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 import java.util.Objects;
36
37 /**
38 * Abstract base class for reference objects. This class defines the
39 * operations common to all reference objects. Because reference objects are
40 * implemented in close cooperation with the garbage collector, this class may
41 * not be subclassed directly.
42 *
43 * <div class="preview-block">
44 * <div class="preview-comment">
45 * The referent must have {@linkplain Objects#hasIdentity(Object) object identity}.
46 * When preview features are enabled, attempts to create a reference
47 * to a {@linkplain Class#isValue value object} result in an {@link IdentityException}.
48 * </div>
49 * </div>
50 * @param <T> the type of the referent
51 *
52 * @author Mark Reinhold
53 * @since 1.2
54 * @sealedGraph
55 */
56 @AOTSafeClassInitializer
57 public abstract sealed class Reference<@jdk.internal.RequiresIdentity T>
58 permits PhantomReference, SoftReference, WeakReference, FinalReference {
59
60 /* The state of a Reference object is characterized by two attributes. It
61 * may be either "active", "pending", or "inactive". It may also be
62 * either "registered", "enqueued", "dequeued", or "unregistered".
63 *
64 * Active: Subject to special treatment by the garbage collector. Some
65 * time after the collector detects that the reachability of the
66 * referent has changed to the appropriate state, the collector
67 * "notifies" the reference, changing the state to either "pending" or
68 * "inactive".
69 * referent != null; discovered = null, or in GC discovered list.
70 *
71 * Pending: An element of the pending-Reference list, waiting to be
72 * processed by the ReferenceHandler thread. The pending-Reference
73 * list is linked through the discovered fields of references in the
74 * list.
75 * referent = null; discovered = next element in pending-Reference list.
76 *
77 * Inactive: Neither Active nor Pending.
78 * referent = null.
79 *
80 * Registered: Associated with a queue when created, and not yet added
81 * to the queue.
82 * queue = the associated queue.
83 *
84 * Enqueued: Added to the associated queue, and not yet removed.
85 * queue = ReferenceQueue.ENQUEUE; next = next entry in list, or this to
86 * indicate end of list.
87 *
88 * Dequeued: Added to the associated queue and then removed.
89 * queue = ReferenceQueue.NULL_QUEUE; next = this.
90 *
91 * Unregistered: Not associated with a queue when created.
92 * queue = ReferenceQueue.NULL_QUEUE.
93 *
94 * The collector only needs to examine the referent field and the
95 * discovered field to determine whether a (non-FinalReference) Reference
96 * object needs special treatment. If the referent is non-null and not
97 * known to be live, then it may need to be discovered for possible later
98 * notification. But if the discovered field is non-null, then it has
99 * already been discovered.
100 *
101 * FinalReference (which exists to support finalization) differs from
102 * other references, because a FinalReference is not cleared when
103 * notified. The referent being null or not cannot be used to distinguish
104 * between the active state and pending or inactive states. However,
105 * FinalReferences do not support enqueue(). Instead, the next field of a
106 * FinalReference object is set to "this" when it is added to the
107 * pending-Reference list. The use of "this" as the value of next in the
108 * enqueued and dequeued states maintains the non-active state. An
109 * additional check that the next field is null is required to determine
110 * that a FinalReference object is active.
111 *
112 * Initial states:
113 * [active/registered]
114 * [active/unregistered] [1]
115 *
116 * Transitions:
117 * clear [2]
118 * [active/registered] -------> [inactive/registered]
119 * | |
120 * | | enqueue
121 * | GC enqueue [2] |
122 * | -----------------|
123 * | |
124 * v |
125 * [pending/registered] --- v
126 * | | ReferenceHandler
127 * | enqueue [2] |---> [inactive/enqueued]
128 * v | |
129 * [pending/enqueued] --- |
130 * | | poll/remove
131 * | poll/remove | + clear [4]
132 * | |
133 * v ReferenceHandler v
134 * [pending/dequeued] ------> [inactive/dequeued]
135 *
136 *
137 * clear/enqueue/GC [3]
138 * [active/unregistered] ------
139 * | |
140 * | GC |
141 * | |--> [inactive/unregistered]
142 * v |
143 * [pending/unregistered] ------
144 * ReferenceHandler
145 *
146 * Terminal states:
147 * [inactive/dequeued]
148 * [inactive/unregistered]
149 *
150 * Unreachable states (because enqueue also clears):
151 * [active/enqueued]
152 * [active/dequeued]
153 *
154 * [1] Unregistered is not permitted for FinalReferences.
