("Cannot unregister given BindingModule (not current)")
  }

  private[this] val ref = Atomic(null : BindingModule)
}

HelloWorldController.scala

trait HelloWorldController extends Controller {
  // abstract
  def usersDAL: UsersDAL

  def list = Action { req =>
    val response = usersDAL.list(0, 100).map { list => OK(html.showUsers(list)) }
    Async(response)
  }

  // ...
}

object HelloWorldController extends HelloWorldController with Injectable {
  def bindingModule = Environment.current
  def usersDAL = inject [UsersDAL]
}

LifeCyclePlugin.scala

class LifeCyclePlugin(app: play.Application) extends Plugin with Injectable {
  val bindingModule = new StandardBindings(app.getWrappedApplication)

  override def onStart() {
    Environment.registerCurrentBindings(bindingModule)
  }

  override def onStop() {
    Environment.unregisterBindings(bindingModule)
  }
}

StandardBindings.scala

import com.escalatesoft.subcut.inject._
import play.api.libs.concurrent.Akka
import play.api.{Play, Application}
import akka.actor.ActorSystem
import play.api.libs.concurrent.Execution.Implicits.defaultContext

class StandardBindings(app: => Application) extends NewBindingModule(module => {
  import module._

  lazy val config = Configuration.load()
  lazy val system = Akka.system(app)

  bind [Configuration] toSingle config
  bind [DBConnection] toSingle DBConnection(config.db, system)(app)
  bind [ActorSystem] toSingle system
  bind [Application] toSingle Play.current
  bind [GeoIP] toSingle GeoIP()

  // ...
})