1 /*
2 * Copyright (c) 1997, 2024, 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.misc.Unsafe;
29 import jdk.internal.vm.annotation.ForceInline;
30 import jdk.internal.vm.annotation.IntrinsicCandidate;
31 import jdk.internal.access.JavaLangRefAccess;
32 import jdk.internal.access.SharedSecrets;
33 import jdk.internal.ref.Cleaner;
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
57 public abstract sealed class Reference<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; next = this.
90 *
91 * Unregistered: Not associated with a queue when created.
92 * queue = ReferenceQueue.NULL.
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
175 * unregistered: ReferenceQueue.NULL
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 // pre-load and initialize Cleaner class so that we don't
213 // get into trouble later in the run loop if there's
214 // memory shortage while loading/initializing it lazily.
215 Unsafe.getUnsafe().ensureClassInitialized(Cleaner.class);
216
217 while (true) {
218 processPendingReferences();
219 }
220 }
221 }
222
223 /*
224 * Atomically get and clear (set to null) the VM's pending-Reference list.
225 */
226 private static native Reference<?> getAndClearReferencePendingList();
227
228 /*
229 * Test whether the VM's pending-Reference list contains any entries.
230 */
231 private static native boolean hasReferencePendingList();
232
233 /*
234 * Wait until the VM's pending-Reference list may be non-null.
235 */
236 private static native void waitForReferencePendingList();
237
238 /*
239 * Enqueue a Reference taken from the pending list. Calling this method
240 * takes us from the Reference<?> domain of the pending list elements to
241 * having a Reference<T> with a correspondingly typed queue.
242 */
243 private void enqueueFromPending() {
244 var q = queue;
245 if (q != ReferenceQueue.NULL) q.enqueue(this);
246 }
247
248 private static final Object processPendingLock = new Object();
249 private static boolean processPendingActive = false;
250
251 private static void processPendingReferences() {
252 // Only the singleton reference processing thread calls
253 // waitForReferencePendingList() and getAndClearReferencePendingList().
254 // These are separate operations to avoid a race with other threads
255 // that are calling waitForReferenceProcessing().
256 waitForReferencePendingList();
257 Reference<?> pendingList;
258 synchronized (processPendingLock) {
259 pendingList = getAndClearReferencePendingList();
260 processPendingActive = true;
261 }
262 while (pendingList != null) {
263 Reference<?> ref = pendingList;
264 pendingList = ref.discovered;
265 ref.discovered = null;
266
267 if (ref instanceof Cleaner) {
268 ((Cleaner)ref).clean();
269 // Notify any waiters that progress has been made.
270 // This improves latency for nio.Bits waiters, which
271 // are the only important ones.
272 synchronized (processPendingLock) {
273 processPendingLock.notifyAll();
274 }
275 } else {
276 ref.enqueueFromPending();
277 }
278 }
279 // Notify any waiters of completion of current round.
280 synchronized (processPendingLock) {
281 processPendingActive = false;
282 processPendingLock.notifyAll();
283 }
284 }
285
286 // Wait for progress in reference processing.
287 //
288 // Returns true after waiting (for notification from the reference
289 // processing thread) if either (1) the VM has any pending
290 // references, or (2) the reference processing thread is
291 // processing references. Otherwise, returns false immediately.
292 private static boolean waitForReferenceProcessing()
293 throws InterruptedException
294 {
295 synchronized (processPendingLock) {
296 if (processPendingActive || hasReferencePendingList()) {
297 // Wait for progress, not necessarily completion.
298 processPendingLock.wait();
299 return true;
300 } else {
301 return false;
302 }
303 }
304 }
305
306 /**
307 * Start the Reference Handler thread as a daemon thread.
