/**
 *  Copyright (C) 2009-2013 Typesafe Inc. <http://www.typesafe.com>
 */

package akka.actor

import java.io.Closeable
import java.util.concurrent.{ ConcurrentHashMap, ThreadFactory, CountDownLatch, TimeoutException, RejectedExecutionException }
import java.util.concurrent.TimeUnit.MILLISECONDS
import com.typesafe.config.{ Config, ConfigFactory }
import akka.event._
import akka.dispatch._
import akka.dispatch.sysmsg.{ SystemMessageList, EarliestFirstSystemMessageList, LatestFirstSystemMessageList, SystemMessage }
import akka.japi.Util.immutableSeq
import akka.actor.dungeon.ChildrenContainer
import akka.util._
import scala.annotation.tailrec
import scala.collection.immutable
import scala.concurrent.duration.{ FiniteDuration, Duration }
import scala.concurrent.{ Await, Awaitable, CanAwait, Future, ExecutionContext }
import scala.util.{ Failure, Success }
import scala.util.control.{ NonFatal, ControlThrowable }

object ActorSystem {

  val Version: String = "2.2-SNAPSHOT"

  val EnvHome: Option[String] = System.getenv("AKKA_HOME") match {
    case null | "" | "." ⇒ None
    case value           ⇒ Some(value)
  }

  val SystemHome: Option[String] = System.getProperty("akka.home") match {
    case null | "" ⇒ None
    case value     ⇒ Some(value)
  }

  val GlobalHome: Option[String] = SystemHome orElse EnvHome

  /**
   * Creates a new ActorSystem with the name "default",
   * obtains the current ClassLoader by first inspecting the current threads' getContextClassLoader,
   * then tries to walk the stack to find the callers class loader, then falls back to the ClassLoader
   * associated with the ActorSystem class.
   * Then it loads the default reference configuration using the ClassLoader.
   */
  def create(): ActorSystem = apply()

  /**
   * Creates a new ActorSystem with the specified name,
   * obtains the current ClassLoader by first inspecting the current threads' getContextClassLoader,
   * then tries to walk the stack to find the callers class loader, then falls back to the ClassLoader
   * associated with the ActorSystem class.
   * Then it loads the default reference configuration using the ClassLoader.
   */
  def create(name: String): ActorSystem = apply(name)

  /**
   * Creates a new ActorSystem with the name "default", and the specified Config, then
   * obtains the current ClassLoader by first inspecting the current threads' getContextClassLoader,
   * then tries to walk the stack to find the callers class loader, then falls back to the ClassLoader
   * associated with the ActorSystem class.
   *
   * @see <a href="http://typesafehub.github.com/config/v0.4.1/" target="_blank">The Typesafe Config Library API Documentation</a>
   */
  def create(name: String, config: Config): ActorSystem = apply(name, config)

  /**
   * Creates a new ActorSystem with the name "default", the specified Config, and specified ClassLoader
   *
   * @see <a href="http://typesafehub.github.com/config/v0.4.1/" target="_blank">The Typesafe Config Library API Documentation</a>
   */
  def create(name: String, config: Config, classLoader: ClassLoader): ActorSystem = apply(name, config, classLoader)

  /**
   * Creates a new ActorSystem with the name "default",
   * obtains the current ClassLoader by first inspecting the current threads' getContextClassLoader,
   * then tries to walk the stack to find the callers class loader, then falls back to the ClassLoader
   * associated with the ActorSystem class.
   * Then it loads the default reference configuration using the ClassLoader.
   */
  def apply(): ActorSystem = apply("default")

  /**
   * Creates a new ActorSystem with the specified name,
   * obtains the current ClassLoader by first inspecting the current threads' getContextClassLoader,
   * then tries to walk the stack to find the callers class loader, then falls back to the ClassLoader
   * associated with the ActorSystem class.
   * Then it loads the default reference configuration using the ClassLoader.
   */
  def apply(name: String): ActorSystem = {
    val classLoader = findClassLoader()
    apply(name, ConfigFactory.load(classLoader), classLoader)
  }