155 *
156 * [2] These transitions are not possible for FinalReferences, making
157 * [pending/enqueued], [pending/dequeued], and [inactive/registered]
158 * unreachable.
159 *
160 * [3] The garbage collector may directly transition a Reference
161 * from [active/unregistered] to [inactive/unregistered],
162 * bypassing the pending-Reference list.
163 *
164 * [4] The queue handler for FinalReferences also clears the reference.
165 */
166
167 private T referent; /* Treated specially by GC */
168
169 /* The queue this reference gets enqueued to by GC notification or by
170 * calling enqueue().
171 *
172 * When registered: the queue with which this reference is registered.
173 * enqueued: ReferenceQueue.ENQUEUE
174 * dequeued: ReferenceQueue.NULL_QUEUE
175 * unregistered: ReferenceQueue.NULL_QUEUE
176 */
177 volatile ReferenceQueue<? super T> queue;
178
179 /* The link in a ReferenceQueue's list of Reference objects.
180 *
181 * When registered: null
182 * enqueued: next element in queue (or this if last)
183 * dequeued: this (marking FinalReferences as inactive)
184 * unregistered: null
185 */
186 @SuppressWarnings("rawtypes")
187 volatile Reference next;
188
189 /* Used by the garbage collector to accumulate Reference objects that need
190 * to be revisited in order to decide whether they should be notified.
191 * Also used as the link in the pending-Reference list. The discovered
192 * field and the next field are distinct to allow the enqueue() method to
193 * be applied to a Reference object while it is either in the
194 * pending-Reference list or in the garbage collector's discovered set.
195 *
196 * When active: null or next element in a discovered reference list
197 * maintained by the GC (or this if last)
198 * pending: next element in the pending-Reference list (null if last)
199 * inactive: null
200 */
201 private transient Reference<?> discovered;
202
203
204 /* High-priority thread to enqueue pending References
205 */
206 private static class ReferenceHandler extends Thread {
207 ReferenceHandler(ThreadGroup g, String name) {
208 super(g, null, name, 0, false);
209 }
210
211 public void run() {
212 while (true) {
213 processPendingReferences();
214 }
215 }
216 }
217
218 /*
219 * Atomically get and clear (set to null) the VM's pending-Reference list.
220 */
221 private static native Reference<?> getAndClearReferencePendingList();
222
223 /*
224 * Test whether the VM's pending-Reference list contains any entries.
225 */
226 private static native boolean hasReferencePendingList();
227
228 /*
229 * Wait until the VM's pending-Reference list may be non-null.
230 */
231 private static native void waitForReferencePendingList();
232
233 /*
234 * Enqueue a Reference taken from the pending list. Calling this method
235 * takes us from the Reference<?> domain of the pending list elements to
236 * having a Reference<T> with a correspondingly typed queue.
237 */
238 private void enqueueFromPending() {
239 var q = queue;
240 if (q != ReferenceQueue.NULL_QUEUE) q.enqueue(this);
241 }
242
243 private static final Object processPendingLock = new Object();
244 private static boolean processPendingActive = false;
245
246 private static void processPendingReferences() {
247 // Only the singleton reference processing thread calls
248 // waitForReferencePendingList() and getAndClearReferencePendingList().
249 // These are separate operations to avoid a race with other threads
250 // that are calling waitForReferenceProcessing().