308 */
309 static void startReferenceHandlerThread(ThreadGroup tg) {
310 Thread handler = new ReferenceHandler(tg, "Reference Handler");
311 /* If there were a special system-only priority greater than
312 * MAX_PRIORITY, it would be used here
313 */
314 handler.setPriority(Thread.MAX_PRIORITY);
315 handler.setDaemon(true);
316 handler.start();
317 }
318
319 static {
320 // provide access in SharedSecrets
321 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
322 @Override
323 public void startThreads() {
324 ThreadGroup tg = Thread.currentThread().getThreadGroup();
325 for (ThreadGroup tgn = tg;
326 tgn != null;
327 tg = tgn, tgn = tg.getParent());
328 Reference.startReferenceHandlerThread(tg);
329 Finalizer.startFinalizerThread(tg);
330 }
331
332 @Override
333 public boolean waitForReferenceProcessing()
334 throws InterruptedException
335 {
336 return Reference.waitForReferenceProcessing();
337 }
338
339 @Override
340 public void runFinalization() {
341 Finalizer.runFinalization();
342 }
343 });
344 }
345
346 /* -- Referent accessor and setters -- */
347
348 /**
349 * Returns this reference object's referent. If this reference object has
350 * been cleared, either by the program or by the garbage collector, then
351 * this method returns {@code null}.
352 *
353 * @apiNote
354 * This method returns a strong reference to the referent. This may cause
355 * the garbage collector to treat it as strongly reachable until some later
356 * collection cycle. The {@link #refersTo(Object) refersTo} method can be
357 * used to avoid such strengthening when testing whether some object is
358 * the referent of a reference object; that is, use {@code ref.refersTo(obj)}
359 * rather than {@code ref.get() == obj}.
360 *
361 * @return The object to which this reference refers, or
362 * {@code null} if this reference object has been cleared
363 * @see #refersTo
364 */
365 @IntrinsicCandidate
366 public T get() {
367 return this.referent;
368 }
369
370 /**
371 * Tests if the referent of this reference object is {@code obj}.
372 * Using a {@code null} {@code obj} returns {@code true} if the
373 * reference object has been cleared.
374 *
375 * @param obj the object to compare with this reference object's referent
376 * @return {@code true} if {@code obj} is the referent of this reference object
377 * @since 16
378 */
379 public final boolean refersTo(T obj) {
380 return refersToImpl(obj);
381 }
382
383 /* Implementation of refersTo(), overridden for phantom references.
384 * This method exists only to avoid making refersTo0() virtual. Making
385 * refersTo0() virtual has the undesirable effect of C2 often preferring
386 * to call the native implementation over the intrinsic.
387 */
388 boolean refersToImpl(T obj) {
389 return refersTo0(obj);
390 }
391
392 @IntrinsicCandidate
393 private native boolean refersTo0(Object o);
394
395 /**
396 * Clears this reference object. Invoking this method does not enqueue this
397 * object, and the garbage collector will not clear or enqueue this object.
398 *
399 * <p>When the garbage collector or the {@link #enqueue()} method clear
400 * references they do so directly, without invoking this method.
401 *
402 * @apiNote
403 * There is a potential race condition with the garbage collector. When this
404 * method is called, the garbage collector may already be in the process of
405 * (or already completed) clearing and/or enqueueing this reference.
406 * Avoid this race by ensuring the referent remains strongly reachable until
407 * after the call to clear(), using {@link #reachabilityFence(Object)} if
408 * necessary.
409 */
410 public void clear() {
411 clearImpl();
412 }
413
414 /* Implementation of clear(). A simple assignment of the referent field
415 * won't do for some garbage collectors. There is the override for phantom
416 * references, which requires different semantics. This method is also
417 * used by enqueue().
418 *
419 * <p>This method exists only to avoid making clear0() virtual. Making
420 * clear0() virtual has the undesirable effect of C2 often preferring
421 * to call the native implementation over the intrinsic.
422 */
423 void clearImpl() {
424 clear0();
425 }
426
427 @IntrinsicCandidate
428 private native void clear0();
429
430 /* -- Operations on inactive FinalReferences -- */
431
432 /* These functions are only used by FinalReference, and must only be
433 * called after the reference becomes inactive. While active, a
434 * FinalReference is considered weak but the referent is not normally
435 * accessed. Once a FinalReference becomes inactive it is considered a
436 * strong reference. These functions are used to bypass the
437 * corresponding weak implementations, directly accessing the referent
438 * field with strong semantics.
439 */
440
441 /**
442 * Load referent with strong semantics.
443 */
444 T getFromInactiveFinalReference() {
445 assert this instanceof FinalReference;
446 assert next != null; // I.e. FinalReference is inactive
447 return this.referent;
448 }
449
450 /**
451 * Clear referent with strong semantics.
452 */
453 void clearInactiveFinalReference() {
454 assert this instanceof FinalReference;
455 assert next != null; // I.e. FinalReference is inactive
456 this.referent = null;
457 }
458
459 /* -- Queue operations -- */
460
461 /**
462 * Tests if this reference object is in its associated queue, if any.