  /**
   * Creates a new ActorSystem with the name "default", and the specified Config, then
   * obtains the current ClassLoader by first inspecting the current threads' getContextClassLoader,
   * then tries to walk the stack to find the callers class loader, then falls back to the ClassLoader
   * associated with the ActorSystem class.
   *
   * @see <a href="http://typesafehub.github.com/config/v0.4.1/" target="_blank">The Typesafe Config Library API Documentation</a>
   */
  def apply(name: String, config: Config): ActorSystem = apply(name, config, findClassLoader())

  /**
   * Creates a new ActorSystem with the name "default", the specified Config, and specified ClassLoader
   *
   * @see <a href="http://typesafehub.github.com/config/v0.4.1/" target="_blank">The Typesafe Config Library API Documentation</a>
   */
  def apply(name: String, config: Config, classLoader: ClassLoader): ActorSystem = new ActorSystemImpl(name, config, classLoader).start()

  /**
   * Settings are the overall ActorSystem Settings which also provides a convenient access to the Config object.
   *
   * For more detailed information about the different possible configuration options, look in the Akka Documentation under "Configuration"
   *
   * @see <a href="http://typesafehub.github.com/config/v0.4.1/" target="_blank">The Typesafe Config Library API Documentation</a>
   */
  class Settings(classLoader: ClassLoader, cfg: Config, final val name: String) {

    /**
     * The backing Config of this ActorSystem's Settings
     *
     * @see <a href="http://typesafehub.github.com/config/v0.4.1/" target="_blank">The Typesafe Config Library API Documentation</a>
     */
    final val config: Config = {
      val config = cfg.withFallback(ConfigFactory.defaultReference(classLoader))
      config.checkValid(ConfigFactory.defaultReference(classLoader), "akka")
      config
    }

    import scala.collection.JavaConverters._
    import config._

    final val ConfigVersion: String = getString("akka.version")
    final val ProviderClass: String = getString("akka.actor.provider")
    final val SupervisorStrategyClass: String = getString("akka.actor.guardian-supervisor-strategy")
    final val CreationTimeout: Timeout = Timeout(Duration(getMilliseconds("akka.actor.creation-timeout"), MILLISECONDS))
    final val UnstartedPushTimeout: Timeout = Timeout(Duration(getMilliseconds("akka.actor.unstarted-push-timeout"), MILLISECONDS))

    final val SerializeAllMessages: Boolean = getBoolean("akka.actor.serialize-messages")
    final val SerializeAllCreators: Boolean = getBoolean("akka.actor.serialize-creators")

    final val LogLevel: String = getString("akka.loglevel")
    final val StdoutLogLevel: String = getString("akka.stdout-loglevel")
    final val Loggers: immutable.Seq[String] = immutableSeq(getStringList("akka.loggers"))
    final val LoggerStartTimeout: Timeout = Timeout(Duration(getMilliseconds("akka.logger-startup-timeout"), MILLISECONDS))
    @deprecated("use Loggers)", "2.2")
    final val EventHandlers: immutable.Seq[String] = immutableSeq(getStringList("akka.event-handlers"))
    @deprecated("use LoggerStartTimeout)", "2.2")
    final val EventHandlerStartTimeout: Timeout = Timeout(Duration(getMilliseconds("akka.event-handler-startup-timeout"), MILLISECONDS))
    final val LogConfigOnStart: Boolean = config.getBoolean("akka.log-config-on-start")

    final val AddLoggingReceive: Boolean = getBoolean("akka.actor.debug.receive")
    final val DebugAutoReceive: Boolean = getBoolean("akka.actor.debug.autoreceive")
    final val DebugLifecycle: Boolean = getBoolean("akka.actor.debug.lifecycle")
    final val FsmDebugEvent: Boolean = getBoolean("akka.actor.debug.fsm")
    final val DebugEventStream: Boolean = getBoolean("akka.actor.debug.event-stream")
    final val DebugUnhandledMessage: Boolean = getBoolean("akka.actor.debug.unhandled")
    final val DebugRouterMisconfiguration: Boolean = getBoolean("akka.actor.debug.router-misconfiguration")

