上一遍博客中，我们分析了网络链接建立的过程，一旦建立就可以正常的收发消息了。发送消息的细节不再分析，因为对于本地的actor来说这个过程相对简单，它只是创立链接然后给指定的netty网路服务发送消息就好了。接收消息就比较麻烦了，因为这对于actor来说是透明的，netty收到消息后如何把消息分发给指定的actor呢？这个分发的过程值得研究研究。
之前分析过，在监听创立的过程中，有一个对象非常关键：TcpServerHandler。它负责链接建立、消息收发等功能。TcpServerHandler继承了ServerHandler
private[netty] abstract class ServerHandler( protected final val transport: NettyTransport, private final val associationListenerFuture: Future[AssociationEventListener]) extends NettyServerHelpers with CommonHandlers
ServerHandler继承了NettyServerHelpers
private[netty] trait NettyServerHelpers extends SimpleChannelUpstreamHandler with NettyHelpers {
final override def messageReceived(ctx: ChannelHandlerContext, e: MessageEvent): Unit = {
super.messageReceived(ctx, e)
onMessage(ctx, e)
}
final override def exceptionCaught(ctx: ChannelHandlerContext, e: ExceptionEvent): Unit = transformException(ctx, e)
final override def channelConnected(ctx: ChannelHandlerContext, e: ChannelStateEvent): Unit = {
super.channelConnected(ctx, e)
onConnect(ctx, e)
}
final override def channelOpen(ctx: ChannelHandlerContext, e: ChannelStateEvent): Unit = {
super.channelOpen(ctx, e)
onOpen(ctx, e)
}
final override def channelDisconnected(ctx: ChannelHandlerContext, e: ChannelStateEvent): Unit = {
super.channelDisconnected(ctx, e)
onDisconnect(ctx, e)
}
}
很明显NettyServerHelpers有一个messageReceived应该就是收到消息时回调的方法，那onMessage在哪里实现呢？TcpServerHandler还继承了TcpHandlers，我们来看看TcpHandlers的onMessage方法。
override def onMessage(ctx: ChannelHandlerContext, e: MessageEvent): Unit = {
val bytes: Array[Byte] = e.getMessage.asInstanceOf[ChannelBuffer].array()
if (bytes.length > 0) notifyListener(e.getChannel, InboundPayload(ByteString(bytes)))
}
它最终用InboundPayload封装了收到的数据，并调用了ChannelLocalActor.notifyListener方法。
private[remote] object ChannelLocalActor extends ChannelLocal[Option[HandleEventListener]] {
override def initialValue(channel: Channel): Option[HandleEventListener] = None
def notifyListener(channel: Channel, msg: HandleEvent): Unit = get(channel) foreach { _ notify msg }
}
ChannelLocalActor可以先把它理解成一个ThreadLocal对象，其他的技术细节读者可以自行谷歌。notifyListener只调用了get，那具体是在哪里set的呢？通过channel变量get到的Option[HandleEventListener]又是在哪里赋值的呢？
override def registerListener(
channel: Channel,
listener: HandleEventListener,
msg: ChannelBuffer,
remoteSocketAddress: InetSocketAddress): Unit = ChannelLocalActor.set(channel, Some(listener))
很显然是在registerListener时set的值，那registerListener在哪里调用呢？如果读过上一篇的文章，一定会知道ServerHandler.initInbound函数，这个函数调用了CommonHandlers.init
final protected def init(channel: Channel, remoteSocketAddress: SocketAddress, remoteAddress: Address, msg: ChannelBuffer)(
op: (AssociationHandle ⇒ Any)): Unit = {
import transport._
NettyTransport.addressFromSocketAddress(channel.getLocalAddress, schemeIdentifier, system.name, Some(settings.Hostname), None) match {
case Some(localAddress) ⇒
val handle = createHandle(channel, localAddress, remoteAddress)
handle.readHandlerPromise.future.foreach {
listener ⇒
registerListener(channel, listener, msg, remoteSocketAddress.asInstanceOf[InetSocketAddress])
channel.setReadable(true)
}
op(handle)
case _ ⇒ NettyTransport.gracefulClose(channel)
}
}
看到没，上面的函数中调用了registerListener，那listener具体在哪里创建的呢，或者是哪个变量对应的值呢？这就需要研究createHandle对象及其返回值是什么了。经过分析还是找到了TcpHandlers这个trait，里面有createHandle的具体实现。
override def createHandle(channel: Channel, localAddress: Address, remoteAddress: Address): AssociationHandle =
new TcpAssociationHandle(localAddress, remoteAddress, transport, channel)
TcpAssociationHandle源码如下
