Akka源码分析-Actor创建(续)

在上一遍博客中，我们已经分析了actor创建的大致过程，但只是涉及到了Dipatcher/Mailbox/ActorCell/InternalActorRef等对象的创建，并没有介绍我们自定义的继承Actor特质的类如何完成初始化。这篇文章对这一部分内容进行简单的补充。
在akka.actor.dungeon.init代码中，有一段代码我们当时没有分析，此处对此代码进行深入分析，然后才能找到Actor子类完成创建的真实过程。

上面是init的代码片段，其中有一个局部变量createMessage，根据前后分析，它的值应该是Create这个case
class。最后mailbox.systemEnqueue(self,
createMessage)这个代码给actor对应的邮箱发送了该消息。

/**
 * INTERNAL API
 */
@SerialVersionUID(1L)
private[akka] final case class Create(failure: Option[ActorInitializationException]) extends SystemMessage // sent to self from Dispatcher.register

根据Create类名以及前后上下文分析，这应该是指示Actor完成初始化的。那么我们要分析一下actor是如何对该消息响应的。那么究竟是哪段代码对这个消息进行响应的呢？
如果读过之前的文章，你肯定能想起来Mailbox在循环处理消息时，有一个processAllSystemMessages方法，这个方法里面调用了actor的systemInvoke方法。具体源码如下：

/**
   * Will at least try to process all queued system messages: in case of
   * failure simply drop and go on to the next, because there is nothing to
   * restart here (failure is in ActorCell somewhere …). In case the mailbox
   * becomes closed (because of processing a Terminate message), dump all
   * already dequeued message to deadLetters.
   */
  final def processAllSystemMessages() {
    var interruption: Throwable = null
    var messageList = systemDrain(SystemMessageList.LNil)
    while ((messageList.nonEmpty) && !isClosed) {
      val msg = messageList.head
      messageList = messageList.tail
      msg.unlink()
      if (debug) println(actor.self + " processing system message " + msg + " with " + actor.childrenRefs)
      // we know here that systemInvoke ensures that only "fatal" exceptions get rethrown
      actor systemInvoke msg
      if (Thread.interrupted())
        interruption = new InterruptedException("Interrupted while processing system messages")
      // don’t ever execute normal message when system message present!
      if ((messageList.isEmpty) && !isClosed) messageList = systemDrain(SystemMessageList.LNil)
    }
    /*
     * if we closed the mailbox, we must dump the remaining system messages
     * to deadLetters (this is essential for DeathWatch)
     */
    // 忽略剩余源码
  }

我们来研究一下systemInvoke的代码

/*
   * MESSAGE PROCESSING
   */
  //Memory consistency is handled by the Mailbox (reading mailbox status then processing messages, then writing mailbox status
  final def systemInvoke(message: SystemMessage): Unit = {
    /*
     * When recreate/suspend/resume are received while restarting (i.e. between
     * preRestart and postRestart, waiting for children to terminate), these
     * must not be executed immediately, but instead queued and released after
     * finishRecreate returns. This can only ever be triggered by
     * ChildTerminated, and ChildTerminated is not one of the queued message
     * types (hence the overwrite further down). Mailbox sets message.next=null
     * before systemInvoke, so this will only be non-null during such a replay.
     */

    def calculateState: Int =
      if (waitingForChildrenOrNull ne null) SuspendedWaitForChildrenState
      else if (mailbox.isSuspended) SuspendedState
      else DefaultState

    @tailrec def sendAllToDeadLetters(messages: EarliestFirstSystemMessageList): Unit =
      if (messages.nonEmpty) {
        val tail = messages.tail
        val msg = messages.head
        msg.unlink()
        provider.deadLetters ! msg
        sendAllToDeadLetters(tail)
      }

    def shouldStash(m: SystemMessage, state: Int): Boolean =
      (state: @switch) match {
        case DefaultState                  ⇒ false
        case SuspendedState                ⇒ m.isInstanceOf[StashWhenFailed]
        case SuspendedWaitForChildrenState ⇒ m.isInstanceOf[StashWhenWaitingForChildren]
      }

