1 /*

     2  * Copyright 1997-2006 Sun Microsystems, Inc.  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.  Sun designates this

     8  * particular file as subject to the "Classpath" exception as provided

     9  * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

    22  * CA 95054 USA or visit www.sun.com if you need additional information or

    23  * have any questions.

    24  */

    25 

    26 package java.lang.ref;

    27 

    28 import sun.misc.Cleaner;

    29 

    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 = Following instance in queue, or this if at end of list.

    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 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.

    88      */

    89 

    90     private T referent;         /* Treated specially by GC */

    91 

    92     ReferenceQueue<? super T> queue;

    93 

    94     Reference next;

    95     transient private Reference<T> discovered;  /* used by VM */

    96 

    97 

    98     /* Object used to synchronize with the garbage collector.  The collector

    99      * must acquire this lock at the beginning of each collection cycle.  It is

   100      * therefore critical that any code holding this lock complete as quickly

   101      * as possible, allocate no new objects, and avoid calling user code.

   102      */

   103     static private class Lock { };

   104     private static Lock lock = new Lock();

   105 

   106 

   107     /* List of References waiting to be enqueued.  The collector adds

   108      * References to this list, while the Reference-handler thread removes

   109      * them.  This list is protected by the above lock object.

   110      */

   111     private static Reference pending = null;

   112 

   113     /* High-priority thread to enqueue pending References

   114      */

   115     private static class ReferenceHandler extends Thread {

   116 

   117         ReferenceHandler(ThreadGroup g, String name) {

   118             super(g, name);

   119         }

   120 

   121         public void run() {

   122             for (;;) {

   123 

   124                 Reference r;

   125                 synchronized (lock) {

   126                     if (pending != null) {

   127                         r = pending;

   128                         Reference rn = r.next;

   129                         pending = (rn == r) ? null : rn;

   130                         r.next = r;

   131                     } else {

   132                         try {

   133                             lock.wait();

   134                         } catch (InterruptedException x) { }

   135                         continue;

   136                     }

   137                 }

   138 

   139                 // Fast path for cleaners

   140                 if (r instanceof Cleaner) {

   141                     ((Cleaner)r).clean();

   142                     continue;

   143                 }

   144 

   145                 ReferenceQueue q = r.queue;

   146                 if (q != ReferenceQueue.NULL) q.enqueue(r);

   147             }

   148         }

   149     }

   150 

   151     static {

   152         ThreadGroup tg = Thread.currentThread().getThreadGroup();

   153         for (ThreadGroup tgn = tg;

   154              tgn != null;

   155              tg = tgn, tgn = tg.getParent());

   156         Thread handler = new ReferenceHandler(tg, "Reference Handler");

   157         /* If there were a special system-only priority greater than

   158          * MAX_PRIORITY, it would be used here

   159          */

   160         handler.setPriority(Thread.MAX_PRIORITY);

   161         handler.setDaemon(true);

   162         handler.start();

   163     }

   164 

   165 

   166     /* -- Referent accessor and setters -- */

   167 

   168     /**

   169      * Returns this reference object's referent.  If this reference object has

   170      * been cleared, either by the program or by the garbage collector, then

   171      * this method returns <code>null</code>.

   172      *

   173      * @return   The object to which this reference refers, or

   174      *           <code>null</code> if this reference object has been cleared

   175      */

   176     public T get() {

   177         return this.referent;

   178     }

   179 

   180     /**

   181      * Clears this reference object.  Invoking this method will not cause this

   182      * object to be enqueued.

   183      *

   184      * <p> This method is invoked only by Java code; when the garbage collector

   185      * clears references it does so directly, without invoking this method.

   186      */

   187     public void clear() {

   188         this.referent = null;

   189     }

   190 

   191 

   192     /* -- Queue operations -- */

   193 

   194     /**

   195      * Tells whether or not this reference object has been enqueued, either by

   196      * the program or by the garbage collector.  If this reference object was

   197      * not registered with a queue when it was created, then this method will

   198      * always return <code>false</code>.

   199      *

   200      * @return   <code>true</code> if and only if this reference object has

   201      *           been enqueued

   202      */

   203     public boolean isEnqueued() {

   204         /* In terms of the internal states, this predicate actually tests

   205            whether the instance is either Pending or Enqueued */

   206         synchronized (this) {

   207             return (this.queue != ReferenceQueue.NULL) && (this.next != null);

   208         }

   209     }

   210 

   211     /**

   212      * Adds this reference object to the queue with which it is registered,

   213      * if any.

   214      *

   215      * <p> This method is invoked only by Java code; when the garbage collector

   216      * enqueues references it does so directly, without invoking this method.

   217      *

   218      * @return   <code>true</code> if this reference object was successfully

   219      *           enqueued; <code>false</code> if it was already enqueued or if

   220      *           it was not registered with a queue when it was created

   221      */

   222     public boolean enqueue() {

   223         return this.queue.enqueue(this);

   224     }

   225 

   226 

   227     /* -- Constructors -- */

   228 

   229     Reference(T referent) {

   230         this(referent, null);

   231     }

   232 

   233     Reference(T referent, ReferenceQueue<? super T> queue) {

   234         this.referent = referent;

   235         this.queue = (queue == null) ? ReferenceQueue.NULL : queue;

   236     }

   237 

   238 }