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   /*
    * Copyright 2014 The Netty Project
    *
    * The Netty Project licenses this file to you under the Apache License,
    * version 2.0 (the "License"); you may not use this file except in compliance
    * with the License. You may obtain a copy of the License at:
    *
    *   http://www.apache.org/licenses/LICENSE-2.0
    *
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
   * License for the specific language governing permissions and limitations
   * under the License.
   */
  
  package io.netty.util;
  
  import io.netty.util.concurrent.DefaultThreadFactory;
  import io.netty.util.internal.StringUtil;
  import io.netty.util.internal.SystemPropertyUtil;
  import io.netty.util.internal.logging.InternalLogger;
  import io.netty.util.internal.logging.InternalLoggerFactory;
  
  import java.security.AccessController;
  import java.security.PrivilegedAction;
  import java.util.ArrayList;
  import java.util.List;
  import java.util.Queue;
  import java.util.concurrent.ConcurrentLinkedQueue;
  import java.util.concurrent.ThreadFactory;
  import java.util.concurrent.TimeUnit;
  import java.util.concurrent.atomic.AtomicBoolean;
  
  /**
   * Checks if a thread is alive periodically and runs a task when a thread dies.
   * <p>
   * This thread starts a daemon thread to check the state of the threads being watched and to invoke their
   * associated {@link Runnable}s.  When there is no thread to watch (i.e. all threads are dead), the daemon thread
   * will terminate itself, and a new daemon thread will be started again when a new watch is added.
   * </p>
   *
   * @deprecated will be removed in the next major release
   */
  @Deprecated
  public final class ThreadDeathWatcher {
  
      private static final InternalLogger logger = InternalLoggerFactory.getInstance(ThreadDeathWatcher.class);
      // visible for testing
      static final ThreadFactory threadFactory;
  
      // Use a MPMC queue as we may end up checking isEmpty() from multiple threads which may not be allowed to do
      // concurrently depending on the implementation of it in a MPSC queue.
      private static final Queue<Entry> pendingEntries = new ConcurrentLinkedQueue<Entry>();
      private static final Watcher watcher = new Watcher();
      private static final AtomicBoolean started = new AtomicBoolean();
      private static volatile Thread watcherThread;
  
      static {
          String poolName = "threadDeathWatcher";
          String serviceThreadPrefix = SystemPropertyUtil.get("io.netty.serviceThreadPrefix");
          if (!StringUtil.isNullOrEmpty(serviceThreadPrefix)) {
              poolName = serviceThreadPrefix + poolName;
          }
          // because the ThreadDeathWatcher is a singleton, tasks submitted to it can come from arbitrary threads and
          // this can trigger the creation of a thread from arbitrary thread groups; for this reason, the thread factory
          // must not be sticky about its thread group
          threadFactory = new DefaultThreadFactory(poolName, true, Thread.MIN_PRIORITY, null);
      }
  
      /**
       * Schedules the specified {@code task} to run when the specified {@code thread} dies.
       *
       * @param thread the {@link Thread} to watch
       * @param task the {@link Runnable} to run when the {@code thread} dies
       *
       * @throws IllegalArgumentException if the specified {@code thread} is not alive
       */
      public static void watch(Thread thread, Runnable task) {
          if (thread == null) {
              throw new NullPointerException("thread");
          }
          if (task == null) {
              throw new NullPointerException("task");
          }
          if (!thread.isAlive()) {
              throw new IllegalArgumentException("thread must be alive.");
          }
  
          schedule(thread, task, true);
      }
  
      /**
       * Cancels the task scheduled via {@link #watch(Thread, Runnable)}.
       */
      public static void unwatch(Thread thread, Runnable task) {
          if (thread == null) {
              throw new NullPointerException("thread");
          }
         if (task == null) {
             throw new NullPointerException("task");
         }
 
         schedule(thread, task, false);
     }
 
     private static void schedule(Thread thread, Runnable task, boolean isWatch) {
         pendingEntries.add(new Entry(thread, task, isWatch));
 
         if (started.compareAndSet(false, true)) {
             final Thread watcherThread = threadFactory.newThread(watcher);
             // Set to null to ensure we not create classloader leaks by holds a strong reference to the inherited
             // classloader.
             // See:
             // - https://github.com/netty/netty/issues/7290
             // - https://bugs.openjdk.java.net/browse/JDK-7008595
             AccessController.doPrivileged(new PrivilegedAction<Void>() {
                 @Override
                 public Void run() {
                     watcherThread.setContextClassLoader(null);
                     return null;
                 }
             });
 
             watcherThread.start();
             ThreadDeathWatcher.watcherThread = watcherThread;
         }
     }
 