251 waitForReferencePendingList();
252 Reference<?> pendingList;
253 synchronized (processPendingLock) {
254 pendingList = getAndClearReferencePendingList();
255 processPendingActive = true;
256 }
257 while (pendingList != null) {
258 Reference<?> ref = pendingList;
259 pendingList = ref.discovered;
260 ref.discovered = null;
261 ref.enqueueFromPending();
262 }
263 // Notify any waiters of completion of current round.
264 synchronized (processPendingLock) {
265 processPendingActive = false;
266 processPendingLock.notifyAll();
267 }
268 }
269
270 // Wait for progress in reference processing.
271 //
272 // Returns true after waiting (for notification from the reference
273 // processing thread) if either (1) the VM has any pending
274 // references, or (2) the reference processing thread is
275 // processing references. Otherwise, returns false immediately.
276 private static boolean waitForReferenceProcessing()
277 throws InterruptedException
278 {
279 synchronized (processPendingLock) {
280 if (processPendingActive || hasReferencePendingList()) {
281 // Wait for progress, not necessarily completion.
282 processPendingLock.wait();
283 return true;
284 } else {
285 return false;
286 }
287 }
288 }
289
290 /**
291 * Start the Reference Handler thread as a daemon thread.
292 */
293 static void startReferenceHandlerThread(ThreadGroup tg) {
294 Thread handler = new ReferenceHandler(tg, "Reference Handler");
295 /* If there were a special system-only priority greater than
296 * MAX_PRIORITY, it would be used here
297 */
298 handler.setPriority(Thread.MAX_PRIORITY);
299 handler.setDaemon(true);
300 handler.start();
301 }
302
303 static {
304 runtimeSetup();
305 }
306
307 @AOTRuntimeSetup
308 private static void runtimeSetup() {
309 // provide access in SharedSecrets
310 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
311 @Override
312 public void startThreads() {
313 ThreadGroup tg = Thread.currentThread().getThreadGroup();
314 for (ThreadGroup tgn = tg;
315 tgn != null;
316 tg = tgn, tgn = tg.getParent());
317 Reference.startReferenceHandlerThread(tg);
318 Finalizer.startFinalizerThread(tg);
319 }
320
321 @Override
322 public boolean waitForReferenceProcessing()
323 throws InterruptedException
324 {
325 return Reference.waitForReferenceProcessing();
326 }
327
328 @Override
329 public void runFinalization() {
330 Finalizer.runFinalization();
331 }
332 });
333 }
334
335 /* -- Referent accessor and setters -- */
336
337 /**
338 * Returns this reference object's referent. If this reference object has
339 * been cleared, either by the program or by the garbage collector, then
340 * this method returns {@code null}.
341 *
342 * @apiNote
343 * This method returns a strong reference to the referent. This may cause
344 * the garbage collector to treat it as strongly reachable until some later
345 * collection cycle. The {@link #refersTo(Object) refersTo} method can be
346 * used to avoid such strengthening when testing whether some object is
347 * the referent of a reference object; that is, use {@code ref.refersTo(obj)}
348 * rather than {@code ref.get() == obj}.
349 *
350 * @return The object to which this reference refers, or
351 * {@code null} if this reference object has been cleared
352 * @see #refersTo
353 */
354 public T get() {
355 return get0();
356 }
357
358 /* Implementation of get(). This method exists to avoid making get() all
359 * of virtual, native, and intrinsic candidate. That could have the
360 * undesirable effect of having the native method used instead of the
361 * intrinsic when devirtualization fails.
362 */
363 @IntrinsicCandidate
364 private native T get0();
365
366 /**
367 * Tests if the referent of this reference object is {@code obj}.
368 * Using a {@code null} {@code obj} returns {@code true} if the
369 * reference object has been cleared.
370 *
371 * @param obj the object to compare with this reference object's referent
372 * @return {@code true} if {@code obj} is the referent of this reference object
373 * @since 16
374 */
375 public final boolean refersTo(T obj) {
376 return refersToImpl(obj);
377 }
378
379 /* Implementation of refersTo(), overridden for phantom references.
