1 /*
  2  * Copyright (c) 1997, 2025, 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 jdk.internal.vm.annotation.AOTRuntimeSetup;
 29 import jdk.internal.vm.annotation.AOTSafeClassInitializer;
 30 import jdk.internal.vm.annotation.ForceInline;
 31 import jdk.internal.vm.annotation.IntrinsicCandidate;
 32 import jdk.internal.access.JavaLangRefAccess;
 33 import jdk.internal.access.SharedSecrets;
 34 
 35 /**
 36  * Abstract base class for reference objects.  This class defines the
 37  * operations common to all reference objects.  Because reference objects are
 38  * implemented in close cooperation with the garbage collector, this class may
 39  * not be subclassed directly.
 40  * @param <T> the type of the referent
 41  *
 42  * @author   Mark Reinhold
 43  * @since    1.2
 44  * @sealedGraph
 45  */
 46 @AOTSafeClassInitializer
 47 public abstract sealed class Reference<@jdk.internal.RequiresIdentity T>
 48     permits PhantomReference, SoftReference, WeakReference, FinalReference {
 49 
 50     /* The state of a Reference object is characterized by two attributes.  It
 51      * may be either "active", "pending", or "inactive".  It may also be
 52      * either "registered", "enqueued", "dequeued", or "unregistered".
 53      *
 54      *   Active: Subject to special treatment by the garbage collector.  Some
 55      *   time after the collector detects that the reachability of the
 56      *   referent has changed to the appropriate state, the collector
 57      *   "notifies" the reference, changing the state to either "pending" or
 58      *   "inactive".
 59      *   referent != null; discovered = null, or in GC discovered list.
 60      *
 61      *   Pending: An element of the pending-Reference list, waiting to be
 62      *   processed by the ReferenceHandler thread.  The pending-Reference
 63      *   list is linked through the discovered fields of references in the
 64      *   list.
 65      *   referent = null; discovered = next element in pending-Reference list.
 66      *
 67      *   Inactive: Neither Active nor Pending.
 68      *   referent = null.
 69      *
 70      *   Registered: Associated with a queue when created, and not yet added
 71      *   to the queue.
 72      *   queue = the associated queue.
 73      *
 74      *   Enqueued: Added to the associated queue, and not yet removed.
 75      *   queue = ReferenceQueue.ENQUEUE; next = next entry in list, or this to
 76      *   indicate end of list.
 77      *
 78      *   Dequeued: Added to the associated queue and then removed.
 79      *   queue = ReferenceQueue.NULL_QUEUE; next = this.
 80      *
 81      *   Unregistered: Not associated with a queue when created.
 82      *   queue = ReferenceQueue.NULL_QUEUE.
 83      *
 84      * The collector only needs to examine the referent field and the
 85      * discovered field to determine whether a (non-FinalReference) Reference
 86      * object needs special treatment.  If the referent is non-null and not
 87      * known to be live, then it may need to be discovered for possible later
 88      * notification.  But if the discovered field is non-null, then it has
 89      * already been discovered.
 90      *
 91      * FinalReference (which exists to support finalization) differs from
 92      * other references, because a FinalReference is not cleared when
 93      * notified.  The referent being null or not cannot be used to distinguish
 94      * between the active state and pending or inactive states.  However,
 95      * FinalReferences do not support enqueue().  Instead, the next field of a
 96      * FinalReference object is set to "this" when it is added to the
 97      * pending-Reference list.  The use of "this" as the value of next in the
 98      * enqueued and dequeued states maintains the non-active state.  An
 99      * additional check that the next field is null is required to determine
100      * that a FinalReference object is active.