    final val Home: Option[String] = config.getString("akka.home") match {
      case "" ⇒ None
      case x  ⇒ Some(x)
    }

    final val SchedulerClass: String = getString("akka.scheduler.implementation")
    final val Daemonicity: Boolean = getBoolean("akka.daemonic")
    final val JvmExitOnFatalError: Boolean = getBoolean("akka.jvm-exit-on-fatal-error")

    final val DefaultVirtualNodesFactor: Int = getInt("akka.actor.deployment.default.virtual-nodes-factor")

    if (ConfigVersion != Version)
      throw new akka.ConfigurationException("Akka JAR version [" + Version + "] does not match the provided config version [" + ConfigVersion + "]")

    /**
     * Returns the String representation of the Config that this Settings is backed by
     */
    override def toString: String = config.root.render
  }

  /**
   * INTERNAL API
   */
  private[akka] def findClassLoader(): ClassLoader = {
    def findCaller(get: Int ⇒ Class[_]): ClassLoader =
      Iterator.from(2 /*is the magic number, promise*/ ).map(get) dropWhile { c ⇒
        c != null &&
          (c.getName.startsWith("akka.actor.ActorSystem") ||
            c.getName.startsWith("scala.Option") ||
            c.getName.startsWith("scala.collection.Iterator") ||
            c.getName.startsWith("akka.util.Reflect"))
      } next () match {
        case null ⇒ getClass.getClassLoader
        case c    ⇒ c.getClassLoader
      }

    Option(Thread.currentThread.getContextClassLoader) orElse
      (Reflect.getCallerClass map findCaller) getOrElse
      getClass.getClassLoader
  }
}

/**
 * An actor system is a hierarchical group of actors which share common
 * configuration, e.g. dispatchers, deployments, remote capabilities and
 * addresses. It is also the entry point for creating or looking up actors.
 *
 * There are several possibilities for creating actors (see [[akka.actor.Props]]
 * for details on `props`):
 *
 * {{{
 * // Java or Scala
 * system.actorOf(props, "name")
 * system.actorOf(props)
 *
 * // Scala
 * system.actorOf(Props[MyActor], "name")
 * system.actorOf(Props[MyActor])
 * system.actorOf(Props(new MyActor(...)))
 *
 * // Java
 * system.actorOf(MyActor.class);
 * system.actorOf(Props(new Creator<MyActor>() {
 *   public MyActor create() { ... }
 * });
 * system.actorOf(Props(new Creator<MyActor>() {
 *   public MyActor create() { ... }
 * }, "name");
 * }}}
 *
 * Where no name is given explicitly, one will be automatically generated.
 *
 * <b><i>Important Notice:</i></b>
 *
 * This class is not meant to be extended by user code. If you want to
 * actually roll your own Akka, it will probably be better to look into
 * extending [[akka.actor.ExtendedActorSystem]] instead, but beware that you
 * are completely on your own in that case!
 */
abstract class ActorSystem extends ActorRefFactory {
  import ActorSystem._

  /**
   * The name of this actor system, used to distinguish multiple ones within
   * the same JVM & class loader.
   */
  def name: String

  /**
   * The core settings extracted from the supplied configuration.
   */
  def settings: Settings

  /**
   * Log the configuration.
   */
  def logConfiguration(): Unit

  /**
   * Construct a path below the application guardian to be used with [[ActorSystem.actorFor]].
   */
  def /(name: String): ActorPath

  /**
   * Java API: Create a new child actor path.
   */
  def child(child: String): ActorPath = /(child)

  /**
   * Construct a path below the application guardian to be used with [[ActorSystem.actorFor]].
   */
  def /(name: Iterable[String]): ActorPath

