def start(): Unit = synchronized {
    state match {
      case INITIALIZED =>
        startSite.set(DStream.getCreationSite())
        StreamingContext.ACTIVATION_LOCK.synchronized {
          StreamingContext.assertNoOtherContextIsActive()
          try {
            validate()

            // Start the streaming scheduler in a new thread, so that thread local properties
            // like call sites and job groups can be reset without affecting those of the
            // current thread.
            //Spark Streaming内部启动的线程，用于整个作业的调度
            ThreadUtils.runInNewThread("streaming-start") {
              sparkContext.setCallSite(startSite.get)
              sparkContext.clearJobGroup()
              sparkContext.setLocalProperty(SparkContext.SPARK_JOB_INTERRUPT_ON_CANCEL, "false")
              scheduler.start()
            }
            state = StreamingContextState.ACTIVE
          } catch {
            case NonFatal(e) =>
              logError("Error starting the context, marking it as stopped", e)
              scheduler.stop(false)
              state = StreamingContextState.STOPPED
              throw e
          }
          StreamingContext.setActiveContext(this)
        }
        shutdownHookRef = ShutdownHookManager.addShutdownHook(
          StreamingContext.SHUTDOWN_HOOK_PRIORITY)(stopOnShutdown)
        // Registering Streaming Metrics at the start of the StreamingContext
        assert(env.metricsSystem != null)
        env.metricsSystem.registerSource(streamingSource)
        uiTab.foreach(_.attach())
        logInfo("StreamingContext started")
      case ACTIVE =>
        logWarning("StreamingContext has already been started")
      case STOPPED =>
        throw new IllegalStateException("StreamingContext has already been stopped")
    }
  }

/**
   JobScheduler负责逻辑层面的Job，并将其物理级别的运行在Spark之上
 * This class schedules jobs to be run on Spark. It uses the JobGenerator to generate
 * the jobs and runs them using a thread pool.

 */
private[streaming]
class JobScheduler(val ssc: StreamingContext) extends Logging {

  //通过JobSet集合，不断地存放接收到的Job
  private val jobSets: java.util.Map[Time, JobSet] = new ConcurrentHashMap[Time, JobSet]
  //设置并行度，默认为1，想要修改作业运行的并行度在spark-conf或者应用程序中修改此值就中
  为什么要修改并发度呢？
  答：有时候应用程序中有多个输出，会导致多个job的执行，都是在一个batchDurations里面，job之间执行无需互相等待，所以可以通过设置此值并发执行！
     不同的Batch，线程池中有很多的线程，也可以并发运行！
  private val numConcurrentJobs = ssc.conf.getInt("spark.streaming.concurrentJobs", 1)
  //将逻辑级别的Job转化为物理级别的job就是通过newDaemonFixedThreadPool线程实现的
  private val jobExecutor =
    ThreadUtils.newDaemonFixedThreadPool(numConcurrentJobs, "streaming-job-executor")
  //实例化JobGenerator
  private val jobGenerator = new JobGenerator(this)
  val clock = jobGenerator.clock
  val listenerBus = new StreamingListenerBus()
   
  //下面三个是说在JobScheduler启动时实例化
  // These two are created only when scheduler starts.
  // eventLoop not being null means the scheduler has been started and not stopped
  var receiverTracker: ReceiverTracker = null
  // A tracker to track all the input stream information as well as processed record number
  var inputInfoTracker: InputInfoTracker = null

  private var eventLoop: EventLoop[JobSchedulerEvent] = null

  def start(): Unit = synchronized {
    if (eventLoop != null) return // scheduler has already been started

    logDebug("Starting JobScheduler")
    eventLoop = new EventLoop[JobSchedulerEvent]("JobScheduler") {
      override protected def onReceive(event: JobSchedulerEvent): Unit = processEvent(event)

      override protected def onError(e: Throwable): Unit = reportError("Error in job scheduler", e)
    }
    eventLoop.start()

    // attach rate controllers of input streams to receive batch completion updates
    for {
      inputDStream <- ssc.graph.getInputStreams
      rateController <- inputDStream.rateController
    } ssc.addStreamingListener(rateController)

    listenerBus.start(ssc.sparkContext)
    receiverTracker = new ReceiverTracker(ssc)
    inputInfoTracker = new InputInfoTracker(ssc)
    receiverTracker.start()
    jobGenerator.start()
    logInfo("Started JobScheduler")
  }

/**
 * Print the first ten elements of each RDD generated in this DStream. This is an output
 * operator, so this DStream will be registered as an output stream and there materialized.
 */
def print(): Unit = ssc.withScope {
  print(10)
}

/**
 * Print the first num elements of each RDD generated in this DStream. This is an output
 * operator, so this DStream will be registered as an output stream and there materialized.
 */
def print(num: Int): Unit = ssc.withScope {
  def foreachFunc: (RDD[T], Time) => Unit = {
    (rdd: RDD[T], time: Time) => {
      val firstNum = rdd.take(num + 1)
      // scalastyle:off println
      println("-------------------------------------------")
      println("Time: " + time)
      println("-------------------------------------------")
      firstNum.take(num).foreach(println)
      if (firstNum.length > num) println("...")
      println()
      // scalastyle:on println
    }
  }
  foreachRDD(context.sparkContext.clean(foreachFunc), displayInnerRDDOps = false)
}

