Scala Actor是Scala里多线程的基础,核心思想是用消息传递来进行线程间的信息共享和同步。
Scala Actor线程模型可以这样理解:所有Actor共享一个线程池,总的线程个数可以配置,也可以根据CPU个数决定;当一个Actor启动之后,Scala分配一个线程给它使用,如果使用receive模型,这个线程就一直为该Actor所有,如果使用react模型,Scala执行完react方法后抛出异常,则该线程就可以被其它Actor使用。
- def start(): Actor = synchronized {
- // Reset various flags.
- //
- // Note that we do *not* reset `trapExit`. The reason is that
- // users should be able to set the field in the constructor
- // and before `act` is called.
-
- exitReason = 'normal
- exiting = false
- shouldExit = false
-
- scheduler execute {
- ActorGC.newActor(Actor.this)
- (new Reaction(Actor.this)).run()
- }
-
- this
- }
复制代码
其中Reaction实现Runnable接口,scheduler基本相当于是一个线程池,所以调用start方法之后会有一个线程来为该Actor服务。
使用receive模型。
- def receive[R](f: PartialFunction[Any, R]): R = {
- assert(Actor.self == this, "receive from channel belonging to other actor")
- this.synchronized {
- if (shouldExit) exit() // links
- val qel = mailbox.extractFirst((m: Any) => f.isDefinedAt(m))
- if (null eq qel) {
- waitingFor = f.isDefinedAt
- isSuspended = true
- suspendActor()
- } else {
- received = Some(qel.msg)
- sessions = qel.session :: sessions
- }
- waitingFor = waitingForNone
- isSuspended = false
- }
- val result = f(received.get)
- sessions = sessions.tail
- result
复制代码
如果当前mailbox里面没有可以处理的消息,调用suspendActor,该方法会调用wait;如果有消息,这调用PartialFunction进行处理。
使用react模型。
- def react(f: PartialFunction[Any, Unit]): Nothing = {
- assert(Actor.self == this, "react on channel belonging to other actor")
- this.synchronized {
- if (shouldExit) exit() // links
- val qel = mailbox.extractFirst((m: Any) => f.isDefinedAt(m))
- if (null eq qel) {
- waitingFor = f.isDefinedAt
- continuation = f
- isDetached = true
- } else {
- sessions = List(qel.session)
- scheduleActor(f, qel.msg)
- }
- throw new SuspendActorException
- }
复制代码
如果当前mailbox没有可以处理的消息,设置waitingFor和continuation,这两个变量会在接收到消息的时候使用;如果有消息,则调用scheduleActor,该方法会在线程池里选择一个新的线程来处理,具体的处理方法也是由PartialFunction决定。不管是哪条路径,react都会立即返回,或者说是立即抛出异常,结束该线程的执行,这样该线程就可以被其它Actor使用。
再来看看接收消息的处理代码。
- def send(msg: Any, replyTo: OutputChannel[Any]) = synchronized {
- if (waitingFor(msg)) {
- received = Some(msg)
-
- if (isSuspended)
- sessions = replyTo :: sessions
- else
- sessions = List(replyTo)
-
- waitingFor = waitingForNone
-
- if (!onTimeout.isEmpty) {
- onTimeout.get.cancel()
- onTimeout = None
- }
-
- if (isSuspended)
- resumeActor()
- else // assert continuation != null
- scheduler.execute(new Reaction(this, continuation, msg))
- } else {
- mailbox.append(msg, replyTo)
- }
复制代码
如果当前没有在等待消息或者接收到的消息不能处理,就丢到mailbox里去;相反,则进行消息的处理。这里对于receive模型和react模型就有了分支:如果isSuspended为true,表示是receive模型,并且线程在wait,就调用resumeActor,该方法会调用notify;否则就是react模型,同样在线程池里选择一个线程进行处理。
这样,相信大家对Scala Actor就有了一个基本的认识。