接着上一篇的 tree路径匹配抽象（1），我们开始看如何对tree进行索引，akka的路径匹配包含了远程节点的匹配，这样就得引入多个通信机制（akka采用消息），为了简化，我们先假设只在一个本地tree中进行索引:

object ActorSelection {//...省略  /**   * Construct an ActorSelection from the given string representing a path   * relative to the given target. This operation has to create all the   * matching magic, so it is preferable to cache its result if the   * intention is to send messages frequently.   */  def apply(anchorRef: ActorRef, path: String): ActorSelection = apply(anchorRef, path.split("/+"))//ActorRef可以被当作当前路径的引用  /**   * Construct an ActorSelection from the given string representing a path   * relative to the given target. This operation has to create all the   * matching magic, so it is preferable to cache its result if the   * intention is to send messages frequently.   */  def apply(anchorRef: ActorRef, elements: Iterable[String]): ActorSelection = {    val compiled: immutable.IndexedSeq[SelectionPathElement] = elements.collect({      case x if !x.isEmpty ⇒        if ((x.indexOf('?') != -1) || (x.indexOf('*') != -1)) SelectChildPattern(x)        else if (x == "..") SelectParent        else SelectChildName(x)    })(scala.collection.breakOut)//直接转换为immutable.IndexedSeq[SelectionPathElement]    new ActorSelection with ScalaActorSelection {      override val anchor = anchorRef      override val path = compiled    }  }//------上面是将字符串转化为SelectionPathElement，下面则是根据SelectionPathElement进行索引tree值  /**   * INTERNAL API   * The receive logic for ActorSelectionMessage. The idea is to recursively descend as far as possible   * with local refs and hand over to that “foreign” child when we encounter it.   */  private[akka] def deliverSelection(anchor: InternalActorRef, sender: ActorRef, sel: ActorSelectionMessage): Unit =    if (sel.elements.isEmpty)      anchor.tell(sel.msg, sender)//自己    else {      val iter = sel.elements.iterator      @tailrec def rec(ref: InternalActorRef): Unit = {        ref match {          case refWithCell: ActorRefWithCell ⇒            def emptyRef = new EmptyLocalActorRef(refWithCell.provider, anchor.path / sel.elements.map(_.toString),              refWithCell.underlying.system.eventStream)            iter.next() match {              case SelectParent ⇒                val parent = ref.getParent                if (iter.isEmpty)                  parent.tell(sel.msg, sender)                else                  rec(parent)              case SelectChildName(name) ⇒                val child = refWithCell.getSingleChild(name)                if (child == Nobody) {                  // don't send to emptyRef after wildcard fan-out                  if (!sel.wildcardFanOut) emptyRef.tell(sel, sender)                } else if (iter.isEmpty)                  child.tell(sel.msg, sender)                else                  rec(child)              case p: SelectChildPattern ⇒                // fan-out when there is a wildcard                val chldr = refWithCell.children                if (iter.isEmpty) {                  // leaf                  val matchingChildren = chldr.filter(c ⇒ p.pattern.matcher(c.path.name).matches)                  if (matchingChildren.isEmpty && !sel.wildcardFanOut)                    emptyRef.tell(sel, sender)                  else                    matchingChildren.foreach(_.tell(sel.msg, sender))                } else {                  val matchingChildren = chldr.filter(c ⇒ p.pattern.matcher(c.path.name).matches)                  // don't send to emptyRef after wildcard fan-out                   if (matchingChildren.isEmpty && !sel.wildcardFanOut)                    emptyRef.tell(sel, sender)                  else {                    val m = sel.copy(elements = iter.toVector,                      wildcardFanOut = sel.wildcardFanOut || matchingChildren.size > 1)                    matchingChildren.foreach(c ⇒ deliverSelection(c.asInstanceOf[InternalActorRef], sender, m))                  }                }            }          //case _ ⇒            // foreign ref, continue by sending ActorSelectionMessage to it with remaining elements            //ref.tell(sel.copy(elements = iter.toVector), sender)        }      }      rec(anchor)    }}

既然path允许正则索引，那么path最好有个命名规则，akka的命名规则为：

/**   * This Regular Expression is used to validate a path element (Actor Name).   * Since Actors form a tree, it is addressable using an URL, therefore an Actor Name has to conform to:   * http://www.ietf.org/rfc/rfc2396.txt   */  val ElementRegex = """(?:[-\w:@&=+,.!~*'_;]|%\p{XDigit}{2})(?:[-\w:@&=+,.!~*'$_;]|%\p{XDigit}{2})*""".r