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.JavaLangRefAccess;
  30 import sun.misc.SharedSecrets;
  31 
  32 /**
  33  * Abstract base class for reference objects.  This class defines the
  34  * operations common to all reference objects.  Because reference objects are
  35  * implemented in close cooperation with the garbage collector, this class may
  36  * not be subclassed directly.
  37  *
  38  * @author   Mark Reinhold
  39  * @since    1.2
  40  */
  41 
  42 public abstract class Reference<T> {
  43 
  44     /* A Reference instance is in one of four possible internal states:
  45      *
  46      *     Active: Subject to special treatment by the garbage collector.  Some
  47      *     time after the collector detects that the reachability of the
  48      *     referent has changed to the appropriate state, it changes the
  49      *     instance's state to either Pending or Inactive, depending upon
  50      *     whether or not the instance was registered with a queue when it was
  51      *     created.  In the former case it also adds the instance to the
  52      *     pending-Reference list.  Newly-created instances are Active.
  53      *
  54      *     Pending: An element of the pending-Reference list, waiting to be
  55      *     enqueued by the Reference-handler thread.  Unregistered instances
  56      *     are never in this state.
  57      *
  58      *     Enqueued: An element of the queue with which the instance was
  59      *     registered when it was created.  When an instance is removed from
  60      *     its ReferenceQueue, it is made Inactive.  Unregistered instances are
  61      *     never in this state.
  62      *
  63      *     Inactive: Nothing more to do.  Once an instance becomes Inactive its
  64      *     state will never change again.
  65      *
  66      * The state is encoded in the queue and next fields as follows:
  67      *
  68      *     Active: queue = ReferenceQueue with which instance is registered, or
  69      *     ReferenceQueue.NULL if it was not registered with a queue; next =
  70      *     null.
  71      *
  72      *     Pending: queue = ReferenceQueue with which instance is registered;
  73      *     next = this
  74      *
  75      *     Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
  76      *     in queue, or this if at end of list.
  77      *
  78      *     Inactive: queue = ReferenceQueue.NULL; next = this.
  79      *
  80      * With this scheme the collector need only examine the next field in order
  81      * to determine whether a Reference instance requires special treatment: If
  82      * the next field is null then the instance is active; if it is non-null,
  83      * then the collector should treat the instance normally.
  84      *
  85      * To ensure that a concurrent collector can discover active Reference
  86      * objects without interfering with application threads that may apply
  87      * the enqueue() method to those objects, collectors should link
  88      * discovered objects through the discovered field. The discovered
  89      * field is also used for linking Reference objects in the pending list.
  90      */
  91 
  92     private T referent;         /* Treated specially by GC */
  93 
  94     volatile ReferenceQueue<? super T> queue;
  95 
  96     /* When active:   NULL
  97      *     pending:   this
  98      *    Enqueued:   next reference in queue (or this if last)
  99      *    Inactive:   this
 100      */
 101     @SuppressWarnings("rawtypes")
 102     Reference next;
 103 
 104     /* When active:   next element in a discovered reference list maintained by GC (or this if last)
 105      *     pending:   next element in the pending list (or null if last)
 106      *   otherwise:   NULL
 107      */
 108     transient private Reference<T> discovered;  /* used by VM */
 109 
 110 
 111     /* Object used to synchronize with the garbage collector.  The collector
 112      * must acquire this lock at the beginning of each collection cycle.  It is
 113      * therefore critical that any code holding this lock complete as quickly
 114      * as possible, allocate no new objects, and avoid calling user code.
 115      */
 116     static private class Lock { }
 117     private static Lock lock = new Lock();
 118 
 119 
 120     /* List of References waiting to be enqueued.  The collector adds
 121      * References to this list, while the Reference-handler thread removes
 122      * them.  This list is protected by the above lock object. The
 123      * list uses the discovered field to link its elements.
 124      */
 125     private static Reference<Object> pending = null;
 126 
 127     /* High-priority thread to enqueue pending References
 128      */
 129     private static class ReferenceHandler extends Thread {
 130 
 131         private static void ensureClassInitialized(Class<?> clazz) {
 132             try {
 133                 Class.forName(clazz.getName(), true, clazz.getClassLoader());
 134             } catch (ClassNotFoundException e) {
 135                 throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
 136             }
 137         }
 138 
 139         static {
 140             // pre-load and initialize InterruptedException and Cleaner classes
 141             // so that we don't get into trouble later in the run loop if there's
 142             // memory shortage while loading/initializing them lazily.
 143             ensureClassInitialized(InterruptedException.class);
 144             ensureClassInitialized(Cleaner.class);
 145         }
 146 
 147         ReferenceHandler(ThreadGroup g, String name) {
 148             super(g, name);
 149         }
 150 
 151         public void run() {
 152             while (true) {
 153                 tryHandlePending(true);
 154             }
 155         }
 156     }
 157 
 158     /**
 159      * Try handle pending {@link Reference} if there is one.<p>
 160      * Return {@code true} as a hint that there might be another
 161      * {@link Reference} pending or {@code false} when there are no more pending
 162      * {@link Reference}s at the moment and the program can do some other
 163      * useful work instead of looping.