101      *
102      * Initial states:
103      *   [active/registered]
104      *   [active/unregistered] [1]
105      *
106      * Transitions:
107      *                            clear [2]
108      *   [active/registered]     ------->   [inactive/registered]
109      *          |                                 |
110      *          |                                 | enqueue
111      *          | GC              enqueue [2]     |
112      *          |                -----------------|
113      *          |                                 |
114      *          v                                 |
115      *   [pending/registered]    ---              v
116      *          |                   | ReferenceHandler
117      *          | enqueue [2]       |--->   [inactive/enqueued]
118      *          v                   |             |
119      *   [pending/enqueued]      ---              |
120      *          |                                 | poll/remove
121      *          | poll/remove                     | + clear [4]
122      *          |                                 |
123      *          v            ReferenceHandler     v
124      *   [pending/dequeued]      ------>    [inactive/dequeued]
125      *
126      *
127      *                           clear/enqueue/GC [3]
128      *   [active/unregistered]   ------
129      *          |                      |
130      *          | GC                   |
131      *          |                      |--> [inactive/unregistered]
132      *          v                      |
133      *   [pending/unregistered]  ------
134      *                           ReferenceHandler
135      *
136      * Terminal states:
137      *   [inactive/dequeued]
138      *   [inactive/unregistered]
139      *
140      * Unreachable states (because enqueue also clears):
141      *   [active/enqueued]
142      *   [active/dequeued]
143      *
144      * [1] Unregistered is not permitted for FinalReferences.
145      *
146      * [2] These transitions are not possible for FinalReferences, making
147      * [pending/enqueued], [pending/dequeued], and [inactive/registered]
148      * unreachable.
149      *
150      * [3] The garbage collector may directly transition a Reference
151      * from [active/unregistered] to [inactive/unregistered],
152      * bypassing the pending-Reference list.
153      *
154      * [4] The queue handler for FinalReferences also clears the reference.
155      */
156 
157     private T referent;         /* Treated specially by GC */
158 
159     /* The queue this reference gets enqueued to by GC notification or by
160      * calling enqueue().
161      *
162      * When registered: the queue with which this reference is registered.
163      *        enqueued: ReferenceQueue.ENQUEUE
164      *        dequeued: ReferenceQueue.NULL_QUEUE
165      *    unregistered: ReferenceQueue.NULL_QUEUE
166      */
167     volatile ReferenceQueue<? super T> queue;
168 
169     /* The link in a ReferenceQueue's list of Reference objects.
170      *
171      * When registered: null
172      *        enqueued: next element in queue (or this if last)
173      *        dequeued: this (marking FinalReferences as inactive)
174      *    unregistered: null
175      */
176     @SuppressWarnings("rawtypes")
177     volatile Reference next;
178 
179     /* Used by the garbage collector to accumulate Reference objects that need
180      * to be revisited in order to decide whether they should be notified.
181      * Also used as the link in the pending-Reference list.  The discovered
182      * field and the next field are distinct to allow the enqueue() method to
183      * be applied to a Reference object while it is either in the
184      * pending-Reference list or in the garbage collector's discovered set.
185      *
186      * When active: null or next element in a discovered reference list
187      *              maintained by the GC (or this if last)
188      *     pending: next element in the pending-Reference list (null if last)
189      *    inactive: null
190      */
191     private transient Reference<?> discovered;
192 
193 
194     /* High-priority thread to enqueue pending References
195      */
196     private static class ReferenceHandler extends Thread {
197         ReferenceHandler(ThreadGroup g, String name) {
198             super(g, null, name, 0, false);
199         }
200 
201         public void run() {
202             while (true) {
203                 processPendingReferences();
204             }
205         }
206     }
207 
208     /*
209      * Atomically get and clear (set to null) the VM's pending-Reference list.
210      */
211     private static native Reference<?> getAndClearReferencePendingList();
212 
213     /*
214      * Test whether the VM's pending-Reference list contains any entries.
215      */
216     private static native boolean hasReferencePendingList();
217 
218     /*
219      * Wait until the VM's pending-Reference list may be non-null.
220      */
221     private static native void waitForReferencePendingList();
222 
223     /*
224      * Enqueue a Reference taken from the pending list.  Calling this method
225      * takes us from the Reference<?> domain of the pending list elements to
226      * having a Reference<T> with a correspondingly typed queue.
227      */
228     private void enqueueFromPending() {
229         var q = queue;
230         if (q != ReferenceQueue.NULL_QUEUE) q.enqueue(this);
231     }
232 
233     private static final Object processPendingLock = new Object();
234     private static boolean processPendingActive = false;
235 
236     private static void processPendingReferences() {
237         // Only the singleton reference processing thread calls
238         // waitForReferencePendingList() and getAndClearReferencePendingList().
