1. UnboundedMailbox is the default unbounded MailboxType used by Akka Actors
”无界邮箱“ 是akka actors默认使用的邮箱，
UnboundedMailbox继承了MailboxType
/**
* MailboxType is a factory to create MessageQueues for an optionally
* provided ActorContext.
*
* <b>Possibly Important Notice</b>
*
* When implementing a custom mailbox type, be aware that there is special
* semantics attached to `system.actorOf()` in that sending to the returned
* ActorRef may—for a short period of time—enqueue the messages first in a
* dummy queue. Top-level actors are created in two steps, and only after the
* guardian actor has performed that second step will all previously sent
* messages be transferred from the dummy queue into the real mailbox.
*/
trait MailboxType {
def create(owner: Option[ActorRef], system: Option[ActorSystem]): MessageQueue
}
和ProducesMessageQueue
trait ProducesMessageQueue[T <: MessageQueue]
MailboxType特质的create方法接受两个参数owner和system，owner表示“消息队列”的所有者，system表示 ？？？
UnboundedMailbox的定义如下：
/**
* UnboundedMailbox is the default unbounded MailboxType used by Akka Actors.
*/
case class UnboundedMailbox() extends MailboxType with ProducesMessageQueue[UnboundedMailbox.MessageQueue] {
// 使用this关键字来调用构造函数。
// 它演示了如何从其他构造函数调用构造函数。必须确保this必须放在构造函数中的第一个语句，同时调用其他构造函数this()，否则编译器会抛出错误。
// “=”等号右边调用的this()是指向的UnboundedMailBox()类
def this(settings: ActorSystem.Settings, config: Config) = this()
// 重写MailboxType的create()方法，并创建UnboundedMailbox.MessgaeQueue对象
final override def create(owner: Option[ActorRef], system: Option[ActorSystem]): MessageQueue =
new UnboundedMailbox.MessageQueue
}
UnboundedMailbox中的create()方法中创建的UnboundedMailbox.MessgaeQueue对象是
object UnboundedMailbox {
class MessageQueue extends ConcurrentLinkedQueue[Envelope] with UnboundedQueueBasedMessageQueue {
final def queue: Queue[Envelope] = this
}
}
MessageQueue继承了ConCurrentLinkedQueue，并且ConCurrentLinkedQueue是存放的Envelope
final case class Envelope private (val message: Any, val sender: ActorRef)
(信封)对象，信封对象中包含一个message消息和发送者sender的ActorRef。
此外，MessageQueue还with了一个UnboundedQueueBasedMessageQueue
/**
* BoundedMessageQueueSemantics adds bounded semantics to a QueueBasedMessageQueue,
* i.e. blocking enqueue with timeout.
*/
trait BoundedMessageQueueSemantics {
def pushTimeOut: Duration
}
trait UnboundedQueueBasedMessageQueue extends QueueBasedMessageQueue with UnboundedMessageQueueSemantics {
// 入队，这里是入队了一个信封，貌似receiver没有用到？不明白为什么要有这个receiver，可能是为了明显标记是发给那个Actor的吧
def enqueue(receiver: ActorRef, handle: Envelope): Unit = queue add handle
// 出队，出队就是从队列中取出一个元素（一封信）
def dequeue(): Envelope = queue.poll()
}
UnboundedQueueBasedMessageQueue中实现了从QueueBasedMessageQueue继承来的enqueue()和dequeue()方法。更确切的话是从MessageQueue继承来的，因为QueueBasedMessageQueue是继承了MessageQueue，代码如下:
/**
* This is a marker trait for message queues which support multiple consumers,
* as is required by the BalancingDispatcher.
*/
trait MultipleConsumerSemantics
/**
* A QueueBasedMessageQueue is a MessageQueue backed by a java.util.Queue.
*/
trait QueueBasedMessageQueue extends MessageQueue with MultipleConsumerSemantics {
def queue: Queue[Envelope]
// 消息的数量 （队列的元素个数）
def numberOfMessages = queue.size
// 是否包含消息 （队列是否为空）
def hasMessages = !queue.isEmpty
// 清空消息 （清空队列）
def cleanUp(owner: ActorRef, deadLetters: MessageQueue): Unit = {
if (hasMessages) {
// 获取第一个消息
var envelope = dequeue
while (envelope ne null) {
// 如果消息不为空，则不断向deadLetters发送消息
// deadLetters也是一个消息队列actor.dispatcher.mailboxes.deadLetterMailbox.messageQueue
deadLetters.enqueue(owner, envelope)
envelope = dequeue
}
}
}
}
从QueueBasedMessageQueue的定义来看，其中包含了几个重要的方法。还定义了一个queue，回顾一下UnboundedMailbox的定义
object UnboundedMailbox {
class MessageQueue extends ConcurrentLinkedQueue[Envelope] with UnboundedQueueBasedMessageQueue {
final def queue: Queue[Envelope] = this
}
}
其中的queue是重写了QueueBasedMessageQueue中定义的queue，def queue: Queue[Envelope]。
3. 邮箱是akka里面是怎么使用的？
如果我们是使用的ActorRef去创建一个actor，例如:
val testActor: ActorRef = context.actorOf(
TestActor.props().withDispatcher("test-actor-dispatcher"),
TestActor.Name)