 164      *
 165      * @param waitForNotify if {@code true} and there was no pending
 166      *                      {@link Reference}, wait until notified from VM
 167      *                      or interrupted; if {@code false}, return immediately
 168      *                      when there is no pending {@link Reference}.
 169      * @return {@code true} if there was a {@link Reference} pending and it
 170      *         was processed, or we waited for notification and either got it
 171      *         or thread was interrupted before being notified;
 172      *         {@code false} otherwise.
 173      */
 174     static boolean tryHandlePending(boolean waitForNotify) {
 175         Reference<Object> r;
 176         Cleaner c;
 177         try {
 178             synchronized (lock) {
 179                 if (pending != null) {
 180                     r = pending;
 181                     // 'instanceof' might throw OutOfMemoryError sometimes
 182                     // so do this before un-linking 'r' from the 'pending' chain...
 183                     c = r instanceof Cleaner ? (Cleaner) r : null;
 184                     // unlink 'r' from 'pending' chain
 185                     pending = r.discovered;
 186                     r.discovered = null;
 187                 } else {
 188                     // The waiting on the lock may cause an OutOfMemoryError
 189                     // because it may try to allocate exception objects.
 190                     if (waitForNotify) {
 191                         lock.wait();
 192                     }
 193                     // retry if waited
 194                     return waitForNotify;
 195                 }
 196             }
 197         } catch (OutOfMemoryError x) {
 198             // Give other threads CPU time so they hopefully drop some live references
 199             // and GC reclaims some space.
 200             // Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above
 201             // persistently throws OOME for some time...
 202             Thread.yield();
 203             // retry
 204             return true;
 205         } catch (InterruptedException x) {
 206             // retry
 207             return true;
 208         }
 209 
 210         // Fast path for cleaners
 211         if (c != null) {
 212             c.clean();
 213             return true;
 214         }
 215 
 216         ReferenceQueue<? super Object> q = r.queue;
 217         if (q != ReferenceQueue.NULL) q.enqueue(r);
 218         return true;
 219     }
 220 
 221     static {
 222         ThreadGroup tg = Thread.currentThread().getThreadGroup();
 223         for (ThreadGroup tgn = tg;
 224              tgn != null;
 225              tg = tgn, tgn = tg.getParent());
 226         Thread handler = new ReferenceHandler(tg, "Reference Handler");
 227         /* If there were a special system-only priority greater than
 228          * MAX_PRIORITY, it would be used here
 229          */
 230         handler.setPriority(Thread.MAX_PRIORITY);
 231         handler.setDaemon(true);
 232         handler.start();
 233 
 234         // provide access in SharedSecrets
 235         SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
 236             @Override
 237             public boolean tryHandlePendingReference() {
 238                 return tryHandlePending(false);
 239             }
 240         });
 241     }
 242 
 243     /* -- Referent accessor and setters -- */
 244 
 245     /**
 246      * Returns this reference object's referent.  If this reference object has
 247      * been cleared, either by the program or by the garbage collector, then
 248      * this method returns <code>null</code>.
 249      *
 250      * @return   The object to which this reference refers, or
 251      *           <code>null</code> if this reference object has been cleared
 252      */
 253     public T get() {
 254         return this.referent;
 255     }
 256 
 257     /**
 258      * Clears this reference object.  Invoking this method will not cause this
 259      * object to be enqueued.
 260      *
 261      * <p> This method is invoked only by Java code; when the garbage collector
 262      * clears references it does so directly, without invoking this method.
 263      */
 264     public void clear() {
 265         this.referent = null;
 266     }
 267 
 268 
 269     /* -- Queue operations -- */
 270 
 271     /**
 272      * Tells whether or not this reference object has been enqueued, either by
 273      * the program or by the garbage collector.  If this reference object was
 274      * not registered with a queue when it was created, then this method will
 275      * always return <code>false</code>.
 276      *
 277      * @return   <code>true</code> if and only if this reference object has
 278      *           been enqueued
 279      */
 280     public boolean isEnqueued() {
 281         return (this.queue == ReferenceQueue.ENQUEUED);
 282     }
 283 
 284     /**
 285      * Adds this reference object to the queue with which it is registered,
 286      * if any.
 287      *
 288      * <p> This method is invoked only by Java code; when the garbage collector
 289      * enqueues references it does so directly, without invoking this method.
 290      *
 291      * @return   <code>true</code> if this reference object was successfully
 292      *           enqueued; <code>false</code> if it was already enqueued or if
 293      *           it was not registered with a queue when it was created
 294      */
 295     public boolean enqueue() {
 296         return this.queue.enqueue(this);
 297     }
 298 
 299 
 300     /* -- Constructors -- */
 301 
 302     Reference(T referent) {
 303         this(referent, null);
 304     }
 305 
 306     Reference(T referent, ReferenceQueue<? super T> queue) {
 307         this.referent = referent;
 308         this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
 309     }
 310 
 311 }