SparkApplication是应用程序启动的万能入口，下面分析之

1.org.apache.spark.deploy.SparkSubmit#main 内调用 submit()

override def main(args: Array[String]): Unit = {
     ......
    appArgs.action match {
      case SparkSubmitAction.SUBMIT => submit(appArgs, uninitLog) //传入参数为SUBMIT，则往下调用submit方法
      case SparkSubmitAction.KILL => kill(appArgs)
      case SparkSubmitAction.REQUEST_STATUS => requestStatus(appArgs)
    }
  }

2.org.apache.spark.deploy.SparkSubmit#submit 内调用 runMain()

private def submit(args: SparkSubmitArguments, uninitLog: Boolean): Unit = {
    def doRunMain(): Unit = {
      if (args.proxyUser != null) {
        ......
        try {
          proxyUser.doAs(new PrivilegedExceptionAction[Unit]() {
            override def run(): Unit = {
              runMain(args, uninitLog) 
            }
          })
        } catch {
          ......
      } else {
        runMain(args, uninitLog)
      }
    }

3.org.apache.spark.deploy.SparkSubmit#runMain 中 调用 prepareSubmitEnvironment()获取 childMainClass，并创建SparkApplication启动之

 private def runMain(args: SparkSubmitArguments, uninitLog: Boolean): Unit = {
    1.val (childArgs, childClasspath, sparkConf, childMainClass) = prepareSubmitEnvironment(args) // 获取childMainClass，这里也是区分各种集群模式各种部署模式的主要逻辑所在

    2. try {
          mainClass = Utils.classForName(childMainClass) //获取mainClass的Class对象
        } catch {
        ......
        }
      }

     3. val app: SparkApplication = if (classOf[SparkApplication].isAssignableFrom(mainClass)) {
      mainClass.newInstance().asInstanceOf[SparkApplication] //通过反射mainClass创建SparkApplication实例
    } else {
      // SPARK-4170
      if (classOf[scala.App].isAssignableFrom(mainClass)) {
        printWarning("Subclasses of scala.App may not work correctly. Use a main() method instead.")
      }
      new JavaMainApplication(mainClass) //通过创建JavaMainApplication包装mainClass, JavaMainApplication也是SparkApplication的实现
    }

     4. try {
          app.start(childArgs.toArray, sparkConf) // 调用start方法，SparkApplication启动
       }
  }

流程还是非常清晰简单，下面还会继续学习不同模式下的具体源码，收工！！