在《Spark2.1.0之运行环境准备》一文介绍了如何准备基本的Spark运行环境，并在《Spark2.1.0之初体验》一文通过在spark-shell中执行word count的过程，让读者了解到可以使用spark-shell提交Spark作业。现在读者应该很想知道spark-shell究竟做了什么呢？

脚本分析

        在Spark安装目录的bin文件夹下可以找到spark-shell，其中有代码清单1-1所示的一段脚本。

代码清单1-1       spark-shell脚本

function main() {
  if $cygwin; then
    stty -icanon min 1 -echo > /dev/null 2>&1
    export SPARK_SUBMIT_OPTS="$SPARK_SUBMIT_OPTS -Djline.terminal=unix"
    "${SPARK_HOME}"/bin/spark-submit --class org.apache.spark.repl.Main --name "Spark shell" "$@"
    stty icanon echo > /dev/null 2>&1
  else
    export SPARK_SUBMIT_OPTS
    "${SPARK_HOME}"/bin/spark-submit --class org.apache.spark.repl.Main --name "Spark shell" "$@"
  fi
}

我们看到脚本spark-shell里执行了spark-submit脚本，那么打开spark-submit脚本，发现代码清单1-2中所示的脚本。

代码清单1-2        spark-submit脚本

if [ -z "${SPARK_HOME}" ]; then
  source "$(dirname "$0")"/find-spark-home
fi

# disable randomized hash for string in Python 3.3+
export PYTHONHASHSEED=0

exec "${SPARK_HOME}"/bin/spark-class org.apache.spark.deploy.SparkSubmit "$@"

可以看到spark-submit中又执行了脚本spark-class。打开脚本spark-class，首先发现以下一段脚本：

# Find the java binary
if [ -n "${JAVA_HOME}" ]; then
  RUNNER="${JAVA_HOME}/bin/java"
else
  if [ "$(command -v java)" ]; then
    RUNNER="java"
  else
    echo "JAVA_HOME is not set" >&2
    exit 1
  fi
fi

上面的脚本是为了找到Java命令。在spark-class脚本中还会找到以下内容：

build_command() {
  "$RUNNER" -Xmx128m -cp "$LAUNCH_CLASSPATH" org.apache.spark.launcher.Main "$@"
  printf "%d\0" $?
}

CMD=()
while IFS= read -d '' -r ARG; do
  CMD+=("$ARG")
done < <(build_command "$@")

根据代码清单1-2，脚本spark-submit在执行spark-class脚本时，给它增加了参数SparkSubmit 。所以读到这，应该知道Spark启动了以SparkSubmit为主类的JVM进程。

远程监控

        为便于在本地对Spark进程进行远程监控，在spark-shell脚本中找到以下配置：

SPARK_SUBMIT_OPTS="$SPARK_SUBMIT_OPTS -Dscala.usejavacp=true"

并追加以下jmx配置：

-Dcom.sun.management.jmxremote -Dcom.sun.management.jmxremote.port=10207 -Dcom.sun.management.jmxremote.authenticate=false -Dcom.sun.management.jmxremote.ssl=false

如果Spark安装在其他机器，那么在本地打开jvisualvm后需要添加远程主机，如图1所示：

右键单击已添加的远程主机，添加JMX连接，如图2：

        如果Spark安装在本地，那么打开jvisualvm后就会在应用程序窗口看到org.apache.spark.deploy.SparkSubmit进程，只需双击即可。

