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.util import java.io.File import java.util.PriorityQueue import scala.util.Try import org.apache.hadoop.fs.FileSystem import 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() }