380 * This method exists only to avoid making refersTo0() virtual. Making
381 * refersTo0() virtual has the undesirable effect of C2 often preferring
382 * to call the native implementation over the intrinsic.
383 */
384 boolean refersToImpl(T obj) {
385 return refersTo0(obj);
386 }
387
388 @IntrinsicCandidate
389 private native boolean refersTo0(Object o);
390
391 /**
392 * Clears this reference object. Invoking this method does not enqueue this
393 * object, and the garbage collector will not clear or enqueue this object.
394 *
395 * <p>When the garbage collector or the {@link #enqueue()} method clear
396 * references they do so directly, without invoking this method.
397 *
398 * @apiNote
399 * There is a potential race condition with the garbage collector. When this
400 * method is called, the garbage collector may already be in the process of
401 * (or already completed) clearing and/or enqueueing this reference.
402 * Avoid this race by ensuring the referent remains strongly reachable until
403 * after the call to clear(), using {@link #reachabilityFence(Object)} if
404 * necessary.
405 */
406 public void clear() {
407 clearImpl();
408 }
409
410 /* Implementation of clear(). A simple assignment of the referent field
411 * won't do for some garbage collectors. There is the override for phantom
412 * references, which requires different semantics. This method is also
413 * used by enqueue().
414 *
415 * <p>This method exists only to avoid making clear0() virtual. Making
416 * clear0() virtual has the undesirable effect of C2 often preferring
417 * to call the native implementation over the intrinsic.
418 */
419 void clearImpl() {
420 clear0();
421 }
422
423 @IntrinsicCandidate
424 private native void clear0();
425
426 /* -- Operations on inactive FinalReferences -- */
427
428 /* These functions are only used by FinalReference, and must only be
429 * called after the reference becomes inactive. While active, a
430 * FinalReference is considered weak but the referent is not normally
431 * accessed. Once a FinalReference becomes inactive it is considered a
432 * strong reference. These functions are used to bypass the
433 * corresponding weak implementations, directly accessing the referent
434 * field with strong semantics.
435 */
436
437 /**
438 * Load referent with strong semantics.
439 */
440 T getFromInactiveFinalReference() {
441 assert this instanceof FinalReference;
442 assert next != null; // I.e. FinalReference is inactive
443 return this.referent;
444 }
445
446 /**
447 * Clear referent with strong semantics.
448 */
449 void clearInactiveFinalReference() {
450 assert this instanceof FinalReference;
451 assert next != null; // I.e. FinalReference is inactive
452 this.referent = null;
453 }
454
455 /* -- Queue operations -- */
456
457 /**
458 * Tests if this reference object is in its associated queue, if any.
459 * This method returns {@code true} only if all of the following conditions
460 * are met:
461 * <ul>
462 * <li>this reference object was registered with a queue when it was created; and
463 * <li>the garbage collector has added this reference object to the queue
464 * or {@link #enqueue()} is called; and
465 * <li>this reference object is not yet removed from the queue.
466 * </ul>
467 * Otherwise, this method returns {@code false}.
468 * This method may return {@code false} if this reference object has been cleared
469 * but not enqueued due to the race condition.
470 *
471 * @deprecated
472 * This method was originally specified to test if a reference object has
473 * been cleared and enqueued but was never implemented to do this test.
474 * This method could be misused due to the inherent race condition
475 * or without an associated {@code ReferenceQueue}.
476 * An application relying on this method to release critical resources
477 * could cause serious performance issue.
478 * An application should use {@link ReferenceQueue} to reliably determine
479 * what reference objects that have been enqueued or
480 * {@link #refersTo(Object) refersTo(null)} to determine if this reference
481 * object has been cleared.
482 *
483 * @return {@code true} if and only if this reference object is
484 * in its associated queue (if any).
485 */
486 @Deprecated(since="16")
487 public boolean isEnqueued() {
488 return (this.queue == ReferenceQueue.ENQUEUED);
489 }
490
491 /**
492 * Clears this reference object, then attempts to add it to the queue with
493 * which it is registered, if any.
