1 /*
   2  * Copyright (c) 1997, 2013, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.lang.ref;
  27 
  28 import sun.misc.Cleaner;
  29 import sun.misc.Unsafe;
  30 
  31 /**
  32  * Abstract base class for reference objects.  This class defines the
  33  * operations common to all reference objects.  Because reference objects are
  34  * implemented in close cooperation with the garbage collector, this class may
  35  * not be subclassed directly.
  36  *
  37  * @author   Mark Reinhold
  38  * @since    1.2
  39  */
  40 
  41 public abstract class Reference<T> {
  42 
  43     /* A Reference instance is in one of four possible internal states:
  44      *
  45      *     Active: Subject to special treatment by the garbage collector.  Some
  46      *     time after the collector detects that the reachability of the
  47      *     referent has changed to the appropriate state, it changes the
  48      *     instance's state to either Pending or Inactive, depending upon
  49      *     whether or not the instance was registered with a queue when it was
  50      *     created.  In the former case it also adds the instance to the
  51      *     pending-Reference list.  Newly-created instances are Active.
  52      *
  53      *     Pending: An element of the pending-Reference list, waiting to be
  54      *     enqueued by the Reference-handler thread.  Unregistered instances
  55      *     are never in this state.
  56      *
  57      *     Enqueued: An element of the queue with which the instance was
  58      *     registered when it was created.  When an instance is removed from
  59      *     its ReferenceQueue, it is made Inactive.  Unregistered instances are
  60      *     never in this state.
  61      *
  62      *     Inactive: Nothing more to do.  Once an instance becomes Inactive its
  63      *     state will never change again.
  64      *
  65      * The state is encoded in the queue and next fields as follows:
  66      *
  67      *     Active: queue = ReferenceQueue with which instance is registered, or
  68      *     ReferenceQueue.NULL if it was not registered with a queue; next =
  69      *     null.
  70      *
  71      *     Pending: queue = ReferenceQueue with which instance is registered;
  72      *     next = this
  73      *
  74      *     Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
  75      *     in queue, or this if at end of list.
  76      *
  77      *     Inactive: queue = ReferenceQueue.NULL; next = this.
  78      *
  79      * With this scheme the collector need only examine the next field in order
  80      * to determine whether a Reference instance requires special treatment: If
  81      * the next field is null then the instance is active; if it is non-null,
  82      * then the collector should treat the instance normally.
  83      *
  84      * To ensure that a concurrent collector can discover active Reference
  85      * objects without interfering with application threads that may apply
  86      * the enqueue() method to those objects, collectors should link
  87      * discovered objects through the discovered field. The discovered
  88      * field is also used for linking Reference objects in the pending list.
  89      */
  90 
  91     private T referent;         /* Treated specially by GC */
  92 
  93     volatile ReferenceQueue<? super T> queue;
  94 
  95     /* When active:   NULL
  96      *     pending:   this
  97      *    Enqueued:   next reference in queue (or this if last)
  98      *    Inactive:   this
  99      */
 100     @SuppressWarnings("rawtypes")
 101     Reference next;
 102 
 103     /* When active:   next element in a discovered reference list maintained by GC (or this if last)
 104      *     pending:   next element in the pending list (or null if last)
 105      *   otherwise:   NULL
 106      */
 107     transient private Reference<T> discovered;  /* used by VM */
 108 
 109 
 110     /* Object used to synchronize with the garbage collector.  The collector
 111      * must acquire this lock at the beginning of each collection cycle.  It is
 112      * therefore critical that any code holding this lock complete as quickly
 113      * as possible, allocate no new objects, and avoid calling user code.
 114      */
 115     static private class Lock { };
 116     private static Lock lock = new Lock();
 117 
 118 
 119     /* List of References waiting to be enqueued.  The collector adds
 120      * References to this list, while the Reference-handler thread removes
 121      * them.  This list is protected by the above lock object. The
 122      * list uses the discovered field to link its elements.
 123      */
 124     private static Reference<Object> pending = null;
 125 
 126     /* High-priority thread to enqueue pending References
 127      */
 128     private static class ReferenceHandler extends Thread {
 129 
 130         static {
 131             // pre-load and initialize InterruptedException and Cleaner classes
 132             // so that we don't get into trouble later in the run loop if there's
 133             // memory shortage while loading/initializing them lazily.
 134             Unsafe unsafe = Unsafe.getUnsafe();
 135             unsafe.ensureClassInitialized(InterruptedException.class);
 136             unsafe.ensureClassInitialized(Cleaner.class);
 137         }
 138 
 139         ReferenceHandler(ThreadGroup g, String name) {
 140             super(g, name);
 141         }
 142 
 143         public void run() {
 144             for (;;) {
 145                 Reference<Object> r;
 146                 Cleaner c;
 147                 try {
 148                     synchronized (lock) {

