SparkContext的初始化

本栏目基于spark2.1.1

SparkDriver 用于提交用户的应用程序，

一、SparkConf

负责SparkContext的配置参数加载， 主要通过ConcurrentHashMap来维护各种spark.*的配置属性

class SparkConf(loadDefaults: Boolean) extends Cloneable with Logging with Serializable {
    import SparkConf._

    /** Create a SparkConf that loads defaults from system properties and the classpath */
    def this() = this(true)

    /**  维护一个ConcurrentHashMap 来存储spark配置  */
    private val settings = new ConcurrentHashMap[String, String]()
    if (loadDefaults) {
        loadFromSystemProperties(false)
    }

    /**
     * 加载spark.*的配置 
     */
    private[spark] def loadFromSystemProperties(silent: Boolean): SparkConf = {
        // Load any spark.* system properties, 只加载spark.*的配置
        for ((key, value)