494 *
495 * <p>If this reference is registered with a queue but not yet enqueued,
496 * the reference is added to the queue; this method is
497 * <b><i>successful</i></b> and returns true.
498 * If this reference is not registered with a queue, or was already enqueued
499 * (by the garbage collector, or a previous call to {@code enqueue}), this
500 * method is <b><i>unsuccessful</i></b> and returns false.
501 *
502 * <p>{@linkplain java.lang.ref##MemoryConsistency Memory consistency effects}:
503 * Actions in a thread prior to a <b><i>successful</i></b> call to {@code enqueue}
504 * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happen-before</i></a>
505 * the reference is removed from the queue by {@link ReferenceQueue#poll}
506 * or {@link ReferenceQueue#remove}. <b><i>Unsuccessful</i></b> calls to
507 * {@code enqueue} have no specified memory consistency effects.
508 *
509 * <p> When this method clears references it does so directly, without
510 * invoking the {@link #clear()} method. When the garbage collector clears
511 * and enqueues references it does so directly, without invoking the
512 * {@link #clear()} method or this method.
513 *
514 * @apiNote
515 * Use of this method allows the registered queue's
516 * {@link ReferenceQueue#poll} and {@link ReferenceQueue#remove} methods
517 * to return this reference even though the referent may still be strongly
518 * reachable.
519 *
520 * @return {@code true} if this reference object was successfully
521 * enqueued; {@code false} if it was already enqueued or if
522 * it was not registered with a queue when it was created
523 */
524 public boolean enqueue() {
525 clearImpl(); // Intentionally clearImpl() to dispatch to overridden method, if needed
526 return this.queue.enqueue(this);
527 }
528
529 /**
530 * Throws {@link CloneNotSupportedException}. A {@code Reference} cannot be
531 * meaningfully cloned. Construct a new {@code Reference} instead.
532 *
533 * @return never returns normally
534 * @throws CloneNotSupportedException always
535 */
536 @Override
537 protected Object clone() throws CloneNotSupportedException {
538 throw new CloneNotSupportedException();
539 }
540
541 /* -- Constructors -- */
542
543 Reference(T referent) {
544 this(referent, null);
545 }
546
547 Reference(T referent, ReferenceQueue<? super T> queue) {
548 if (referent != null) {
549 Objects.requireIdentity(referent);
550 }
551 this.referent = referent;
552 this.queue = (queue == null) ? ReferenceQueue.NULL_QUEUE : queue;
553 }
554
555 /**
556 * Ensures that the given object remains
557 * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>.
558 * This reachability is assured regardless of any optimizing transformations
559 * the virtual machine may perform that might otherwise allow the object to
560 * become unreachable (see JLS {@jls 12.6.1}). Thus, the given object is not
561 * reclaimable by garbage collection at least until after the invocation of
562 * this method. References to the given object will not be cleared (or
563 * enqueued, if applicable) by the garbage collector until after invocation
564 * of this method.
565 * Invocation of this method does not itself initiate reference processing,
566 * garbage collection, or finalization.
567 *
568 * <p> This method establishes an ordering for <em>strong reachability</em>
569 * with respect to garbage collection. It controls relations that are
570 * otherwise only implicit in a program -- the reachability conditions
571 * triggering garbage collection. This method is applicable only
572 * when reclamation may have visible effects,
573 * such as for objects that use finalizers or {@link Cleaner}, or code that
574 * performs {@linkplain java.lang.ref reference processing}.
575 *
576 * <p>{@linkplain java.lang.ref##MemoryConsistency Memory consistency effects}:
577 * Actions in a thread prior to calling {@code reachabilityFence(x)}
578 * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happen-before</i></a>
579 * the garbage collector clears any reference to {@code x}.
