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1   /*
2    * Copyright 2017 The Netty Project
3    *
4    * The Netty Project licenses this file to you under the Apache License,
5    * version 2.0 (the "License"); you may not use this file except in compliance
6    * with the License. You may obtain a copy of the License at:
7    *
8    *   http://www.apache.org/licenses/LICENSE-2.0
9    *
10   * Unless required by applicable law or agreed to in writing, software
11   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
12   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
13   * License for the specific language governing permissions and limitations
14   * under the License.
15   */
16  package io.netty.util.internal;
17  
18  import io.netty.util.concurrent.FastThreadLocalThread;
19  
20  import java.lang.ref.ReferenceQueue;
21  import java.lang.ref.WeakReference;
22  import java.security.AccessController;
23  import java.security.PrivilegedAction;
24  import java.util.Set;
25  import java.util.concurrent.atomic.AtomicBoolean;
26  
27  import static io.netty.util.internal.SystemPropertyUtil.getInt;
28  import static java.lang.Math.max;
29  
30  /**
31   * Allows a way to register some {@link Runnable} that will executed once there are no references to an {@link Object}
32   * anymore.
33   */
34  public final class ObjectCleaner {
35      private static final int REFERENCE_QUEUE_POLL_TIMEOUT_MS =
36              max(500, getInt("io.netty.util.internal.ObjectCleaner.refQueuePollTimeout", 10000));
37  
38      // Package-private for testing
39      static final String CLEANER_THREAD_NAME = ObjectCleaner.class.getSimpleName() + "Thread";
40      // This will hold a reference to the AutomaticCleanerReference which will be removed once we called cleanup()
41      private static final Set<AutomaticCleanerReference> LIVE_SET = new ConcurrentSet<AutomaticCleanerReference>();
42      private static final ReferenceQueue<Object> REFERENCE_QUEUE = new ReferenceQueue<Object>();
43      private static final AtomicBoolean CLEANER_RUNNING = new AtomicBoolean(false);
44      private static final Runnable CLEANER_TASK = new Runnable() {
45          @Override
46          public void run() {
47              boolean interrupted = false;
48              for (;;) {
49                  // Keep on processing as long as the LIVE_SET is not empty and once it becomes empty
50                  // See if we can let this thread complete.
51                  while (!LIVE_SET.isEmpty()) {
52                      final AutomaticCleanerReference reference;
53                      try {
54                          reference = (AutomaticCleanerReference) REFERENCE_QUEUE.remove(REFERENCE_QUEUE_POLL_TIMEOUT_MS);
55                      } catch (InterruptedException ex) {
56                          // Just consume and move on
57                          interrupted = true;
58                          continue;
59                      }
60                      if (reference != null) {
61                          try {
62                              reference.cleanup();
63                          } catch (Throwable ignored) {
64                              // ignore exceptions, and don't log in case the logger throws an exception, blocks, or has
65                              // other unexpected side effects.
66                          }
67                          LIVE_SET.remove(reference);
68                      }
69                  }
70                  CLEANER_RUNNING.set(false);
71  
72                  // Its important to first access the LIVE_SET and then CLEANER_RUNNING to ensure correct
73                  // behavior in multi-threaded environments.
74                  if (LIVE_SET.isEmpty() || !CLEANER_RUNNING.compareAndSet(false, true)) {
75                      // There was nothing added after we set STARTED to false or some other cleanup Thread
76                      // was started already so its safe to let this Thread complete now.
77                      break;
78                  }
79              }
80              if (interrupted) {
81                  // As we caught the InterruptedException above we should mark the Thread as interrupted.
82                  Thread.currentThread().interrupt();
83              }
84          }
85      };
86  
87      /**
88       * Register the given {@link Object} for which the {@link Runnable} will be executed once there are no references
89       * to the object anymore.
90       *
91       * This should only be used if there are no other ways to execute some cleanup once the Object is not reachable
92       * anymore because it is not a cheap way to handle the cleanup.
93       */
94      public static void register(Object object, Runnable cleanupTask) {
95          AutomaticCleanerReference reference = new AutomaticCleanerReference(object,
96                  ObjectUtil.checkNotNull(cleanupTask, "cleanupTask"));
97          // Its important to add the reference to the LIVE_SET before we access CLEANER_RUNNING to ensure correct
98          // behavior in multi-threaded environments.
99          LIVE_SET.add(reference);
100 
101         // Check if there is already a cleaner running.
102         if (CLEANER_RUNNING.compareAndSet(false, true)) {
103             final Thread cleanupThread = new FastThreadLocalThread(CLEANER_TASK);
104             cleanupThread.setPriority(Thread.MIN_PRIORITY);
105             // Set to null to ensure we not create classloader leaks by holding a strong reference to the inherited
106             // classloader.
107             // See:
108             // - https://github.com/netty/netty/issues/7290
109             // - https://bugs.openjdk.java.net/browse/JDK-7008595
110             AccessController.doPrivileged(new PrivilegedAction<Void>() {
111                 @Override
112                 public Void run() {
113                     cleanupThread.setContextClassLoader(null);
114                     return null;
115                 }
116             });
117             cleanupThread.setName(CLEANER_THREAD_NAME);
118 
119             // Mark this as a daemon thread to ensure that we the JVM can exit if this is the only thread that is
120             // running.
121             cleanupThread.setDaemon(true);
122             cleanupThread.start();
123         }
124     }
125 
126     public static int getLiveSetCount() {
127         return LIVE_SET.size();
128     }
129 
130     private ObjectCleaner() {
131         // Only contains a static method.
132     }
133 
134     private static final class AutomaticCleanerReference extends WeakReference<Object> {
135         private final Runnable cleanupTask;
136 
137         AutomaticCleanerReference(Object referent, Runnable cleanupTask) {
138             super(referent, REFERENCE_QUEUE);
139             this.cleanupTask = cleanupTask;
140         }
141 
142         void cleanup() {
143             cleanupTask.run();
144         }
145 
146         @Override
147         public Thread get() {
148             return null;
149         }
150 
151         @Override
152         public void clear() {
153             LIVE_SET.remove(this);
154             super.clear();
155         }
156     }
157 }