        选择右侧的“线程”选项卡，选择main线程，然后点击“线程Dump”按钮，如图3。

图3 查看Spark线程

从线程Dump的内容中找到线程main的信息如代码清单1-3所示。

代码清单1-3       main线程的Dump信息

"main" #1 prio=5 os_prio=31 tid=0x00007fa012802000 nid=0x1303 runnable [0x000000010d11c000]
   java.lang.Thread.State: RUNNABLE
	at java.io.FileInputStream.read0(Native Method)
	at java.io.FileInputStream.read(FileInputStream.java:207)
	at jline.internal.NonBlockingInputStream.read(NonBlockingInputStream.java:169)
	- locked <0x00000007837a8ab8> (a jline.internal.NonBlockingInputStream)
	at jline.internal.NonBlockingInputStream.read(NonBlockingInputStream.java:137)
	at jline.internal.NonBlockingInputStream.read(NonBlockingInputStream.java:246)
	at jline.internal.InputStreamReader.read(InputStreamReader.java:261)
	- locked <0x00000007837a8ab8> (a jline.internal.NonBlockingInputStream)
	at jline.internal.InputStreamReader.read(InputStreamReader.java:198)
	- locked <0x00000007837a8ab8> (a jline.internal.NonBlockingInputStream)
	at jline.console.ConsoleReader.readCharacter(ConsoleReader.java:2145)
	at jline.console.ConsoleReader.readLine(ConsoleReader.java:2349)
	at jline.console.ConsoleReader.readLine(ConsoleReader.java:2269)
	at scala.tools.nsc.interpreter.jline.InteractiveReader.readOneLine(JLineReader.scala:57)
	at scala.tools.nsc.interpreter.InteractiveReader$$anonfun$readLine$2.apply(InteractiveReader.scala:37)
	at scala.tools.nsc.interpreter.InteractiveReader$$anonfun$readLine$2.apply(InteractiveReader.scala:37)
	at scala.tools.nsc.interpreter.InteractiveReader$.restartSysCalls(InteractiveReader.scala:44)
	at scala.tools.nsc.interpreter.InteractiveReader$class.readLine(InteractiveReader.scala:37)
	at scala.tools.nsc.interpreter.jline.InteractiveReader.readLine(JLineReader.scala:28)
	at scala.tools.nsc.interpreter.ILoop.readOneLine(ILoop.scala:404)
	at scala.tools.nsc.interpreter.ILoop.loop(ILoop.scala:413)
	at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply$mcZ$sp(ILoop.scala:923)
	at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:909)
	at scala.tools.nsc.interpreter.ILoop$$anonfun$process$1.apply(ILoop.scala:909)
	at scala.reflect.internal.util.ScalaClassLoader$.savingContextLoader(ScalaClassLoader.scala:97)
	at scala.tools.nsc.interpreter.ILoop.process(ILoop.scala:909)
	at org.apache.spark.repl.Main$.doMain(Main.scala:68)
	at org.apache.spark.repl.Main$.main(Main.scala:51)
	at org.apache.spark.repl.Main.main(Main.scala)
	at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
	at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
	at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
	at java.lang.reflect.Method.invoke(Method.java:498)
	at org.apache.spark.deploy.SparkSubmit$.org$apache$spark$deploy$SparkSubmit$$runMain(SparkSubmit.scala:738)
	at org.apache.spark.deploy.SparkSubmit$.doRunMain$1(SparkSubmit.scala:187)
	at org.apache.spark.deploy.SparkSubmit$.submit(SparkSubmit.scala:212)
	at org.apache.spark.deploy.SparkSubmit$.main(SparkSubmit.scala:126)
	at org.apache.spark.deploy.SparkSubmit.main(SparkSubmit.scala)

从main线程的栈信息中看出程序的调用顺序：SparkSubmit.main→repl.Main→Iloop.process。

源码分析

我们根据上面的线索，直接阅读Iloop的process方法的源码（Iloop是Scala语言自身的类库中的用于实现交互式shell的实现类，提供对REPL（Read-eval-print-loop）的实现），见代码清单1-4。

  def process(settings: Settings): Boolean = savingContextLoader {
    this.settings = settings
    createInterpreter()