463 * This method returns {@code true} only if all of the following conditions
464 * are met:
465 * <ul>
466 * <li>this reference object was registered with a queue when it was created; and
467 * <li>the garbage collector has added this reference object to the queue
468 * or {@link #enqueue()} is called; and
469 * <li>this reference object is not yet removed from the queue.
470 * </ul>
471 * Otherwise, this method returns {@code false}.
472 * This method may return {@code false} if this reference object has been cleared
473 * but not enqueued due to the race condition.
474 *
475 * @deprecated
476 * This method was originally specified to test if a reference object has
477 * been cleared and enqueued but was never implemented to do this test.
478 * This method could be misused due to the inherent race condition
479 * or without an associated {@code ReferenceQueue}.
480 * An application relying on this method to release critical resources
481 * could cause serious performance issue.
482 * An application should use {@link ReferenceQueue} to reliably determine
483 * what reference objects that have been enqueued or
484 * {@link #refersTo(Object) refersTo(null)} to determine if this reference
485 * object has been cleared.
486 *
487 * @return {@code true} if and only if this reference object is
488 * in its associated queue (if any).
489 */
490 @Deprecated(since="16")
491 public boolean isEnqueued() {
492 return (this.queue == ReferenceQueue.ENQUEUED);
493 }
494
495 /**
496 * Clears this reference object, then attempts to add it to the queue with
497 * which it is registered, if any.
498 *
499 * <p>If this reference is registered with a queue but not yet enqueued,
500 * the reference is added to the queue; this method is
501 * <b><i>successful</i></b> and returns true.
502 * If this reference is not registered with a queue, or was already enqueued
503 * (by the garbage collector, or a previous call to {@code enqueue}), this
504 * method is <b><i>unsuccessful</i></b> and returns false.
505 *
506 * <p>{@linkplain java.lang.ref##MemoryConsistency Memory consistency effects}:
507 * Actions in a thread prior to a <b><i>successful</i></b> call to {@code enqueue}
508 * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happen-before</i></a>
509 * the reference is removed from the queue by {@link ReferenceQueue#poll}
510 * or {@link ReferenceQueue#remove}. <b><i>Unsuccessful</i></b> calls to
511 * {@code enqueue} have no specified memory consistency effects.
512 *
513 * <p> When this method clears references it does so directly, without
514 * invoking the {@link #clear()} method. When the garbage collector clears
515 * and enqueues references it does so directly, without invoking the
516 * {@link #clear()} method or this method.
517 *
518 * @apiNote
519 * Use of this method allows the registered queue's
520 * {@link ReferenceQueue#poll} and {@link ReferenceQueue#remove} methods
521 * to return this reference even though the referent may still be strongly
522 * reachable.
523 *
524 * @return {@code true} if this reference object was successfully
525 * enqueued; {@code false} if it was already enqueued or if
526 * it was not registered with a queue when it was created
527 */
528 public boolean enqueue() {
529 clearImpl(); // Intentionally clearImpl() to dispatch to overridden method, if needed
530 return this.queue.enqueue(this);
531 }
532
533 /**
534 * Throws {@link CloneNotSupportedException}. A {@code Reference} cannot be
535 * meaningfully cloned. Construct a new {@code Reference} instead.
536 *
537 * @return never returns normally
538 * @throws CloneNotSupportedException always
539 */
540 @Override
541 protected Object clone() throws CloneNotSupportedException {
542 throw new CloneNotSupportedException();
543 }
544
545 /* -- Constructors -- */
546
547 Reference(T referent) {
548 this(referent, null);
549 }
550
551 Reference(T referent, ReferenceQueue<? super T> queue) {
552 if (referent != null) {
553 Objects.requireIdentity(referent);
554 }
555 this.referent = referent;
556 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
557 }
558
559 /**
560 * Ensures that the given object remains
561 * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>.
562 * This reachability is assured regardless of any optimizing transformations
563 * the virtual machine may perform that might otherwise allow the object to
564 * become unreachable (see JLS {@jls 12.6.1}). Thus, the given object is not
565 * reclaimable by garbage collection at least until after the invocation of
566 * this method. References to the given object will not be cleared (or
567 * enqueued, if applicable) by the garbage collector until after invocation
568 * of this method.