  /**
   * Java API: Recursively create a descendant’s path by appending all child names.
   */
  def descendant(names: java.lang.Iterable[String]): ActorPath = /(immutableSeq(names))

  /**
   * Start-up time in milliseconds since the epoch.
   */
  val startTime: Long = System.currentTimeMillis

  /**
   * Up-time of this actor system in seconds.
   */
  def uptime: Long = (System.currentTimeMillis - startTime) / 1000

  /**
   * Main event bus of this actor system, used for example for logging.
   */
  def eventStream: EventStream

  /**
   * Convenient logging adapter for logging to the [[ActorSystem.eventStream]].
   */
  def log: LoggingAdapter

  /**
   * Actor reference where messages are re-routed to which were addressed to
   * stopped or non-existing actors. Delivery to this actor is done on a best
   * effort basis and hence not strictly guaranteed.
   */
  def deadLetters: ActorRef
  //#scheduler
  /**
   * Light-weight scheduler for running asynchronous tasks after some deadline
   * in the future. Not terribly precise but cheap.
   */
  def scheduler: Scheduler
  //#scheduler

  /**
   * Helper object for looking up configured dispatchers.
   */
  def dispatchers: Dispatchers

  /**
   * Default dispatcher as configured. This dispatcher is used for all actors
   * in the actor system which do not have a different dispatcher configured
   * explicitly.
   * Importing this member will place the default MessageDispatcher in scope.
   */
  implicit def dispatcher: ExecutionContext

  /**
   * Register a block of code (callback) to run after ActorSystem.shutdown has been issued and
   * all actors in this actor system have been stopped.
   * Multiple code blocks may be registered by calling this method multiple times.
   * The callbacks will be run sequentially in reverse order of registration, i.e.
   * last registration is run first.
   *
   * @throws a RejectedExecutionException if the System has already shut down or if shutdown has been initiated.
   *
   * Scala API
   */
  def registerOnTermination[T](code: ⇒ T): Unit

  /**
   * Java API: Register a block of code (callback) to run after ActorSystem.shutdown has been issued and
   * all actors in this actor system have been stopped.
   * Multiple code blocks may be registered by calling this method multiple times.
   * The callbacks will be run sequentially in reverse order of registration, i.e.
   * last registration is run first.
   *
   * @throws a RejectedExecutionException if the System has already shut down or if shutdown has been initiated.
   */
  def registerOnTermination(code: Runnable): Unit

  /**
   * Block current thread until the system has been shutdown, or the specified
   * timeout has elapsed. This will block until after all on termination
   * callbacks have been run.
   *
   * @throws TimeoutException in case of timeout
   */
  def awaitTermination(timeout: Duration): Unit

  /**
   * Block current thread until the system has been shutdown. This will
   * block until after all on termination callbacks have been run.
   */
  def awaitTermination(): Unit

  /**
   * Stop this actor system. This will stop the guardian actor, which in turn
   * will recursively stop all its child actors, then the system guardian
   * (below which the logging actors reside) and the execute all registered
   * termination handlers (see [[ActorSystem.registerOnTermination]]).
   */
  def shutdown(): Unit

  /**
   * Query the termination status: if it returns true, all callbacks have run
   * and the ActorSystem has been fully stopped, i.e.
   * `awaitTermination(0 seconds)` would return normally. If this method
   * returns `false`, the status is actually unknown, since it might have
   * changed since you queried it.
   */
  def isTerminated: Boolean

  /**
   * Registers the provided extension and creates its payload, if this extension isn't already registered
   * This method has putIfAbsent-semantics, this method can potentially block, waiting for the initialization
   * of the payload, if is in the process of registration from another Thread of execution
   */
  def registerExtension[T <: Extension](ext: ExtensionId[T]): T