    @tailrec
    def invokeAll(messages: EarliestFirstSystemMessageList, currentState: Int): Unit = {
      val rest = messages.tail
      val message = messages.head
      message.unlink()
      try {
        message match {
          case message: SystemMessage if shouldStash(message, currentState) ⇒ stash(message)
          case f: Failed ⇒ handleFailure(f)
          case DeathWatchNotification(a, ec, at) ⇒ watchedActorTerminated(a, ec, at)
          case Create(failure) ⇒ create(failure)
          case Watch(watchee, watcher) ⇒ addWatcher(watchee, watcher)
          case Unwatch(watchee, watcher) ⇒ remWatcher(watchee, watcher)
          case Recreate(cause) ⇒ faultRecreate(cause)
          case Suspend() ⇒ faultSuspend()
          case Resume(inRespToFailure) ⇒ faultResume(inRespToFailure)
          case Terminate() ⇒ terminate()
          case Supervise(child, async) ⇒ supervise(child, async)
          case NoMessage ⇒ // only here to suppress warning
        }
      } catch handleNonFatalOrInterruptedException { e ⇒
        handleInvokeFailure(Nil, e)
      }
      val newState = calculateState
      // As each state accepts a strict subset of another state, it is enough to unstash if we "walk up" the state
      // chain
      val todo = if (newState < currentState) unstashAll() reverse_::: rest else rest

      if (isTerminated) sendAllToDeadLetters(todo)
      else if (todo.nonEmpty) invokeAll(todo, newState)
    }

    invokeAll(new EarliestFirstSystemMessageList(message), calculateState)
  }

由于我们只是准备分析actor的创建过程，所以上面的代码，我们只关注对Create消息的处理：create(failure)。也就是说调用了create函数。

protected def create(failure: Option[ActorInitializationException]): Unit = {

    def clearOutActorIfNonNull(): Unit = {
      if (actor != null) {
        clearActorFields(actor, recreate = false)
        actor = null // ensure that we know that we failed during creation
      }
    }

    failure.foreach { throw _ }

    try {
      val created = newActor()
      actor = created
      created.aroundPreStart()
      checkReceiveTimeout
      if (system.settings.DebugLifecycle) publish(Debug(self.path.toString, clazz(created), "started (" + created + ")"))
    } catch {
      case e: InterruptedException ⇒
        clearOutActorIfNonNull()
        Thread.currentThread().interrupt()
        throw ActorInitializationException(self, "interruption during creation", e)
      case NonFatal(e) ⇒
        clearOutActorIfNonNull()
        e match {
          case i: InstantiationException ⇒ throw ActorInitializationException(
            self,
            """exception during creation, this problem is likely to occur because the class of the Actor you tried to create is either,
               a non-static inner class (in which case make it a static inner class or use Props(new ...) or Props( new Creator ... )
               or is missing an appropriate, reachable no-args constructor.
              """, i.getCause)
          case x ⇒ throw ActorInitializationException(self, "exception during creation", x)
        }
    }
  }

我们来分析一下这个create函数。其中主要的逻辑都在try中，首先调用newActor函数，创建了Actor实例，然后赋值给actor字段。actor字段我们已经知道，这是ActorCell的最终actor实例。

/*
   * ACTOR INSTANCE HANDLING
   */

  //This method is in charge of setting up the contextStack and create a new instance of the Actor
  protected def newActor(): Actor = {
    contextStack.set(this :: contextStack.get)
    try {
      behaviorStack = emptyBehaviorStack
      val instance = props.newActor()

      if (instance eq null)
        throw ActorInitializationException(self, "Actor instance passed to actorOf can't be 'null'")

      // If no becomes were issued, the actors behavior is its receive method
      behaviorStack = if (behaviorStack.isEmpty) instance.receive :: behaviorStack else behaviorStack
      instance
    } finally {
      val stackAfter = contextStack.get
      if (stackAfter.nonEmpty)
        contextStack.set(if (stackAfter.head eq null) stackAfter.tail.tail else stackAfter.tail) // pop null marker plus our context
    }
  }

newActor函数源码如上，抛去其他代码，该函数调用了props.newActor创建了最终的Actor实例，也就是我们自定义的Actor子类。通过源码注释我们知道behaviorStack是actor当前行为的一个栈。如果读者用过become的话，对这段代码应该比较好理解。我们在actor内部使用become方法改变当前actor实例的时候，其实是把新的receive函数压入栈顶，mailbox在调用receive时，其实是取出当前栈顶的receive函数进行处理的。当然这是akka以前版本的默认行为。为什么这样说呢？因为新版本默认行为就是简单的把最新的receive函数替换旧receive函数，如果想恢复旧receive函数，需要开发者在编码时，再次调用become用旧receive函数替换当前receive。为什么要这么做？当然是为了防止开发者恶意或者无意中胡乱调用become，造成栈溢出喽。
props.newActor我们不再深入分析，这应该就是通过反射创建Actor特质的子类，也就是我们自定义的actor。
至此，我们自定义的actor就真正完成了初始化。细心的读者一定会发现，就连actor最终的实例化，都是异步的。因为newActor是通过Create消息触发的，而Mailbox对所有消息的处理都是在单独的线程处理的。如果actor的创建过程中有一些线程不安全的代码，就需要注意喽。