239         // These are separate operations to avoid a race with other threads
240         // that are calling waitForReferenceProcessing().
241         waitForReferencePendingList();
242         Reference<?> pendingList;
243         synchronized (processPendingLock) {
244             pendingList = getAndClearReferencePendingList();
245             processPendingActive = true;
246         }
247         while (pendingList != null) {
248             Reference<?> ref = pendingList;
249             pendingList = ref.discovered;
250             ref.discovered = null;
251             ref.enqueueFromPending();
252         }
253         // Notify any waiters of completion of current round.
254         synchronized (processPendingLock) {
255             processPendingActive = false;
256             processPendingLock.notifyAll();
257         }
258     }
259 
260     // Wait for progress in reference processing.
261     //
262     // Returns true after waiting (for notification from the reference
263     // processing thread) if either (1) the VM has any pending
264     // references, or (2) the reference processing thread is
265     // processing references. Otherwise, returns false immediately.
266     private static boolean waitForReferenceProcessing()
267         throws InterruptedException
268     {
269         synchronized (processPendingLock) {
270             if (processPendingActive || hasReferencePendingList()) {
271                 // Wait for progress, not necessarily completion.
272                 processPendingLock.wait();
273                 return true;
274             } else {
275                 return false;
276             }
277         }
278     }
279 
280     /**
281      * Start the Reference Handler thread as a daemon thread.
282      */
283     static void startReferenceHandlerThread(ThreadGroup tg) {
284         Thread handler = new ReferenceHandler(tg, "Reference Handler");
285         /* If there were a special system-only priority greater than
286          * MAX_PRIORITY, it would be used here
287          */
288         handler.setPriority(Thread.MAX_PRIORITY);
289         handler.setDaemon(true);
290         handler.start();
291     }
292 
293     static {
294         runtimeSetup();
295     }
296 
297     @AOTRuntimeSetup
298     private static void runtimeSetup() {
299         // provide access in SharedSecrets
300         SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
301             @Override
302             public void startThreads() {
303                 ThreadGroup tg = Thread.currentThread().getThreadGroup();
304                 for (ThreadGroup tgn = tg;
305                      tgn != null;
306                      tg = tgn, tgn = tg.getParent());
307                 Reference.startReferenceHandlerThread(tg);
308                 Finalizer.startFinalizerThread(tg);
309             }
310 
311             @Override
312             public boolean waitForReferenceProcessing()
313                 throws InterruptedException
314             {
315                 return Reference.waitForReferenceProcessing();
316             }
317 
318             @Override
319             public void runFinalization() {
320                 Finalizer.runFinalization();
321             }
322         });
323     }
324 
325     /* -- Referent accessor and setters -- */
326 
327     /**
328      * Returns this reference object's referent.  If this reference object has
329      * been cleared, either by the program or by the garbage collector, then
330      * this method returns {@code null}.
331      *
332      * @apiNote
333      * This method returns a strong reference to the referent. This may cause
334      * the garbage collector to treat it as strongly reachable until some later
335      * collection cycle.  The {@link #refersTo(Object) refersTo} method can be
336      * used to avoid such strengthening when testing whether some object is
337      * the referent of a reference object; that is, use {@code ref.refersTo(obj)}
338      * rather than {@code ref.get() == obj}.
339      *
340      * @return   The object to which this reference refers, or
341      *           {@code null} if this reference object has been cleared
342      * @see #refersTo
343      */
344     public T get() {
345         return get0();
346     }
347 
348     /* Implementation of get().  This method exists to avoid making get() all
349      * of virtual, native, and intrinsic candidate. That could have the
350      * undesirable effect of having the native method used instead of the
351      * intrinsic when devirtualization fails.
352      */
353     @IntrinsicCandidate
354     private native T get0();
355 
356     /**
357      * Tests if the referent of this reference object is {@code obj}.
358      * Using a {@code null} {@code obj} returns {@code true} if the
359      * reference object has been cleared.