  /**
   * Returns the payload that is associated with the provided extension
   * throws an IllegalStateException if it is not registered.
   * This method can potentially block, waiting for the initialization
   * of the payload, if is in the process of registration from another Thread of execution
   */
  def extension[T <: Extension](ext: ExtensionId[T]): T

  /**
   * Returns whether the specified extension is already registered, this method can potentially block, waiting for the initialization
   * of the payload, if is in the process of registration from another Thread of execution
   */
  def hasExtension(ext: ExtensionId[_ <: Extension]): Boolean
}

/**
 * More powerful interface to the actor system’s implementation which is presented to extensions (see [[akka.actor.Extension]]).
 *
 * <b><i>Important Notice:</i></b>
 *
 * This class is not meant to be extended by user code. If you want to
 * actually roll your own Akka, beware that you are completely on your own in
 * that case!
 */
abstract class ExtendedActorSystem extends ActorSystem {

  /**
   * The ActorRefProvider is the only entity which creates all actor references within this actor system.
   */
  def provider: ActorRefProvider

  /**
   * The top-level supervisor of all actors created using system.actorOf(...).
   */
  def guardian: InternalActorRef

  /**
   * The top-level supervisor of all system-internal services like logging.
   */
  def systemGuardian: InternalActorRef

  /**
   * A ThreadFactory that can be used if the transport needs to create any Threads
   */
  def threadFactory: ThreadFactory

  /**
   * ClassLoader wrapper which is used for reflective accesses internally. This is set
   * to use the context class loader, if one is set, or the class loader which
   * loaded the ActorSystem implementation. The context class loader is also
   * set on all threads created by the ActorSystem, if one was set during
   * creation.
   */
  def dynamicAccess: DynamicAccess

  /**
   * For debugging: traverse actor hierarchy and make string representation.
   * Careful, this may OOM on large actor systems, and it is only meant for
   * helping debugging in case something already went terminally wrong.
   */
  private[akka] def printTree: String
}

private[akka] class ActorSystemImpl(val name: String, applicationConfig: Config, classLoader: ClassLoader) extends ExtendedActorSystem {

  if (!name.matches("""^[a-zA-Z0-9][a-zA-Z0-9-]*$"""))
    throw new IllegalArgumentException(
      "invalid ActorSystem name [" + name +
        "], must contain only word characters (i.e. [a-zA-Z0-9] plus non-leading '-')")

  import ActorSystem._

  final val settings: Settings = new Settings(classLoader, applicationConfig, name)

  protected def uncaughtExceptionHandler: Thread.UncaughtExceptionHandler =
    new Thread.UncaughtExceptionHandler() {
      def uncaughtException(thread: Thread, cause: Throwable): Unit = {
        cause match {
          case NonFatal(_) | _: InterruptedException | _: NotImplementedError | _: ControlThrowable ⇒ log.error(cause, "Uncaught error from thread [{}]", thread.getName)
          case _ ⇒
            if (settings.JvmExitOnFatalError) {
              try {
                log.error(cause, "Uncaught error from thread [{}] shutting down JVM since 'akka.jvm-exit-on-fatal-error' is enabled", thread.getName)
                import System.err
                err.print("Uncaught error from thread [")
                err.print(thread.getName)
                err.print("] shutting down JVM since 'akka.jvm-exit-on-fatal-error' is enabled for ActorSystem[")
                err.print(name)
                err.println("]")
                cause.printStackTrace(System.err)
                System.err.flush()
              } finally {
                System.exit(-1)
              }
            } else {
              log.error(cause, "Uncaught fatal error from thread [{}] shutting down ActorSystem [{}]", thread.getName, name)
              shutdown()
            }
        }
      }
    }

  final val threadFactory: MonitorableThreadFactory =
    MonitorableThreadFactory(name, settings.Daemonicity, Option(classLoader), uncaughtExceptionHandler)

