2.1.4、SparkEnv中创建BroadcastManager

// 此处只是声明， 只有调用initialize， 才会生效
val broadcastManager = new BroadcastManager(isDriver, conf, securityManager)

  // Called by SparkContext or Executor before using Broadcast
  private def initialize() {
    synchronized {
      if (!initialized) {
        broadcastFactory = new TorrentBroadcastFactory
        broadcastFactory.initialize(isDriver, conf, securityManager)
        initialized = true
      }
    }
  }