360      *
361      * @param  obj the object to compare with this reference object's referent
362      * @return {@code true} if {@code obj} is the referent of this reference object
363      * @since 16
364      */
365     public final boolean refersTo(T obj) {
366         return refersToImpl(obj);
367     }
368 
369     /* Implementation of refersTo(), overridden for phantom references.
370      * This method exists only to avoid making refersTo0() virtual. Making
371      * refersTo0() virtual has the undesirable effect of C2 often preferring
372      * to call the native implementation over the intrinsic.
373      */
374     boolean refersToImpl(T obj) {
375         return refersTo0(obj);
376     }
377 
378     @IntrinsicCandidate
379     private native boolean refersTo0(Object o);
380 
381     /**
382      * Clears this reference object. Invoking this method does not enqueue this
383      * object, and the garbage collector will not clear or enqueue this object.
384      *
385      * <p>When the garbage collector or the {@link #enqueue()} method clear
386      * references they do so directly, without invoking this method.
387      *
388      * @apiNote
389      * There is a potential race condition with the garbage collector. When this
390      * method is called, the garbage collector may already be in the process of
391      * (or already completed) clearing and/or enqueueing this reference.
392      * Avoid this race by ensuring the referent remains strongly reachable until
393      * after the call to clear(), using {@link #reachabilityFence(Object)} if
394      * necessary.
395      */
396     public void clear() {
397         clearImpl();
398     }
399 
400     /* Implementation of clear(). A simple assignment of the referent field
401      * won't do for some garbage collectors. There is the override for phantom
402      * references, which requires different semantics. This method is also
403      * used by enqueue().
404      *
405      * <p>This method exists only to avoid making clear0() virtual. Making
406      * clear0() virtual has the undesirable effect of C2 often preferring
407      * to call the native implementation over the intrinsic.
408      */
409     void clearImpl() {
410         clear0();
411     }
412 
413     @IntrinsicCandidate
414     private native void clear0();
415 
416     /* -- Operations on inactive FinalReferences -- */
417 
418     /* These functions are only used by FinalReference, and must only be
419      * called after the reference becomes inactive. While active, a
420      * FinalReference is considered weak but the referent is not normally
421      * accessed. Once a FinalReference becomes inactive it is considered a
422      * strong reference. These functions are used to bypass the
423      * corresponding weak implementations, directly accessing the referent
424      * field with strong semantics.
425      */
426 
427     /**
428      * Load referent with strong semantics.
429      */
430     T getFromInactiveFinalReference() {
431         assert this instanceof FinalReference;
432         assert next != null; // I.e. FinalReference is inactive
433         return this.referent;
434     }
435 
436     /**
437      * Clear referent with strong semantics.
438      */
439     void clearInactiveFinalReference() {
440         assert this instanceof FinalReference;
441         assert next != null; // I.e. FinalReference is inactive
442         this.referent = null;
443     }
444 
445     /* -- Queue operations -- */
446 
447     /**
448      * Tests if this reference object is in its associated queue, if any.
449      * This method returns {@code true} only if all of the following conditions
450      * are met:
451      * <ul>
452      * <li>this reference object was registered with a queue when it was created; and
453      * <li>the garbage collector has added this reference object to the queue
454      *     or {@link #enqueue()} is called; and
455      * <li>this reference object is not yet removed from the queue.
456      * </ul>
457      * Otherwise, this method returns {@code false}.
458      * This method may return {@code false} if this reference object has been cleared
459      * but not enqueued due to the race condition.
460      *
461      * @deprecated
462      * This method was originally specified to test if a reference object has
463      * been cleared and enqueued but was never implemented to do this test.
464      * This method could be misused due to the inherent race condition
465      * or without an associated {@code ReferenceQueue}.
466      * An application relying on this method to release critical resources
467      * could cause serious performance issue.
468      * An application should use {@link ReferenceQueue} to reliably determine
469      * what reference objects that have been enqueued or
470      * {@link #refersTo(Object) refersTo(null)} to determine if this reference
471      * object has been cleared.
472      *
473      * @return   {@code true} if and only if this reference object is
474      *           in its associated queue (if any).
