七周七语言：Scala Day 3

这一天的内容还是比较丰富的，不仅介绍了Scala中的对于XML的处理、正则，更有比较重要的“并发”。

Scala支持并发的方式代表了并行编程领域的一次重大进步。actor模型和线程池都是很受欢迎的改进，并且无需可变状态的并发应用设计能力也绝对是一个巨大的进步。－－摘自《七周七语言》

• 对于sizer程序，如果你没有为每一个要跟踪的链接创建一个新的actor，这段程序的性能会发生怎样的变化？

  感觉会很慢，因为每个actor都包含了react和receive方法，如果没有对于每个链接都创建actor的话，那和sequentially的方法就没有什么区别了。

• 修改sizer程序，增加一个计算页面上链接总和的消息。

  import scala.io._

  import scala.actors._

  import Actor._

  object PageLoader {

      def getPageSize(url: String) = Source.fromURL(url).mkString.length

      // get the contents of the the URL

      def getPageContent(url: String) = Source.fromURL(url).mkString

      // regular expression of a link

      def getUrlNumber(url: String) = {

          val reg = """^(?i)<a[^>]+?href""".r

          reg.findAllIn(getPageContent(url)).size

      }

  }

  // the List of urls

  val urls = List("http://www.baidu.com/",

                  "http://www.seu.edu.cn/",

                  "http://www.fantasyshao.tk/"

                 )

  // Cauculate the time of running

  def timeMethod(method: ()=> Unit) = {

      val start = System.nanoTime

      method()

      val end = System.nanoTime

      println("Method took " + (end - start)/1000000000.0 + " seconds.")

  }

  // not using actors

  def getPageSizeSequentially() = {

      for(url <- urls) {

          println("Size for " + url + ": " + PageLoader.getPageSize(url))

      }

      for(url <- urls) {

          println("Links in " + url + ": " + PageLoader.getUrlNumber(url))

      }

  }

  // concurrency method

  def getPageSizeConcurrently() = {

      val caller = self

      for(url <- urls) {

          actor { caller ! ("Size", url, PageLoader.getPageSize(url))}

          actor { caller ! ("Number", url, PageLoader.getUrlNumber(url))}

      }

      // two actor for each url, so the size became lager

      for(i <- 1 to (urls.size * 2)) {

          receive {

              case ("Size", url, size) =>

                  println("Size for " + url + ": " + size)

              case ("Number", url, number) =>

                  println("Number of " + url + ": " + number)

          }

      }

  }

  println("Sequential run:")

  timeMethod { getPageSizeSequentially }

  println("Concurrent run:")

  timeMethod { getPageSizeConcurrently }