private[remote] class TcpAssociationHandle(
val localAddress: Address,
val remoteAddress: Address,
val transport: NettyTransport,
private val channel: Channel)
extends AssociationHandle {
import transport.executionContext
override val readHandlerPromise: Promise[HandleEventListener] = Promise()
override def write(payload: ByteString): Boolean =
if (channel.isWritable && channel.isOpen) {
channel.write(ChannelBuffers.wrappedBuffer(payload.asByteBuffer))
true
} else false
override def disassociate(): Unit = NettyTransport.gracefulClose(channel)
}
由此可见，readHandlerPromise是一个Promise[HandleEventListener]，并没有具体赋值的逻辑，这就要去使用TcpAssociationHandle的相关代码找相关的赋值逻辑了。TcpAssociationHandle在哪里使用呢？还记得handleInboundAssociation建立连接的过程吗？它最终调用了createAndRegisterEndpoint
private def createAndRegisterEndpoint(handle: AkkaProtocolHandle): Unit = {
val writing = settings.UsePassiveConnections && !endpoints.hasWritableEndpointFor(handle.remoteAddress)
eventPublisher.notifyListeners(AssociatedEvent(handle.localAddress, handle.remoteAddress, inbound = true))
val endpoint = createEndpoint(
handle.remoteAddress,
handle.localAddress,
transportMapping(handle.localAddress),
settings,
Some(handle),
writing)
if (writing)
endpoints.registerWritableEndpoint(handle.remoteAddress, Some(handle.handshakeInfo.uid), endpoint)
else {
endpoints.registerReadOnlyEndpoint(handle.remoteAddress, endpoint, handle.handshakeInfo.uid)
if (!endpoints.hasWritableEndpointFor(handle.remoteAddress))
endpoints.removePolicy(handle.remoteAddress)
}
}
createAndRegisterEndpoint拿着一个连接实例AkkaProtocolHandle创建了一个endpoint，其中有个很关键的字段writing，它是true还是false呢？UsePassiveConnections默认为true，且经分析!endpoints.hasWritableEndpointFor(handle.remoteAddress)应该也是true，所以writing是true
# Reuse inbound connections for outbound messages
use-passive-connections = on
ReliableDeliverySupervisor其实是对EndpointWriter的代理。在创建ReliableDeliverySupervisor的过程中AkkaProtocolHandle是作为参数传入的，也就监听到连接消息后创建的handle。而在创建EndpointWriter的过程中，这个handle又是作为第一个参数传入了EndpointWriter。我们来看看EndpointWriter是如何使用这个handle的。
override def preStart(): Unit = {
handle match {
case Some(h) ⇒
reader = startReadEndpoint(h)
case None ⇒
transport.associate(remoteAddress, refuseUid).map(Handle(_)) pipeTo self
}
}
在preStart时，handle应该是有值的，如果有值，就调用了startReadEndpoint(h)方法。
private def startReadEndpoint(handle: AkkaProtocolHandle): Some[ActorRef] = {
val newReader =
context.watch(context.actorOf(
RARP(context.system).configureDispatcher(EndpointReader.props(localAddress, remoteAddress, transport, settings, codec,
msgDispatch, inbound, handle.handshakeInfo.uid, reliableDeliverySupervisor, receiveBuffers)).withDeploy(Deploy.local),
"endpointReader-" + AddressUrlEncoder(remoteAddress) + "-" + readerId.next()))
handle.readHandlerPromise.success(ActorHandleEventListener(newReader))
Some(newReader)
}
startReadEndpoint做了什么呢？它又创建了一个Actor：EndpointReader！！！好多中间的actor创建。创建之后，调用了handle.readHandlerPromise.success(ActorHandleEventListener(newReader))给handle.readHandlerPromise。还记得ActorHandleEventListener吗，它就是把收到的消息转发了对应的actor，此处就是newReader。
EndpointReader如何处理InboundPayload消息呢？首先解码收到的消息，然后给创建它的reliableDelivery发送ack消息。
override def decodeMessage(
raw: ByteString,
provider: RemoteActorRefProvider,
localAddress: Address): (Option[Ack], Option[Message]) = {
val ackAndEnvelope = AckAndEnvelopeContainer.parseFrom(raw.toArray)
val ackOption = if (ackAndEnvelope.hasAck) {
import scala.collection.JavaConverters._
Some(Ack(SeqNo(ackAndEnvelope.getAck.getCumulativeAck), ackAndEnvelope.getAck.getNacksList.asScala.map(SeqNo(_)).toSet))