/**
 * Apply a function to each RDD in this DStream. This is an output operator, so
 * ‘this‘ DStream will be registered as an output stream and therefore materialized.
 * @param foreachFunc foreachRDD function
 * @param displayInnerRDDOps Whether the detailed callsites and scopes of the RDDs generated
 *                           in the `foreachFunc` to be displayed in the UI. If `false`, then
 *                           only the scopes and callsites of `foreachRDD` will override those
 *                           of the RDDs on the display.
 */
private def foreachRDD(
    foreachFunc: (RDD[T], Time) => Unit,
    displayInnerRDDOps: Boolean): Unit = {
  new ForEachDStream(this,
    context.sparkContext.clean(foreachFunc, false), displayInnerRDDOps).register()
}

/**
 * An internal DStream used to represent output operations like DStream.foreachRDD.
 * @param parent        Parent DStream
 * @param foreachFunc   Function to apply on each RDD generated by the parent DStream
 * @param displayInnerRDDOps Whether the detailed callsites and scopes of the RDDs generated
 *                           by `foreachFunc` will be displayed in the UI; only the scope and
 *                           callsite of `DStream.foreachRDD` will be displayed.
 */
private[streaming]
class ForEachDStream[T: ClassTag] (
    parent: DStream[T],
    foreachFunc: (RDD[T], Time) => Unit,
    displayInnerRDDOps: Boolean
  ) extends DStream[Unit](parent.ssc) {

  override def dependencies: List[DStream[_]] = List(parent)

  override def slideDuration: Duration = parent.slideDuration

  override def compute(validTime: Time): Option[RDD[Unit]] = None
  //根据时间间隔不断的产生Job
  override def generateJob(time: Time): Option[Job] = {
    parent.getOrCompute(time) match {
      case Some(rdd) =>
        val jobFunc = () => createRDDWithLocalProperties(time, displayInnerRDDOps) {
          //基于时间生成的RDD，由于是输出，所以是最后一个RDD，接下来我们只要找出哪儿调用ForEachDStream的generateJob方法，就能知道job最终的生成
          foreachFunc(rdd, time)
        }
        Some(new Job(time, jobFunc))
      case None => None
    }
  }
}

def generateJobs(time: Time): Seq[Job] = {
  logDebug("Generating jobs for time " + time)
  val jobs = this.synchronized {
    //此时的outputStream就是forEachDStream
    outputStreams.flatMap { outputStream =>
      val jobOption = outputStream.generateJob(time)
      jobOption.foreach(_.setCallSite(outputStream.creationSite))
      jobOption
    }
  }
  logDebug("Generated " + jobs.length + " jobs for time " + time)
  jobs
}

private val outputStreams = new ArrayBuffer[DStream[_]]()

通过查看DStream的子类继承结构和上面的ForEachDStream的generateJob方法，得出DStream的子类中只有ForEachDStream override了DStream的generateJob!
最终得出结论：

真正Job的生成是通过ForeachDStream的generateJob来生成的，此时的job是逻辑级别的，真正被物理级别的调用是在JobGenerator中generateJob方法中：

/** Generate jobs and perform checkpoint for the given `time`.  */
  private def generateJobs(time: Time) {
    // Set the SparkEnv in this thread, so that job generation code can access the environment
    // Example: BlockRDDs are created in this thread, and it needs to access BlockManager
    // Update: This is probably redundant after threadlocal stuff in SparkEnv has been removed.
    SparkEnv.set(ssc.env)
    Try {
      jobScheduler.receiverTracker.allocateBlocksToBatch(time) // allocate received blocks to batch
      graph.generateJobs(time) // generate jobs using allocated block
    } match {
      case Success(jobs) =>
        val streamIdToInputInfos = jobScheduler.inputInfoTracker.getInfo(time)
        jobScheduler.submitJobSet(JobSet(time, jobs, streamIdToInputInfos))
      case Failure(e) =>
        jobScheduler.reportError("Error generating jobs for time " + time, e)
    }
    eventLoop.post(DoCheckpoint(time, clearCheckpointDataLater = false))
  }

//将逻辑级别的Job转化为物理级别的job就是通过newDaemonFixedThreadPool线程实现的
  private val jobExecutor =
    ThreadUtils.newDaemonFixedThreadPool(numConcurrentJobs, "streaming-job-executor")

def submitJobSet(jobSet: JobSet) {
    if (jobSet.jobs.isEmpty) {
      logInfo("No jobs added for time " + jobSet.time)
    } else {
      listenerBus.post(StreamingListenerBatchSubmitted(jobSet.toBatchInfo))
      jobSets.put(jobSet.time, jobSet)
      jobSet.jobs.foreach(job => jobExecutor.execute(new JobHandler(job)))
      logInfo("Added jobs for time " + jobSet.time)
    }
  }