scala class和object的区别

Scala中类对象中不可有静态变量和静态方法，但是提供了“伴侣对象”的功能：在和类的同一个文件中定义同名的Object对象：(须在同一文件中；main方法定义在Object对象中)

private[spark] class Client(

    val args: ClientArguments,

    val hadoopConf: Configuration,

    val sparkConf: SparkConf)

  extends Logging {...}

object Client extends Logging {

  def main(argStrings: Array[String]) {

    if (!sys.props.contains("SPARK_SUBMIT")) {

      logWarning("WARNING: This client is deprecated and will be removed in a " +

        "future version of Spark. Use ./bin/spark-submit with \"--master yarn\"")

    }

    // Set an env variable indicating we are running in YARN mode.

    // Note that any env variable with the SPARK_ prefix gets propagated to all (remote) processes

    System.setProperty("SPARK_YARN_MODE", "true")

    val sparkConf = new SparkConf

    val args = new ClientArguments(argStrings, sparkConf)

    // to maintain backwards-compatibility

    if (!Utils.isDynamicAllocationEnabled(sparkConf)) {

      sparkConf.setIfMissing("spark.executor.instances", args.numExecutors.toString)

    }

    new Client(args, sparkConf).run()

  }

 ......

}