  /**
   * This is an extension point: by overriding this method, subclasses can
   * control all reflection activities of an actor system.
   */
  protected def createDynamicAccess(): DynamicAccess = new ReflectiveDynamicAccess(classLoader)

  private val _pm: DynamicAccess = createDynamicAccess()
  def dynamicAccess: DynamicAccess = _pm

  def logConfiguration(): Unit = log.info(settings.toString)

  protected def systemImpl: ActorSystemImpl = this

  private[akka] def systemActorOf(props: Props, name: String): ActorRef = systemGuardian.underlying.attachChild(props, name, systemService = true)

  def actorOf(props: Props, name: String): ActorRef = guardian.underlying.attachChild(props, name, systemService = false)

  def actorOf(props: Props): ActorRef = guardian.underlying.attachChild(props, systemService = false)

  def stop(actor: ActorRef): Unit = {
    val path = actor.path
    val guard = guardian.path
    val sys = systemGuardian.path
    path.parent match {
      case `guard` ⇒ guardian ! StopChild(actor)
      case `sys`   ⇒ systemGuardian ! StopChild(actor)
      case _       ⇒ actor.asInstanceOf[InternalActorRef].stop()
    }
  }

  import settings._

  // this provides basic logging (to stdout) until .start() is called below
  val eventStream: EventStream = new EventStream(DebugEventStream)
  eventStream.startStdoutLogger(settings)

  val log: LoggingAdapter = new BusLogging(eventStream, "ActorSystem(" + name + ")", this.getClass)

  val scheduler: Scheduler = createScheduler()

  val provider: ActorRefProvider = {
    val arguments = Vector(
      classOf[String] -> name,
      classOf[Settings] -> settings,
      classOf[EventStream] -> eventStream,
      classOf[Scheduler] -> scheduler,
      classOf[DynamicAccess] -> dynamicAccess)

    dynamicAccess.createInstanceFor[ActorRefProvider](ProviderClass, arguments).get
  }

  def deadLetters: ActorRef = provider.deadLetters

  //FIXME Why do we need this at all?
  val deadLetterQueue: MessageQueue = new MessageQueue {
    def enqueue(receiver: ActorRef, envelope: Envelope): Unit =
      deadLetters.tell(DeadLetter(envelope.message, envelope.sender, receiver), envelope.sender)
    def dequeue() = null
    def hasMessages = false
    def numberOfMessages = 0
    def cleanUp(owner: ActorRef, deadLetters: MessageQueue): Unit = ()
  }
  //FIXME Why do we need this at all?
  val deadLetterMailbox: Mailbox = new Mailbox(deadLetterQueue) {
    becomeClosed()
    def systemEnqueue(receiver: ActorRef, handle: SystemMessage): Unit =
      deadLetters ! DeadLetter(handle, receiver, receiver)
    def systemDrain(newContents: LatestFirstSystemMessageList): EarliestFirstSystemMessageList = SystemMessageList.ENil
    def hasSystemMessages = false
  }

  val dispatchers: Dispatchers = new Dispatchers(settings, DefaultDispatcherPrerequisites(
    threadFactory, eventStream, deadLetterMailbox, scheduler, dynamicAccess, settings))

  val dispatcher: ExecutionContext = dispatchers.defaultGlobalDispatcher

  def terminationFuture: Future[Unit] = provider.terminationFuture
  def lookupRoot: InternalActorRef = provider.rootGuardian
  def guardian: LocalActorRef = provider.guardian
  def systemGuardian: LocalActorRef = provider.systemGuardian

  def /(actorName: String): ActorPath = guardian.path / actorName
  def /(path: Iterable[String]): ActorPath = guardian.path / path

  private lazy val _start: this.type = {
    // the provider is expected to start default loggers, LocalActorRefProvider does this
    provider.init(this)
    registerOnTermination(stopScheduler())
    loadExtensions()
    if (LogConfigOnStart) logConfiguration()
    this
  }

  def start(): this.type = _start

  private lazy val terminationCallbacks = {
    implicit val d = dispatcher
    val callbacks = new TerminationCallbacks
    terminationFuture onComplete (_ ⇒ callbacks.run)
    callbacks
  }
  def registerOnTermination[T](code: ⇒ T) { registerOnTermination(new Runnable { def run = code }) }
  def registerOnTermination(code: Runnable) { terminationCallbacks.add(code) }
  def awaitTermination(timeout: Duration) { Await.ready(terminationCallbacks, timeout) }
  def awaitTermination() = awaitTermination(Duration.Inf)
  def isTerminated = terminationCallbacks.isTerminated

  def shutdown(): Unit = guardian.stop()