 149                         r = pending;
 150                         if (r != null) {
 151                             // 'instanceof' might throw OOME sometimes so do this before
 152                             // unlinking 'r' from the 'pending' chain...
 153                             c = r instanceof Cleaner ? (Cleaner) r : null;
 154                             // unlink 'r' from 'pending' chain
 155                             pending = r.discovered;
 156                             r.discovered = null;
 157                         } else {
 158                             // The waiting on the lock may cause an OOME because it may try to allocate
 159                             // exception objects.













 160                             lock.wait();


 161                             continue;
 162                         }
 163                     }
 164                 } catch (OutOfMemoryError x) {
 165                     // Catch OOME from 'r instanceof Cleaner' or 'lock.wait()'.
 166                     // Give other threads CPU time so they hopefully drop some live references
 167                     // and GC reclaims some space.
 168                     // Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above
 169                     // persistently throws OOME for some time...
 170                     Thread.yield();
 171                     // retry
 172                     continue;
 173                 } catch (InterruptedException x) {
 174                     // Catch InterruptedException from 'lock.wait()' and retry
 175                     continue;
 176                 }
 177 
 178                 // Fast path for cleaners
 179                 if (c != null) {
 180                     c.clean();
 181                     continue;
 182                 }
 183 
 184                 ReferenceQueue<Object> q = r.queue;
 185                 if (q != ReferenceQueue.NULL) q.enqueue(r);
 186             }
 187         }
 188     }
 189 
 190     static {
 191         ThreadGroup tg = Thread.currentThread().getThreadGroup();
 192         for (ThreadGroup tgn = tg;
 193              tgn != null;
 194              tg = tgn, tgn = tg.getParent());
 195         Thread handler = new ReferenceHandler(tg, "Reference Handler");
 196         /* If there were a special system-only priority greater than
 197          * MAX_PRIORITY, it would be used here
 198          */
 199         handler.setPriority(Thread.MAX_PRIORITY);
 200         handler.setDaemon(true);
 201         handler.start();
 202     }
 203 
 204 
 205     /* -- Referent accessor and setters -- */
 206 
 207     /**
 208      * Returns this reference object's referent.  If this reference object has
 209      * been cleared, either by the program or by the garbage collector, then
 210      * this method returns <code>null</code>.
 211      *
 212      * @return   The object to which this reference refers, or
 213      *           <code>null</code> if this reference object has been cleared
 214      */
 215     public T get() {
 216         return this.referent;
 217     }
 218 
 219     /**
 220      * Clears this reference object.  Invoking this method will not cause this
 221      * object to be enqueued.
 222      *
 223      * <p> This method is invoked only by Java code; when the garbage collector
 224      * clears references it does so directly, without invoking this method.
 225      */
 226     public void clear() {
 227         this.referent = null;
 228     }
 229 
 230 
 231     /* -- Queue operations -- */
 232 
 233     /**
 234      * Tells whether or not this reference object has been enqueued, either by
 235      * the program or by the garbage collector.  If this reference object was
 236      * not registered with a queue when it was created, then this method will
 237      * always return <code>false</code>.
 238      *
 239      * @return   <code>true</code> if and only if this reference object has
 240      *           been enqueued
 241      */
 242     public boolean isEnqueued() {
 243         return (this.queue == ReferenceQueue.ENQUEUED);
 244     }
 245 
 246     /**
 247      * Adds this reference object to the queue with which it is registered,
 248      * if any.
 249      *
 250      * <p> This method is invoked only by Java code; when the garbage collector
 251      * enqueues references it does so directly, without invoking this method.
 252      *
 253      * @return   <code>true</code> if this reference object was successfully
 254      *           enqueued; <code>false</code> if it was already enqueued or if
 255      *           it was not registered with a queue when it was created
 256      */
 257     public boolean enqueue() {
 258         return this.queue.enqueue(this);
 259     }
 260 
 261 
 262     /* -- Constructors -- */
 263 
 264     Reference(T referent) {
 265         this(referent, null);
 266     }
 267 
 268     Reference(T referent, ReferenceQueue<? super T> queue) {
 269         this.referent = referent;
 270         this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
 271     }
 272 
 273 }
--- EOF ---