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 }