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 
  30 /**
  31  * Abstract base class for reference objects.  This class defines the
  32  * operations common to all reference objects.  Because reference objects are
  33  * implemented in close cooperation with the garbage collector, this class may
  34  * not be subclassed directly.
  35  *
  36  * @author   Mark Reinhold
  37  * @since    1.2
  38  */
  39 
  40 public abstract class Reference<T> {
  41 
  42     /* A Reference instance is in one of four possible internal states:
  43      *
  44      *     Active: Subject to special treatment by the garbage collector.  Some
  45      *     time after the collector detects that the reachability of the
  46      *     referent has changed to the appropriate state, it changes the
  47      *     instance's state to either Pending or Inactive, depending upon
  48      *     whether or not the instance was registered with a queue when it was
  49      *     created.  In the former case it also adds the instance to the
  50      *     pending-Reference list.  Newly-created instances are Active.
  51      *
  52      *     Pending: An element of the pending-Reference list, waiting to be
  53      *     enqueued by the Reference-handler thread.  Unregistered instances
  54      *     are never in this state.
  55      *
  56      *     Enqueued: An element of the queue with which the instance was
  57      *     registered when it was created.  When an instance is removed from
  58      *     its ReferenceQueue, it is made Inactive.  Unregistered instances are
  59      *     never in this state.
  60      *
  61      *     Inactive: Nothing more to do.  Once an instance becomes Inactive its
  62      *     state will never change again.
  63      *
  64      * The state is encoded in the queue and next fields as follows:
  65      *
  66      *     Active: queue = ReferenceQueue with which instance is registered, or
  67      *     ReferenceQueue.NULL if it was not registered with a queue; next =
  68      *     null.
  69      *
  70      *     Pending: queue = ReferenceQueue with which instance is registered;
  71      *     next = this
  72      *
  73      *     Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
  74      *     in queue, or this if at end of list.
  75      *
  76      *     Inactive: queue = ReferenceQueue.NULL; next = this.
  77      *
  78      * With this scheme the collector need only examine the next field in order
  79      * to determine whether a Reference instance requires special treatment: If
  80      * the next field is null then the instance is active; if it is non-null,
  81      * then the collector should treat the instance normally.
  82      *
  83      * To ensure that a concurrent collector can discover active Reference
  84      * objects without interfering with application threads that may apply
  85      * the enqueue() method to those objects, collectors should link
  86      * discovered objects through the discovered field. The discovered
  87      * field is also used for linking Reference objects in the pending list.
  88      */
  89 
  90     private T referent;         /* Treated specially by GC */
  91 
  92     volatile ReferenceQueue<? super T> queue;
  93 
  94     /* When active:   NULL
  95      *     pending:   this
  96      *    Enqueued:   next reference in queue (or this if last)
  97      *    Inactive:   this
  98      */
  99     @SuppressWarnings("rawtypes")
 100     Reference next;
 101 
 102     /* When active:   next element in a discovered reference list maintained by GC (or this if last)
 103      *     pending:   next element in the pending list (or null if last)
 104      *   otherwise:   NULL
 105      */
 106     transient private Reference<T> discovered;  /* used by VM */
 107 
 108 
 109     /* Object used to synchronize with the garbage collector.  The collector
 110      * must acquire this lock at the beginning of each collection cycle.  It is
 111      * therefore critical that any code holding this lock complete as quickly
 112      * as possible, allocate no new objects, and avoid calling user code.
 113      */
 114     static private class Lock { };
 115     private static Lock lock = new Lock();
 116 
 117 
 118     /* List of References waiting to be enqueued.  The collector adds
 119      * References to this list, while the Reference-handler thread removes
 120      * them.  This list is protected by the above lock object. The
 121      * list uses the discovered field to link its elements.
 122      */
 123     private static Reference<Object> pending = null;
 124 
 125     /* High-priority thread to enqueue pending References
 126      */
 127     private static class ReferenceHandler extends Thread {
 128 
 129         ReferenceHandler(ThreadGroup g, String name) {
 130             super(g, name);
 131         }
 132 
 133         public void run() {
 134             for (;;) {
 135                 try {
 136                     Reference<Object> r;
 137                     synchronized (lock) {
 138                         if (pending != null) {
 139                             r = pending;
 140                             pending = r.discovered;
 141                             r.discovered = null;
 142                         } else {
 143                             // The waiting on the lock may cause an OOME because it may try to allocate
 144                             // exception objects, so also catch OOME here to avoid silent exit of the
 145                             // reference handler thread.