475      */
476     @Deprecated(since="16")
477     public boolean isEnqueued() {
478         return (this.queue == ReferenceQueue.ENQUEUED);
479     }
480 
481     /**
482      * Clears this reference object, then attempts to add it to the queue with
483      * which it is registered, if any.
484      *
485      * <p>If this reference is registered with a queue but not yet enqueued,
486      * the reference is added to the queue; this method is
487      * <b><i>successful</i></b> and returns true.
488      * If this reference is not registered with a queue, or was already enqueued
489      * (by the garbage collector, or a previous call to {@code enqueue}), this
490      * method is <b><i>unsuccessful</i></b> and returns false.
491      *
492      * <p>{@linkplain java.lang.ref##MemoryConsistency Memory consistency effects}:
493      * Actions in a thread prior to a <b><i>successful</i></b> call to {@code enqueue}
494      * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happen-before</i></a>
495      * the reference is removed from the queue by {@link ReferenceQueue#poll}
496      * or {@link ReferenceQueue#remove}. <b><i>Unsuccessful</i></b> calls to
497      * {@code enqueue} have no specified memory consistency effects.
498      *
499      * <p> When this method clears references it does so directly, without
500      * invoking the {@link #clear()} method. When the garbage collector clears
501      * and enqueues references it does so directly, without invoking the
502      * {@link #clear()} method or this method.
503      *
504      * @apiNote
505      * Use of this method allows the registered queue's
506      * {@link ReferenceQueue#poll} and {@link ReferenceQueue#remove} methods
507      * to return this reference even though the referent may still be strongly
508      * reachable.
509      *
510      * @return   {@code true} if this reference object was successfully
511      *           enqueued; {@code false} if it was already enqueued or if
512      *           it was not registered with a queue when it was created
513      */
514     public boolean enqueue() {
515         clearImpl(); // Intentionally clearImpl() to dispatch to overridden method, if needed
516         return this.queue.enqueue(this);
517     }
518 
519     /**
520      * Throws {@link CloneNotSupportedException}. A {@code Reference} cannot be
521      * meaningfully cloned. Construct a new {@code Reference} instead.
522      *
523      * @return never returns normally
524      * @throws  CloneNotSupportedException always
525      */
526     @Override
527     protected Object clone() throws CloneNotSupportedException {
528         throw new CloneNotSupportedException();
529     }
530 
531     /* -- Constructors -- */
532 
533     Reference(T referent) {
534         this(referent, null);
535     }
536 
537     Reference(T referent, ReferenceQueue<? super T> queue) {
538         this.referent = referent;
539         this.queue = (queue == null) ? ReferenceQueue.NULL_QUEUE : queue;
540     }
541 
542     /**
543      * Ensures that the given object remains
544      * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>.
545      * This reachability is assured regardless of any optimizing transformations
546      * the virtual machine may perform that might otherwise allow the object to
547      * become unreachable (see JLS {@jls 12.6.1}). Thus, the given object is not
548      * reclaimable by garbage collection at least until after the invocation of
549      * this method. References to the given object will not be cleared (or
550      * enqueued, if applicable) by the garbage collector until after invocation
551      * of this method.
552      * Invocation of this method does not itself initiate reference processing,
553      * garbage collection, or finalization.
554      *
555      * <p> This method establishes an ordering for <em>strong reachability</em>
556      * with respect to garbage collection.  It controls relations that are
557      * otherwise only implicit in a program -- the reachability conditions
558      * triggering garbage collection.  This method is applicable only
559      * when reclamation may have visible effects,
560      * such as for objects that use finalizers or {@link Cleaner}, or code that
561      * performs {@linkplain java.lang.ref reference processing}.
562      *
563      * <p>{@linkplain java.lang.ref##MemoryConsistency Memory consistency effects}:
564      * Actions in a thread prior to calling {@code reachabilityFence(x)}
565      * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happen-before</i></a>
566      * the garbage collector clears any reference to {@code x}.