    // sets in to some kind of reader depending on environmental cues
    in = in0.fold(chooseReader(settings))(r => SimpleReader(r, out, interactive = true))
    globalFuture = future {
      intp.initializeSynchronous()
      loopPostInit()
      !intp.reporter.hasErrors
    }
    loadFiles(settings)
    printWelcome()

    try loop() match {
      case LineResults.EOF => out print Properties.shellInterruptedString
      case _               =>
    }
    catch AbstractOrMissingHandler()
    finally closeInterpreter()

    true
  }

根据代码清单1-4，Iloop的process方法调用了loadFiles方法。Spark中的SparkILoop继承了Iloop并重写了loadFiles方法，其实现如下：

  override def loadFiles(settings: Settings): Unit = {
    initializeSpark()
    super.loadFiles(settings)
  }

根据上面展示的代码，loadFiles方法调用了SparkILoop的initializeSpark方法，initializeSpark的实现见代码清单1-5。

代码清单1-5        initializeSpark的实现

  def initializeSpark() {
    intp.beQuietDuring {
      processLine("""
        @transient val spark = if (org.apache.spark.repl.Main.sparkSession != null) {
            org.apache.spark.repl.Main.sparkSession
          } else {
            org.apache.spark.repl.Main.createSparkSession()
          }
        @transient val sc = {
          val _sc = spark.sparkContext
          if (_sc.getConf.getBoolean("spark.ui.reverseProxy", false)) {
            val proxyUrl = _sc.getConf.get("spark.ui.reverseProxyUrl", null)
            if (proxyUrl != null) {
              println(s"Spark Context Web UI is available at ${proxyUrl}/proxy/${_sc.applicationId}")
            } else {
              println(s"Spark Context Web UI is available at Spark Master Public URL")
            }
          } else {
            _sc.uiWebUrl.foreach {
              webUrl => println(s"Spark context Web UI available at ${webUrl}")
            }
          }
          println("Spark context available as 'sc' " +
            s"(master = ${_sc.master}, app id = ${_sc.applicationId}).")
          println("Spark session available as 'spark'.")
          _sc
        }
        """)
      processLine("import org.apache.spark.SparkContext._")
      processLine("import spark.implicits._")
      processLine("import spark.sql")
      processLine("import org.apache.spark.sql.functions._")
      replayCommandStack = Nil // remove above commands from session history.
    }
  }

我们看到initializeSpark向交互式shell发送了一大串代码，Scala的交互式shell将调用org.apache.spark.repl.Main的createSparkSession方法（见代码清单1-6）创建SparkSession。我们看到常量spark将持有SparkSession的引用，并且sc持有SparkSession内部初始化好的SparkContext。所以我们才能够在spark-shell的交互式shell中使用sc和spark。

代码清单1-6        createSparkSession的实现

  def createSparkSession(): SparkSession = {
    val execUri = System.getenv("SPARK_EXECUTOR_URI")
    conf.setIfMissing("spark.app.name", "Spark shell")
    conf.set("spark.repl.class.outputDir", outputDir.getAbsolutePath())
    if (execUri != null) {
      conf.set("spark.executor.uri", execUri)
    }
    if (System.getenv("SPARK_HOME") != null) {
      conf.setSparkHome(System.getenv("SPARK_HOME"))
    }

    val builder = SparkSession.builder.config(conf)
    if (conf.get(CATALOG_IMPLEMENTATION.key, "hive").toLowerCase == "hive") {
      if (SparkSession.hiveClassesArePresent) {
        sparkSession = builder.enableHiveSupport().getOrCreate()
        logInfo("Created Spark session with Hive support")
      } else {
        builder.config(CATALOG_IMPLEMENTATION.key, "in-memory")
        sparkSession = builder.getOrCreate()
        logInfo("Created Spark session")
      }
    } else {
      sparkSession = builder.getOrCreate()
      logInfo("Created Spark session")
    }
    sparkContext = sparkSession.sparkContext
    sparkSession
  }

根据代码清单1-6，createSparkSession方法通过SparkSession的API创建SparkSession实例。本书将有关SparkSession等API的内容在《Spark内核设计的艺术》一书的第10章讲解，初次接触Spark的读者现在只需要了解即可。