 146                             //
 147                             // Explicitly define the order of the two exceptions we catch here
 148                             // when waiting for the lock.
 149                             //
 150                             // We do not want to try to potentially load the InterruptedException class
 151                             // (which would be done if this was its first use, and InterruptedException
 152                             // were checked first) in this situation.
 153                             //
 154                             // This may lead to the VM not ever trying to load the InterruptedException
 155                             // class again.
 156                             try {
 157                                 try {
 158                                     lock.wait();
 159                                 } catch (OutOfMemoryError x) { }
 160                             } catch (InterruptedException x) { }
 161                             continue;
 162                         }
 163                     }
 164 
 165                     // Fast path for cleaners
 166                     if (r instanceof Cleaner) {
 167                         ((Cleaner)r).clean();
 168                         continue;
 169                     }
 170 
 171                     ReferenceQueue<Object> q = r.queue;
 172                     if (q != ReferenceQueue.NULL) q.enqueue(r);
 173 
 174                 } catch (OutOfMemoryError x) { }
 175             }
 176         }
 177     }
 178 
 179     static {
 180         ThreadGroup tg = Thread.currentThread().getThreadGroup();
 181         for (ThreadGroup tgn = tg;
 182              tgn != null;
 183              tg = tgn, tgn = tg.getParent());
 184         Thread handler = new ReferenceHandler(tg, "Reference Handler");
 185         /* If there were a special system-only priority greater than
 186          * MAX_PRIORITY, it would be used here
 187          */
 188         handler.setPriority(Thread.MAX_PRIORITY);
 189         handler.setDaemon(true);
 190         handler.start();
 191     }
 192 
 193 
 194     /* -- Referent accessor and setters -- */
 195 
 196     /**
 197      * Returns this reference object's referent.  If this reference object has
 198      * been cleared, either by the program or by the garbage collector, then
 199      * this method returns <code>null</code>.
 200      *
 201      * @return   The object to which this reference refers, or
 202      *           <code>null</code> if this reference object has been cleared
 203      */
 204     public T get() {
 205         return this.referent;
 206     }
 207 
 208     /**
 209      * Clears this reference object.  Invoking this method will not cause this
 210      * object to be enqueued.
 211      *
 212      * <p> This method is invoked only by Java code; when the garbage collector
 213      * clears references it does so directly, without invoking this method.
 214      */
 215     public void clear() {
 216         this.referent = null;
 217     }
 218 
 219 
 220     /* -- Queue operations -- */
 221 
 222     /**
 223      * Tells whether or not this reference object has been enqueued, either by
 224      * the program or by the garbage collector.  If this reference object was
 225      * not registered with a queue when it was created, then this method will
 226      * always return <code>false</code>.
 227      *
 228      * @return   <code>true</code> if and only if this reference object has
 229      *           been enqueued
 230      */
 231     public boolean isEnqueued() {
 232         return (this.queue == ReferenceQueue.ENQUEUED);
 233     }
 234 
 235     /**
 236      * Adds this reference object to the queue with which it is registered,
 237      * if any.
 238      *
 239      * <p> This method is invoked only by Java code; when the garbage collector
 240      * enqueues references it does so directly, without invoking this method.
 241      *
 242      * @return   <code>true</code> if this reference object was successfully
 243      *           enqueued; <code>false</code> if it was already enqueued or if
 244      *           it was not registered with a queue when it was created
 245      */
 246     public boolean enqueue() {
 247         return this.queue.enqueue(this);
 248     }
 249 
 250 
 251     /* -- Constructors -- */
 252 
 253     Reference(T referent) {
 254         this(referent, null);
 255     }
 256 
 257     Reference(T referent, ReferenceQueue<? super T> queue) {
 258         this.referent = referent;
 259         this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
 260     }
 261 
 262 }