567      *
568      * @apiNote
569      * Reference processing or finalization can occur after an object becomes
570      * unreachable. An object can become unreachable when the virtual machine
571      * detects that there is no further need for the object (other than for
572      * running a finalizer). In the course of optimization, the virtual machine
573      * can reorder operations of an object's methods such that the object
574      * becomes unneeded earlier than might naively be expected &mdash;
575      * including while a method of the object is still running. For instance,
576      * the VM can move the loading of <em>values</em> from the object's fields
577      * to occur earlier. The object itself is then no longer needed and becomes
578      * unreachable, and the method can continue running using the obtained values.
579      * This may have surprising and undesirable effects when using a Cleaner or
580      * finalizer for cleanup: there is a race between the
581      * program thread running the method, and the cleanup thread running the
582      * Cleaner or finalizer. The cleanup thread could free a
583      * resource, followed by the program thread (still running the method)
584      * attempting to access the now-already-freed resource.
585      * Use of {@code reachabilityFence} can prevent this race by ensuring that the
586      * object remains strongly reachable.
587      * <p>
588      * The following is an example in which the bookkeeping associated with a class is
589      * managed through array indices.  Here, method {@code action} uses a
590      * {@code reachabilityFence} to ensure that the {@code Resource} object is
591      * not reclaimed before bookkeeping on an associated
592      * {@code ExternalResource} has been performed; specifically, to
593      * ensure that the array slot holding the {@code ExternalResource} is not
594      * nulled out in method {@link Object#finalize}, which may otherwise run
595      * concurrently.
596      *
597      * {@snippet :
598      * class Resource {
599      *   private static ExternalResource[] externalResourceArray = ...
600      *
601      *   int myIndex;
602      *   Resource(...) {
603      *     this.myIndex = ...
604      *     externalResourceArray[myIndex] = ...;
605      *     ...
606      *   }
607      *   protected void finalize() {
608      *     externalResourceArray[this.myIndex] = null;
609      *     ...
610      *   }
611      *   public void action() {
612      *     try {
613      *       // ...
614      *       int i = this.myIndex; // last use of 'this' Resource in action()
615      *       Resource.update(externalResourceArray[i]);
616      *     } finally {
617      *       Reference.reachabilityFence(this);
618      *     }
619      *   }
620      *   private static void update(ExternalResource ext) {
621      *     ext.status = ...;
622      *   }
623      * }
624      * }
625      *
626      * The invocation of {@code reachabilityFence} is
627      * placed <em>after</em> the call to {@code update}, to ensure that the
628      * array slot is not nulled out by {@link Object#finalize} before the
629      * update, even if the call to {@code action} was the last use of this
630      * object.  This might be the case if, for example, a usage in a user program
631      * had the form {@code new Resource().action();} which retains no other
632      * reference to this {@code Resource}.
633      * The {@code reachabilityFence} call is placed in a {@code finally} block to
634      * ensure that it is invoked across all paths in the method. A more complex
635      * method might need further precautions to ensure that
636      * {@code reachabilityFence} is encountered along all code paths.
637      *
638      * <p> Method {@code reachabilityFence} is not required in constructions
639      * that themselves ensure reachability.  For example, because objects that
640      * are locked cannot, in general, be reclaimed, it would suffice if all
641      * accesses of the object, in all methods of class {@code Resource}
642      * (including {@code finalize}) were enclosed in {@code synchronized (this)}
643      * blocks.  (Further, such blocks must not include infinite loops, or
644      * themselves be unreachable, which fall into the corner case exceptions to
645      * the "in general" disclaimer.)  However, method {@code reachabilityFence}
646      * remains a better option in cases where synchronization is not as efficient,
647      * desirable, or possible; for example because it would encounter deadlock.
648      *
649      * @param ref the reference to the object to keep strongly reachable. If
650      * {@code null}, this method has no effect.
651      * @since 9
652      */
653     @ForceInline
654     public static void reachabilityFence(Object ref) {
655         // Does nothing. This method is annotated with @ForceInline to eliminate
656         // most of the overhead that using @DontInline would cause with the
657         // HotSpot JVM, when this fence is used in a wide variety of situations.
658         // HotSpot JVM retains the ref and does not GC it before a call to
659         // this method, because the JIT-compilers do not have GC-only safepoints.
660     }
661 }