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   /*
    * Copyright 2017 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:
    *
    *   https://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.internal;
  
  import io.netty.util.concurrent.FastThreadLocalThread;
  
  import java.lang.ref.ReferenceQueue;
  import java.lang.ref.WeakReference;
  import java.security.AccessController;
  import java.security.PrivilegedAction;
  import java.util.Set;
  import java.util.concurrent.atomic.AtomicBoolean;
  
  import static io.netty.util.internal.SystemPropertyUtil.getInt;
  import static java.lang.Math.max;
  
  /**
   * Allows a way to register some {@link Runnable} that will executed once there are no references to an {@link Object}
   * anymore.
   */
  public final class ObjectCleaner {
      private static final int REFERENCE_QUEUE_POLL_TIMEOUT_MS =
              max(500, getInt("io.netty.util.internal.ObjectCleaner.refQueuePollTimeout", 10000));
  
      // Package-private for testing
      static final String CLEANER_THREAD_NAME = ObjectCleaner.class.getSimpleName() + "Thread";
      // This will hold a reference to the AutomaticCleanerReference which will be removed once we called cleanup()
      private static final Set<AutomaticCleanerReference> LIVE_SET = new ConcurrentSet<AutomaticCleanerReference>();
      private static final ReferenceQueue<Object> REFERENCE_QUEUE = new ReferenceQueue<Object>();
      private static final AtomicBoolean CLEANER_RUNNING = new AtomicBoolean(false);
      private static final Runnable CLEANER_TASK = new Runnable() {
          @Override
          public void run() {
              boolean interrupted = false;
              for (;;) {
                  // Keep on processing as long as the LIVE_SET is not empty and once it becomes empty
                  // See if we can let this thread complete.
                  while (!LIVE_SET.isEmpty()) {
                      final AutomaticCleanerReference reference;
                      try {
                          reference = (AutomaticCleanerReference) REFERENCE_QUEUE.remove(REFERENCE_QUEUE_POLL_TIMEOUT_MS);
                      } catch (InterruptedException ex) {
                          // Just consume and move on
                          interrupted = true;
                          continue;
                      }
                      if (reference != null) {
                          try {
                              reference.cleanup();
                          } catch (Throwable ignored) {
                              // ignore exceptions, and don't log in case the logger throws an exception, blocks, or has
                              // other unexpected side effects.
                          }
                          LIVE_SET.remove(reference);
                      }
                  }
                  CLEANER_RUNNING.set(false);
  
                  // Its important to first access the LIVE_SET and then CLEANER_RUNNING to ensure correct
                  // behavior in multi-threaded environments.
                  if (LIVE_SET.isEmpty() || !CLEANER_RUNNING.compareAndSet(false, true)) {
                      // There was nothing added after we set STARTED to false or some other cleanup Thread
                      // was started already so its safe to let this Thread complete now.
                      break;
                  }
              }
              if (interrupted) {
                  // As we caught the InterruptedException above we should mark the Thread as interrupted.
                  Thread.currentThread().interrupt();
              }
          }
      };
  
      /**
       * Register the given {@link Object} for which the {@link Runnable} will be executed once there are no references
       * to the object anymore.
       *
       * This should only be used if there are no other ways to execute some cleanup once the Object is not reachable
       * anymore because it is not a cheap way to handle the cleanup.
       */
      public static void register(Object object, Runnable cleanupTask) {
          AutomaticCleanerReference reference = new AutomaticCleanerReference(object,
                  ObjectUtil.checkNotNull(cleanupTask, "cleanupTask"));
          // Its important to add the reference to the LIVE_SET before we access CLEANER_RUNNING to ensure correct
          // behavior in multi-threaded environments.
          LIVE_SET.add(reference);
 
         // Check if there is already a cleaner running.
         if (CLEANER_RUNNING.compareAndSet(false, true)) {
             final Thread cleanupThread = new FastThreadLocalThread(CLEANER_TASK);
             cleanupThread.setPriority(Thread.MIN_PRIORITY);
             // Set to null to ensure we not create classloader leaks by holding 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() {
                     cleanupThread.setContextClassLoader(null);
                     return null;
                 }
             });
             cleanupThread.setName(CLEANER_THREAD_NAME);
 
             // Mark this as a daemon thread to ensure that we the JVM can exit if this is the only thread that is
             // running.
             cleanupThread.setDaemon(true);
             cleanupThread.start();
         }
     }
 
     public static int getLiveSetCount() {
         return LIVE_SET.size();
     }
 
     private ObjectCleaner() {
         // Only contains a static method.
     }
 
     private static final class AutomaticCleanerReference extends WeakReference<Object> {
         private final Runnable cleanupTask;
 
         AutomaticCleanerReference(Object referent, Runnable cleanupTask) {
             super(referent, REFERENCE_QUEUE);
             this.cleanupTask = cleanupTask;
         }
 
         void cleanup() {
             cleanupTask.run();
         }
 
         @Override
         public Thread get() {
             return null;
         }
 
         @Override
         public void clear() {
             LIVE_SET.remove(this);
             super.clear();
         }
     }
 }