spark学习-61-源代码：ShutdownHookManager虚拟机关闭钩子管理器

　　Java程序经常也会遇到进程挂掉的情况，一些状态没有正确的保存下来，这时候就需要在JVM关掉的时候执行一些清理现场的代码。

　　JAVA中的ShutdownHook提供了比较好的方案。

　　JDK提供了Java.Runtime.addShutdownHook(Thread hook)方法，可以注册一个JVM关闭的钩子，这个钩子可以在一下几种场景中被调用：

１.　程序正常退出
２.　使用System.exit()
３.　终端使用Ctrl+C触发的中断
４.　系统关闭
５.　OutOfMemory宕机
６.　使用Kill pid命令干掉进程（注：在使用kill -9 pid时，是不会被调用的）

下面是JDK1.7中关于钩子的定义：

    public void addShutdownHook(Thread hook)参数：    hook - An initialized but unstarted Thread object 抛出：     IllegalArgumentException - If the specified hook has already been registered, or if it can be determined that the hook is already running or has already been run     IllegalStateException - If the virtual machine is already in the process of shutting down     SecurityException - If a security manager is present and it denies RuntimePermission("shutdownHooks")从以下版本开始：     1.3另请参见：    removeShutdownHook(java.lang.Thread), halt(int), exit(int)

首先来测试第一种，程序正常退出的情况：

package com.hook;  import java.util.concurrent.TimeUnit;  public class HookTest  {      public void start()      {          Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {              @Override             public void run()              {                  System.out.println("Execute Hook.....");              }          }));      }      public static void main(String[] args)      {          new HookTest().start();          System.out.println("The Application is doing something");          try         {              TimeUnit.MILLISECONDS.sleep(5000);          }          catch (InterruptedException e)          {              e.printStackTrace();          }      }  }

运行结果：

The Application is doing something  Execute Hook.....

如上可以看到，当main线程运行结束之后就会调用关闭钩子。

下面再来测试第五种情况（顺序有点乱，表在意这些细节）：

package com.hook;  import java.util.concurrent.TimeUnit;  public class HookTest2  {      public void start()      {          Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {              @Override             public void run()              {                  System.out.println("Execute Hook.....");              }          }));      }      public static void main(String[] args)      {          new HookTest().start();          System.out.println("The Application is doing something");          byte[] b = new byte[500*1024*1024];          try         {              TimeUnit.MILLISECONDS.sleep(5000);          }          catch (InterruptedException e)          {              e.printStackTrace();          }      }  }

运行参数设置为：-Xmx20M 这样可以保证会有OutOfMemoryError的发生。

运行结果：

The Application is doing something  Exception in thread "main" java.lang.OutOfMemoryError: Java heap space      at com.hook.HookTest2.main(HookTest2.java:22)  Execute Hook.....

可以看到程序遇到内存溢出错误后调用关闭钩子，与第一种情况中，程序等待5000ms运行结束之后推出调用关闭钩子不同。

接下来再来测试第三种情况：

package com.hook;  import java.util.concurrent.TimeUnit;  public class HookTest3  {      public void start()      {          Runtime.getRuntime().addShutdownHook(new Thread(new Runnable() {              @Override             public void run()              {                  System.out.println("Execute Hook.....");              }          }));      }      public static void main(String[] args)      {          new HookTest3().start();          Thread thread = new Thread(new Runnable(){              @Override             public void run()              {                  while(true)                  {                      System.out.println("thread is running....");                      try                     {                          TimeUnit.MILLISECONDS.sleep(100);                      }                      catch (InterruptedException e)                      {                          e.printStackTrace();                      }                  }              }          });          thread.start();      }  }

在命令行中编译：javac com/hook/HookTest3.java

在命令行中运行：Java com.hook.HookTest3 （之后按下Ctrl+C）

运行结果：

上面是java的，下面来看看spark的ShutdownHookManager

ShutdownHookManager的创建是在SparkContext中，为了在Spark程序挂掉的时候，处理一些清理工作。

 /** ShutdownHookManager的创建，为了在Spark程序挂掉的时候，处理一些清理工作  */    _shutdownHookRef = ShutdownHookManager.addShutdownHook(      ShutdownHookManager.SPARK_CONTEXT_SHUTDOWN_PRIORITY) { () =>      logInfo("Invoking stop() from shutdown hook")      // 这调用停止方法。关闭SparkContext，我就搞不懂了      stop()    }