  //#create-scheduler
  /**
   * Create the scheduler service. This one needs one special behavior: if
   * Closeable, it MUST execute all outstanding tasks upon .close() in order
   * to properly shutdown all dispatchers.
   *
   * Furthermore, this timer service MUST throw IllegalStateException if it
   * cannot schedule a task. Once scheduled, the task MUST be executed. If
   * executed upon close(), the task may execute before its timeout.
   */
  protected def createScheduler(): Scheduler =
    dynamicAccess.createInstanceFor[Scheduler](settings.SchedulerClass, immutable.Seq(
      classOf[Config] -> settings.config,
      classOf[LoggingAdapter] -> log,
      classOf[ThreadFactory] -> threadFactory.withName(threadFactory.name + "-scheduler"))).get
  //#create-scheduler

  /*
   * This is called after the last actor has signaled its termination, i.e.
   * after the last dispatcher has had its chance to schedule its shutdown
   * action.
   */
  protected def stopScheduler(): Unit = scheduler match {
    case x: Closeable ⇒ x.close()
    case _            ⇒
  }

  private val extensions = new ConcurrentHashMap[ExtensionId[_], AnyRef]

  /**
   * Returns any extension registered to the specified Extension or returns null if not registered
   */
  @tailrec
  private def findExtension[T <: Extension](ext: ExtensionId[T]): T = extensions.get(ext) match {
    case c: CountDownLatch ⇒
      c.await(); findExtension(ext) //Registration in process, await completion and retry
    case other ⇒
      other.asInstanceOf[T] //could be a T or null, in which case we return the null as T
  }

  @tailrec
  final def registerExtension[T <: Extension](ext: ExtensionId[T]): T = {
    findExtension(ext) match {
      case null ⇒ //Doesn't already exist, commence registration
        val inProcessOfRegistration = new CountDownLatch(1)
        extensions.putIfAbsent(ext, inProcessOfRegistration) match { // Signal that registration is in process
          case null ⇒ try { // Signal was successfully sent
            ext.createExtension(this) match { // Create and initialize the extension
              case null ⇒ throw new IllegalStateException("Extension instance created as 'null' for extension [" + ext + "]")
              case instance ⇒
                extensions.replace(ext, inProcessOfRegistration, instance) //Replace our in process signal with the initialized extension
                instance //Profit!
            }
          } catch {
            case t: Throwable ⇒
              extensions.remove(ext, inProcessOfRegistration) //In case shit hits the fan, remove the inProcess signal
              throw t //Escalate to caller
          } finally {
            inProcessOfRegistration.countDown //Always notify listeners of the inProcess signal
          }
          case other ⇒ registerExtension(ext) //Someone else is in process of registering an extension for this Extension, retry
        }
      case existing ⇒ existing.asInstanceOf[T]
    }
  }

  def extension[T <: Extension](ext: ExtensionId[T]): T = findExtension(ext) match {
    case null ⇒ throw new IllegalArgumentException("Trying to get non-registered extension [" + ext + "]")
    case some ⇒ some.asInstanceOf[T]
  }

  def hasExtension(ext: ExtensionId[_ <: Extension]): Boolean = findExtension(ext) != null