     /**
      * Waits until the thread of this watcher has no threads to watch and terminates itself.
      * Because a new watcher thread will be started again on {@link #watch(Thread, Runnable)},
      * this operation is only useful when you want to ensure that the watcher thread is terminated
      * <strong>after</strong> your application is shut down and there's no chance of calling
      * {@link #watch(Thread, Runnable)} afterwards.
      *
      * @return {@code true} if and only if the watcher thread has been terminated
      */
     public static boolean awaitInactivity(long timeout, TimeUnit unit) throws InterruptedException {
         if (unit == null) {
             throw new NullPointerException("unit");
         }
 
         Thread watcherThread = ThreadDeathWatcher.watcherThread;
         if (watcherThread != null) {
             watcherThread.join(unit.toMillis(timeout));
             return !watcherThread.isAlive();
         } else {
             return true;
         }
     }
 
     private ThreadDeathWatcher() { }
 
     private static final class Watcher implements Runnable {
 
         private final List<Entry> watchees = new ArrayList<Entry>();
 
         @Override
         public void run() {
             for (;;) {
                 fetchWatchees();
                 notifyWatchees();
 
                 // Try once again just in case notifyWatchees() triggered watch() or unwatch().
                 fetchWatchees();
                 notifyWatchees();
 
                 try {
                     Thread.sleep(1000);
                 } catch (InterruptedException ignore) {
                     // Ignore the interrupt; do not terminate until all tasks are run.
                 }
 
                 if (watchees.isEmpty() && pendingEntries.isEmpty()) {
 
                     // Mark the current worker thread as stopped.
                     // The following CAS must always success and must be uncontended,
                     // because only one watcher thread should be running at the same time.
                     boolean stopped = started.compareAndSet(true, false);
                     assert stopped;
 
                     // Check if there are pending entries added by watch() while we do CAS above.
                     if (pendingEntries.isEmpty()) {
                         // A) watch() was not invoked and thus there's nothing to handle
                         //    -> safe to terminate because there's nothing left to do
                         // B) a new watcher thread started and handled them all
                         //    -> safe to terminate the new watcher thread will take care the rest
                         break;
                     }
 
                     // There are pending entries again, added by watch()
                     if (!started.compareAndSet(false, true)) {
                         // watch() started a new watcher thread and set 'started' to true.
                         // -> terminate this thread so that the new watcher reads from pendingEntries exclusively.
                         break;
                     }
 
                     // watch() added an entry, but this worker was faster to set 'started' to true.
                     // i.e. a new watcher thread was not started
                     // -> keep this thread alive to handle the newly added entries.
                 }
             }
         }
 
         private void fetchWatchees() {
             for (;;) {
                 Entry e = pendingEntries.poll();
                 if (e == null) {
                     break;
                 }
 
                 if (e.isWatch) {
                     watchees.add(e);
                 } else {
                     watchees.remove(e);
                 }
             }
         }
 
         private void notifyWatchees() {
             List<Entry> watchees = this.watchees;
             for (int i = 0; i < watchees.size();) {
                 Entry e = watchees.get(i);
                 if (!e.thread.isAlive()) {
                     watchees.remove(i);
                     try {
                         e.task.run();
                     } catch (Throwable t) {
                         logger.warn("Thread death watcher task raised an exception:", t);
                     }
                 } else {
                     i ++;
                 }
             }
         }
     }
 
     private static final class Entry {
         final Thread thread;
         final Runnable task;
         final boolean isWatch;
 
         Entry(Thread thread, Runnable task, boolean isWatch) {
             this.thread = thread;
             this.task = task;
             this.isWatch = isWatch;
         }
 
         @Override
         public int hashCode() {
             return thread.hashCode() ^ task.hashCode();
         }
 
         @Override
         public boolean equals(Object obj) {
             if (obj == this) {
                 return true;
             }
 
             if (!(obj instanceof Entry)) {
                 return false;
             }
 
             Entry that = (Entry) obj;
             return thread == that.thread && task == that.task;
         }
     }
 }