来看看整体代码

package org.apache.spark.utilimport java.io.Fileimport java.util.PriorityQueueimport scala.util.Tryimport org.apache.hadoop.fs.FileSystemimport org.apache.spark.internal.Logging/** * Various utility methods used by Spark.  *  * Spark使用的各种实用方法。 */private[spark] object ShutdownHookManager extends Logging {  val DEFAULT_SHUTDOWN_PRIORITY = 100   // 默认的ShutdownHookManager优先级  /**   * The shutdown priority of the SparkContext instance. This is lower than the default   * priority, so that by default hooks are run before the context is shut down.    *    * SparkContext实例的shutdown优先级。这比默认的优先级要低，因此在默认情况下，在关闭上下文之前运行默认的hooks。   */  val SPARK_CONTEXT_SHUTDOWN_PRIORITY = 50  /**   * The shutdown priority of temp directory must be lower than the SparkContext shutdown   * priority. Otherwise cleaning the temp directories while Spark jobs are running can   * throw undesirable errors at the time of shutdown.    *    * temp目录的关闭优先级必须低于SparkContext关闭的优先级。否则，当Spark作业正在运行时，清理temp目录将会在关闭时抛出错误的错误。   */  val TEMP_DIR_SHUTDOWN_PRIORITY = 25  // 懒加载  private lazy val shutdownHooks = {    val manager = new SparkShutdownHookManager()    // 运行所有的hook,并且添加进去    manager.install()    manager  }  private val shutdownDeletePaths = new scala.collection.mutable.HashSet[String]()  // Add a shutdown hook to delete the temp dirs when the JVM exits  // 当JVM退出时，添加一个关闭钩子来删除temp dirs  logDebug("Adding shutdown hook") // force eager creation of logger  addShutdownHook(TEMP_DIR_SHUTDOWN_PRIORITY) { () =>    logInfo("Shutdown hook called")    // we need to materialize the paths to delete because deleteRecursively removes items from    // shutdownDeletePaths as we are traversing through it.    shutdownDeletePaths.toArray.foreach { dirPath =>      try {        logInfo("Deleting directory " + dirPath)        // 递归地删除文件或目录及其内容。 如果删除失败，则抛出异常。        Utils.deleteRecursively(new File(dirPath))      } catch {        case e: Exception => logError(s"Exception while deleting Spark temp dir: $dirPath", e)      }    }  }  // Register the path to be deleted via shutdown hook  // 通过关闭hook注册要删除的路径  def registerShutdownDeleteDir(file: File) {    // 得到文件的绝对路径    val absolutePath = file.getAbsolutePath()    // 假如到要删除文件路径的集合    shutdownDeletePaths.synchronized {      shutdownDeletePaths += absolutePath    }  }  // Remove the path to be deleted via shutdown hook 删除通过关闭hook删除的路径  def removeShutdownDeleteDir(file: File) {    val absolutePath = file.getAbsolutePath()    // 删除文件    shutdownDeletePaths.synchronized {      shutdownDeletePaths.remove(absolutePath)    }  }  // Is the path already registered to be deleted via a shutdown hook ?  // 已经注册的路径是否通过关闭hook被删除?  // 判断shutdownDeletePaths中是否包含给定的路径，如果包含返回true,否则返回false  def hasShutdownDeleteDir(file: File): Boolean = {    val absolutePath = file.getAbsolutePath()    shutdownDeletePaths.synchronized {      shutdownDeletePaths.contains(absolutePath)    }  }  // Note: if file is child of some registered path, while not equal to it, then return true;  // else false. This is to ensure that two shutdown hooks do not try to delete each others  // paths - resulting in IOException and incomplete cleanup.  // 注意:如果文件是某个已注册路径的子元素，而不等于它，则返回true;其他错误的。  // 这是为了确保两个关闭hooks不会试图删除彼此的路径——导致IOException和不完整的清理。  def hasRootAsShutdownDeleteDir(file: File): Boolean = {    val absolutePath = file.getAbsolutePath()    val retval = shutdownDeletePaths.synchronized {      shutdownDeletePaths.exists { path =>        !absolutePath.equals(path) && absolutePath.startsWith(path)      }    }    if (retval) {      logInfo("path = " + file + ", already present as root for deletion.")    }    retval  }  /**   * Detect whether this thread might be executing a shutdown hook. Will always return true if   * the current thread is a running a shutdown hook but may spuriously return true otherwise (e.g.   * if System.exit was just called by a concurrent thread).    *    * 检测此线程是否正在执行关闭hook。如果当前线程是一个正在运行的关闭hook，但可能会错误地返回true(例如，如果系统)，    * 则将始终返回true。退出是由一个并发线程调用的。   *   * Currently, this detects whether the JVM is shutting down by Runtime#addShutdownHook throwing   * an IllegalStateException.    *    * 当前，这检测到JVM是否在Runtime#addShutdownHook，抛出了一个IllegalStateException异常。   */  def inShutdown(): Boolean = {    try {      val hook = new Thread {        override def run() {}      }      // 这一点先加入后移除 是什么意思啊？      // scalastyle:off runtimeaddshutdownhook      Runtime.getRuntime.addShutdownHook(hook)      // scalastyle:on runtimeaddshutdownhook      Runtime.getRuntime.removeShutdownHook(hook)    } catch {      case ise: IllegalStateException => return true    }    false  }  /**   * Adds a shutdown hook with default priority. 添加默认优先级的 shutdown hook。   *   * @param hook The code to run during shutdown.   * @return A handle that can be used to unregister the shutdown hook.   */  def addShutdownHook(hook: () => Unit): AnyRef = {    addShutdownHook(DEFAULT_SHUTDOWN_PRIORITY)(hook)  }  /**   * Adds a shutdown hook with the given priority. Hooks with lower priority values run   * first.    *    * 根据一个指定的优先级添加一个shutdown hook，优先级低的Hooks优先被运行   *   * @param hook The code to run during shutdown.   * @return A handle that can be used to unregister the shutdown hook.   */  def addShutdownHook(priority: Int)(hook: () => Unit): AnyRef = {    shutdownHooks.add(priority, hook)  }  /**   * Remove a previously installed shutdown hook. 删除先前安装的shutdown hook   *   * @param ref A handle returned by `addShutdownHook`.   * @return Whether the hook was removed.   */  def removeShutdownHook(ref: AnyRef): Boolean = {    shutdownHooks.remove(ref)  }}private [util] class SparkShutdownHookManager {  // 权限队列  private val hooks = new PriorityQueue[SparkShutdownHook]()  @volatile private var shuttingDown = false  /**   * Install a hook to run at shutdown and run all registered hooks in order.    * 安装一个hook来运行关闭，并运行所有已注册的hooks。   */  def install(): Unit = {    val hookTask = new Runnable() {      override def run(): Unit = runAll()    }    org.apache.hadoop.util.ShutdownHookManager.get().addShutdownHook(      hookTask, FileSystem.SHUTDOWN_HOOK_PRIORITY + 30)  }  def runAll(): Unit = {    shuttingDown = true    var nextHook: SparkShutdownHook = null    while ({ nextHook = hooks.synchronized { hooks.poll() }; nextHook != null }) {      Try(Utils.logUncaughtExceptions(nextHook.run()))    }  }  def add(priority: Int, hook: () => Unit): AnyRef = {    hooks.synchronized {      if (shuttingDown) {        throw new IllegalStateException("Shutdown hooks cannot be modified during shutdown.")      }      val hookRef = new SparkShutdownHook(priority, hook)      hooks.add(hookRef)      hookRef    }  }  def remove(ref: AnyRef): Boolean = {    hooks.synchronized { hooks.remove(ref) }  }}private class SparkShutdownHook(private val priority: Int, hook: () => Unit)  extends Comparable[SparkShutdownHook] {  override def compareTo(other: SparkShutdownHook): Int = {    other.priority - priority  }  def run(): Unit = hook()}