在SparkEnv中創建BroadcastManager，

// 此處只是聲明， 只有調用initialize， 才會生效
val broadcastManager = new BroadcastManager(isDriver, conf, securityManager)

initialize()

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

2.1.4、SparkEnv中創建BroadcastManager