580 *
581 * @apiNote
582 * Reference processing or finalization can occur after an object becomes
583 * unreachable. An object can become unreachable when the virtual machine
584 * detects that there is no further need for the object (other than for
585 * running a finalizer). In the course of optimization, the virtual machine
586 * can reorder operations of an object's methods such that the object
587 * becomes unneeded earlier than might naively be expected —
588 * including while a method of the object is still running. For instance,
589 * the VM can move the loading of <em>values</em> from the object's fields
590 * to occur earlier. The object itself is then no longer needed and becomes
591 * unreachable, and the method can continue running using the obtained values.
592 * This may have surprising and undesirable effects when using a Cleaner or
593 * finalizer for cleanup: there is a race between the
594 * program thread running the method, and the cleanup thread running the
595 * Cleaner or finalizer. The cleanup thread could free a
596 * resource, followed by the program thread (still running the method)
597 * attempting to access the now-already-freed resource.
598 * Use of {@code reachabilityFence} can prevent this race by ensuring that the
599 * object remains strongly reachable.
600 * <p>
601 * The following is an example in which the bookkeeping associated with a class is
602 * managed through array indices. Here, method {@code action} uses a
603 * {@code reachabilityFence} to ensure that the {@code Resource} object is
604 * not reclaimed before bookkeeping on an associated
605 * {@code ExternalResource} has been performed; specifically, to
606 * ensure that the array slot holding the {@code ExternalResource} is not
607 * nulled out in method {@link Object#finalize}, which may otherwise run
608 * concurrently.
609 *
610 * {@snippet :
611 * class Resource {
612 * private static ExternalResource[] externalResourceArray = ...
613 *
614 * int myIndex;
615 * Resource(...) {
616 * this.myIndex = ...
617 * externalResourceArray[myIndex] = ...;
618 * ...
619 * }
620 * protected void finalize() {
621 * externalResourceArray[this.myIndex] = null;
622 * ...
623 * }
624 * public void action() {
625 * try {
626 * // ...
627 * int i = this.myIndex; // last use of 'this' Resource in action()
628 * Resource.update(externalResourceArray[i]);
629 * } finally {
630 * Reference.reachabilityFence(this);
631 * }
632 * }
633 * private static void update(ExternalResource ext) {
634 * ext.status = ...;
635 * }
636 * }
637 * }
638 *
639 * The invocation of {@code reachabilityFence} is
640 * placed <em>after</em> the call to {@code update}, to ensure that the
641 * array slot is not nulled out by {@link Object#finalize} before the
642 * update, even if the call to {@code action} was the last use of this
643 * object. This might be the case if, for example, a usage in a user program
644 * had the form {@code new Resource().action();} which retains no other
645 * reference to this {@code Resource}.
646 * The {@code reachabilityFence} call is placed in a {@code finally} block to
647 * ensure that it is invoked across all paths in the method. A more complex
648 * method might need further precautions to ensure that
649 * {@code reachabilityFence} is encountered along all code paths.
650 *
651 * <p> Method {@code reachabilityFence} is not required in constructions
652 * that themselves ensure reachability. For example, because objects that
653 * are locked cannot, in general, be reclaimed, it would suffice if all
654 * accesses of the object, in all methods of class {@code Resource}
655 * (including {@code finalize}) were enclosed in {@code synchronized (this)}
656 * blocks. (Further, such blocks must not include infinite loops, or
657 * themselves be unreachable, which fall into the corner case exceptions to
658 * the "in general" disclaimer.) However, method {@code reachabilityFence}
659 * remains a better option in cases where synchronization is not as efficient,
660 * desirable, or possible; for example because it would encounter deadlock.
661 *
662 * @param ref the reference to the object to keep strongly reachable. If
663 * {@code null}, this method has no effect.
664 * @since 9
665 */
666 @ForceInline
667 public static void reachabilityFence(Object ref) {
668 // Does nothing. This method is annotated with @ForceInline to eliminate
669 // most of the overhead that using @DontInline would cause with the
670 // HotSpot JVM, when this fence is used in a wide variety of situations.
671 // HotSpot JVM retains the ref and does not GC it before a call to
672 // this method, because the JIT-compilers do not have GC-only safepoints.
673 }
674 }