  private def loadExtensions() {
    immutableSeq(settings.config.getStringList("akka.extensions")) foreach { fqcn ⇒
      dynamicAccess.getObjectFor[AnyRef](fqcn) recoverWith { case _ ⇒ dynamicAccess.createInstanceFor[AnyRef](fqcn, Nil) } match {
        case Success(p: ExtensionIdProvider) ⇒ registerExtension(p.lookup())
        case Success(p: ExtensionId[_])      ⇒ registerExtension(p)
        case Success(other)                  ⇒ log.error("[{}] is not an 'ExtensionIdProvider' or 'ExtensionId', skipping...", fqcn)
        case Failure(problem)                ⇒ log.error(problem, "While trying to load extension [{}], skipping...", fqcn)
      }
    }
  }

  override def toString: String = lookupRoot.path.root.address.toString

  override def printTree: String = {
    def printNode(node: ActorRef, indent: String): String = {
      node match {
        case wc: ActorRefWithCell ⇒
          val cell = wc.underlying
          (if (indent.isEmpty) "-> " else indent.dropRight(1) + "⌊-> ") +
            node.path.name + " " + Logging.simpleName(node) + " " +
            (cell match {
              case real: ActorCell ⇒ if (real.actor ne null) real.actor.getClass else "null"
              case _               ⇒ Logging.simpleName(cell)
            }) +
            (cell match {
              case real: ActorCell ⇒ " status=" + real.mailbox.status
              case _               ⇒ ""
            }) +
            " " + (cell.childrenRefs match {
              case ChildrenContainer.TerminatingChildrenContainer(_, toDie, reason) ⇒
                "Terminating(" + reason + ")" +
                  (toDie.toSeq.sorted mkString ("\n" + indent + "   |    toDie: ", "\n" + indent + "   |           ", ""))
              case x @ (ChildrenContainer.TerminatedChildrenContainer | ChildrenContainer.EmptyChildrenContainer) ⇒ x.toString
              case n: ChildrenContainer.NormalChildrenContainer ⇒ n.c.size + " children"
              case x ⇒ Logging.simpleName(x)
            }) +
            (if (cell.childrenRefs.children.isEmpty) "" else "\n") +
            ({
              val children = cell.childrenRefs.children.toSeq.sorted
              val bulk = children.dropRight(1) map (printNode(_, indent + "   |"))
              bulk ++ (children.lastOption map (printNode(_, indent + "    ")))
            } mkString ("\n"))
        case _ ⇒
          indent + node.path.name + " " + Logging.simpleName(node)
      }
    }
    printNode(actorFor("/"), "")
  }

  final class TerminationCallbacks extends Runnable with Awaitable[Unit] {
    private val lock = new ReentrantGuard
    private var callbacks: List[Runnable] = _ //non-volatile since guarded by the lock
    lock withGuard { callbacks = Nil }

    private val latch = new CountDownLatch(1)

    final def add(callback: Runnable): Unit = {
      latch.getCount match {
        case 0 ⇒ throw new RejectedExecutionException("Must be called prior to system shutdown.")
        case _ ⇒ lock withGuard {
          if (latch.getCount == 0) throw new RejectedExecutionException("Must be called prior to system shutdown.")
          else callbacks ::= callback
        }
      }
    }

    final def run(): Unit = lock withGuard {
      @tailrec def runNext(c: List[Runnable]): List[Runnable] = c match {
        case Nil ⇒ Nil
        case callback :: rest ⇒
          try callback.run() catch { case NonFatal(e) ⇒ log.error(e, "Failed to run termination callback, due to [{}]", e.getMessage) }
          runNext(rest)
      }
      try { callbacks = runNext(callbacks) } finally latch.countDown()
    }

    final def ready(atMost: Duration)(implicit permit: CanAwait): this.type = {
      if (atMost.isFinite()) {
        if (!latch.await(atMost.length, atMost.unit))
          throw new TimeoutException("Await termination timed out after [%s]" format (atMost.toString))
      } else latch.await()

      this
    }

    final def result(atMost: Duration)(implicit permit: CanAwait): Unit = ready(atMost)

    final def isTerminated: Boolean = latch.getCount == 0
  }
}
