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+/**
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+ * Licensed to the Apache Software Foundation (ASF) under one
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+ * or more contributor license agreements. See the NOTICE file
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+ * distributed with this work for additional information
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+ * regarding copyright ownership. The ASF licenses this file
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+ * to you under the Apache License, Version 2.0 (the
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+ * "License"); you may not use this file except in compliance
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+ * with the License. You may obtain a copy of the License at
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+ *
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+ * http://www.apache.org/licenses/LICENSE-2.0
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+ *
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+ * Unless required by applicable law or agreed to in writing, software
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+ * distributed under the License is distributed on an "AS IS" BASIS,
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+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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+ * See the License for the specific language governing permissions and
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+ * limitations under the License.
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+ */
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+package org.apache.hadoop.util;
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+
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+import org.apache.commons.logging.Log;
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+import org.apache.commons.logging.LogFactory;
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+
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+import java.util.ArrayList;
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+import java.util.Collections;
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+import java.util.Comparator;
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+import java.util.HashSet;
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+import java.util.List;
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+import java.util.Set;
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+import java.util.concurrent.atomic.AtomicBoolean;
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+
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+/**
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+ * The <code>ShutdownHookManager</code> enables running shutdownHook
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+ * in a determistic order, higher priority first.
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+ * <p/>
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+ * The JVM runs ShutdownHooks in a non-deterministic order or in parallel.
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+ * This class registers a single JVM shutdownHook and run all the
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+ * shutdownHooks registered to it (to this class) in order based on their
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+ * priority.
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+ */
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+public class ShutdownHookManager {
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+
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+ private static final ShutdownHookManager MGR = new ShutdownHookManager();
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+
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+ private static final Log LOG = LogFactory.getLog(ShutdownHookManager.class);
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+
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+ static {
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+ Runtime.getRuntime().addShutdownHook(
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+ new Thread() {
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+ @Override
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+ public void run() {
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+ MGR.shutdownInProgress.set(true);
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+ for (Runnable hook: MGR.getShutdownHooksInOrder()) {
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+ try {
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+ hook.run();
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+ } catch (Throwable ex) {
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+ LOG.warn("ShutdownHook '" + hook.getClass().getSimpleName() +
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+ "' failed, " + ex.toString(), ex);
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+ }
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+ }
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+ }
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+ }
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+ );
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+ }
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+
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+ /**
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+ * Return <code>ShutdownHookManager</code> singleton.
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+ *
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+ * @return <code>ShutdownHookManager</code> singleton.
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+ */
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+ public static ShutdownHookManager get() {
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+ return MGR;
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+ }
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+
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+ /**
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+ * Private structure to store ShutdownHook and its priority.
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+ */
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+ private static class HookEntry {
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+ Runnable hook;
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+ int priority;
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+
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+ public HookEntry(Runnable hook, int priority) {
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+ this.hook = hook;
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+ this.priority = priority;
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+ }
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+
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+ @Override
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+ public int hashCode() {
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+ return hook.hashCode();
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+ }
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+
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+ @Override
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+ public boolean equals(Object obj) {
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+ boolean eq = false;
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+ if (obj != null) {
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+ if (obj instanceof HookEntry) {
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+ eq = (hook == ((HookEntry)obj).hook);
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+ }
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+ }
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+ return eq;
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+ }
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+
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+ }
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+
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+ private Set<HookEntry> hooks =
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+ Collections.synchronizedSet(new HashSet<HookEntry>());
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+
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+ private AtomicBoolean shutdownInProgress = new AtomicBoolean(false);
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+
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+ //private to constructor to ensure singularity
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+ private ShutdownHookManager() {
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+ }
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+
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+ /**
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+ * Returns the list of shutdownHooks in order of execution,
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+ * Highest priority first.
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+ *
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+ * @return the list of shutdownHooks in order of execution.
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+ */
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+ List<Runnable> getShutdownHooksInOrder() {
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+ List<HookEntry> list;
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+ synchronized (MGR.hooks) {
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+ list = new ArrayList<HookEntry>(MGR.hooks);
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+ }
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+ Collections.sort(list, new Comparator<HookEntry>() {
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+
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+ //reversing comparison so highest priority hooks are first
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+ @Override
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+ public int compare(HookEntry o1, HookEntry o2) {
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+ return o2.priority - o1.priority;
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+ }
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+ });
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+ List<Runnable> ordered = new ArrayList<Runnable>();
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+ for (HookEntry entry: list) {
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+ ordered.add(entry.hook);
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+ }
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+ return ordered;
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+ }
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+
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+ /**
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+ * Adds a shutdownHook with a priority, the higher the priority
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+ * the earlier will run. ShutdownHooks with same priority run
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+ * in a non-deterministic order.
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+ *
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+ * @param shutdownHook shutdownHook <code>Runnable</code>
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+ * @param priority priority of the shutdownHook.
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+ */
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+ public void addShutdownHook(Runnable shutdownHook, int priority) {
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+ if (shutdownHook == null) {
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+ throw new IllegalArgumentException("shutdownHook cannot be NULL");
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+ }
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+ if (shutdownInProgress.get()) {
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+ throw new IllegalStateException("Shutdown in progress, cannot add a shutdownHook");
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+ }
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+ hooks.add(new HookEntry(shutdownHook, priority));
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+ }
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+
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+ /**
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+ * Removes a shutdownHook.
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+ *
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+ * @param shutdownHook shutdownHook to remove.
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+ * @return TRUE if the shutdownHook was registered and removed,
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+ * FALSE otherwise.
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+ */
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+ public boolean removeShutdownHook(Runnable shutdownHook) {
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+ if (shutdownInProgress.get()) {
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+ throw new IllegalStateException("Shutdown in progress, cannot remove a shutdownHook");
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+ }
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+ return hooks.remove(new HookEntry(shutdownHook, 0));
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+ }
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+
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+ /**
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+ * Indicates if a shutdownHook is registered or nt.
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+ *
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+ * @param shutdownHook shutdownHook to check if registered.
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+ * @return TRUE/FALSE depending if the shutdownHook is is registered.
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+ */
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+ public boolean hasShutdownHook(Runnable shutdownHook) {
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+ return hooks.contains(new HookEntry(shutdownHook, 0));
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+ }
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+
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+}
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Alejandro Abdelnur