} else None
val messageOption = if (ackAndEnvelope.hasEnvelope) {
val msgPdu = ackAndEnvelope.getEnvelope
Some(Message(
recipient = provider.resolveActorRefWithLocalAddress(msgPdu.getRecipient.getPath, localAddress),
recipientAddress = AddressFromURIString(msgPdu.getRecipient.getPath),
serializedMessage = msgPdu.getMessage,
senderOption =
if (msgPdu.hasSender) OptionVal(provider.resolveActorRefWithLocalAddress(msgPdu.getSender.getPath, localAddress))
else OptionVal.None,
seqOption =
if (msgPdu.hasSeq) Some(SeqNo(msgPdu.getSeq)) else None))
} else None
(ackOption, messageOption)
}
上面是decodeMessage的源码，消息最终被decode成了Message对象。
final case class Message(
recipient: InternalActorRef,
recipientAddress: Address,
serializedMessage: SerializedMessage,
senderOption: OptionVal[ActorRef],
seqOption: Option[SeqNo]) extends HasSequenceNumber {
def reliableDeliveryEnabled = seqOption.isDefined
override def seq: SeqNo = seqOption.get
}
默认情况下reliableDeliveryEnabled是false的，因为发送出去的msgPdu是没有getSeq的，因为默认的tcp是保证消息发送的。所以EndpointReader收到消息后调用了msgDispatch.dispatch把消息分发出去了。根据上下文msgDispatch是在EndpointWriter创建的，代码如下。
val msgDispatch = new DefaultMessageDispatcher(extendedSystem, provider, markLog)
DefaultMessageDispatcher.dispatch不再具体分析，它就是把消息tell给了Message.recipient，而recipient是一个InternalActorRef，对的，你没有看错，这就是一个InternalActorRef，是不是很神奇，payload解码之后直接就有目标actor的InternalActorRef了？？！！那我们就得好好看看是如何对payload进行解码的了。
在decodeMessage函数中，有两处代码非常关键：“recipient = provider.resolveActorRefWithLocalAddress(msgPdu.getRecipient.getPath, localAddress)”、“if (msgPdu.hasSender) OptionVal(provider.resolveActorRefWithLocalAddress(msgPdu.getSender.getPath, localAddress))”。都是调用provider.resolveActorRefWithLocalAddress函数通过actor的path转化成了对应actor的ActorRef，很显然provider就是RemoteActorRefProvider。
/**
* INTERNAL API
* Called in deserialization of incoming remote messages where the correct local address is known.
*/
private[akka] def resolveActorRefWithLocalAddress(path: String, localAddress: Address): InternalActorRef = {
path match {
case ActorPathExtractor(address, elems) ⇒
if (hasAddress(address))
local.resolveActorRef(rootGuardian, elems)
else try {
new RemoteActorRef(transport, localAddress, RootActorPath(address) / elems, Nobody, props = None, deploy = None)
} catch {
case NonFatal(e) ⇒
log.warning("Error while resolving ActorRef [{}] due to [{}]", path, e.getMessage)
new EmptyLocalActorRef(this, RootActorPath(address) / elems, eventStream)
}
case _ ⇒
log.debug("Resolve (deserialization) of unknown (invalid) path [{}], using deadLetters.", path)
deadLetters
}
}
resolveActorRefWithLocalAddress也很简单，如果目标address包含在本机范围，就调用local.resolveActorRef，否则就创建RemoteActorRef，关于RemoteActorRef的作用这里不再讲解。
/**
* INTERNAL API
*/
private[akka] def resolveActorRef(ref: InternalActorRef, pathElements: Iterable[String]): InternalActorRef =
if (pathElements.isEmpty) {
log.debug("Resolve (deserialization) of empty path doesn't match an active actor, using deadLetters.")
deadLetters
} else ref.getChild(pathElements.iterator) match {
case Nobody ⇒
if (log.isDebugEnabled)
log.debug(
"Resolve (deserialization) of path [{}] doesn't match an active actor. " +
"It has probably been stopped, using deadLetters.",
pathElements.mkString("/"))
new EmptyLocalActorRef(system.provider, ref.path / pathElements, eventStream)
case x ⇒ x
}
LocalActorRefProvider.resolveActorRef也比较简单，就是调用ref.getChild，而ref是LocalActorRefProvider.rootGuardian，其实就是在本地范围内从root向下查找对应的ActorRef。
至此remote模式下收发消息的过程我们就分析清楚了，如果还有不清楚的小伙伴就再把之前的文章复习一下，当然还可以在下面留言讨论。