569 * Invocation of this method does not itself initiate reference processing,
570 * garbage collection, or finalization.
571 *
572 * <p> This method establishes an ordering for <em>strong reachability</em>
573 * with respect to garbage collection. It controls relations that are
574 * otherwise only implicit in a program -- the reachability conditions
575 * triggering garbage collection. This method is applicable only
576 * when reclamation may have visible effects,
577 * such as for objects that use finalizers or {@link Cleaner}, or code that
578 * performs {@linkplain java.lang.ref reference processing}.
579 *
580 * <p>{@linkplain java.lang.ref##MemoryConsistency Memory consistency effects}:
581 * Actions in a thread prior to calling {@code reachabilityFence(x)}
582 * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happen-before</i></a>
583 * the garbage collector clears any reference to {@code x}.
584 *
585 * @apiNote
586 * Reference processing or finalization can occur after an object becomes
587 * unreachable. An object can become unreachable when the virtual machine
588 * detects that there is no further need for the object (other than for
589 * running a finalizer). In the course of optimization, the virtual machine
590 * can reorder operations of an object's methods such that the object
591 * becomes unneeded earlier than might naively be expected —
592 * including while a method of the object is still running. For instance,
593 * the VM can move the loading of <em>values</em> from the object's fields
594 * to occur earlier. The object itself is then no longer needed and becomes
595 * unreachable, and the method can continue running using the obtained values.
596 * This may have surprising and undesirable effects when using a Cleaner or
597 * finalizer for cleanup: there is a race between the
598 * program thread running the method, and the cleanup thread running the
599 * Cleaner or finalizer. The cleanup thread could free a
600 * resource, followed by the program thread (still running the method)
601 * attempting to access the now-already-freed resource.
602 * Use of {@code reachabilityFence} can prevent this race by ensuring that the
603 * object remains strongly reachable.
604 * <p>
605 * The following is an example in which the bookkeeping associated with a class is
606 * managed through array indices. Here, method {@code action} uses a
607 * {@code reachabilityFence} to ensure that the {@code Resource} object is
608 * not reclaimed before bookkeeping on an associated
609 * {@code ExternalResource} has been performed; specifically, to
610 * ensure that the array slot holding the {@code ExternalResource} is not
611 * nulled out in method {@link Object#finalize}, which may otherwise run
612 * concurrently.
613 *
614 * {@snippet :
615 * class Resource {
616 * private static ExternalResource[] externalResourceArray = ...
617 *
618 * int myIndex;
619 * Resource(...) {
620 * this.myIndex = ...
621 * externalResourceArray[myIndex] = ...;
622 * ...
623 * }
624 * protected void finalize() {
625 * externalResourceArray[this.myIndex] = null;
626 * ...
627 * }
628 * public void action() {
629 * try {
630 * // ...
631 * int i = this.myIndex; // last use of 'this' Resource in action()
632 * Resource.update(externalResourceArray[i]);
633 * } finally {
634 * Reference.reachabilityFence(this);
635 * }
636 * }
637 * private static void update(ExternalResource ext) {
638 * ext.status = ...;
639 * }
640 * }
641 * }
642 *
643 * The invocation of {@code reachabilityFence} is
644 * placed <em>after</em> the call to {@code update}, to ensure that the
645 * array slot is not nulled out by {@link Object#finalize} before the
646 * update, even if the call to {@code action} was the last use of this
647 * object. This might be the case if, for example, a usage in a user program
648 * had the form {@code new Resource().action();} which retains no other
649 * reference to this {@code Resource}.
650 * The {@code reachabilityFence} call is placed in a {@code finally} block to
651 * ensure that it is invoked across all paths in the method. A more complex
652 * method might need further precautions to ensure that
653 * {@code reachabilityFence} is encountered along all code paths.
654 *
655 * <p> Method {@code reachabilityFence} is not required in constructions
656 * that themselves ensure reachability. For example, because objects that
657 * are locked cannot, in general, be reclaimed, it would suffice if all
658 * accesses of the object, in all methods of class {@code Resource}
659 * (including {@code finalize}) were enclosed in {@code synchronized (this)}
660 * blocks. (Further, such blocks must not include infinite loops, or
661 * themselves be unreachable, which fall into the corner case exceptions to
662 * the "in general" disclaimer.) However, method {@code reachabilityFence}
663 * remains a better option in cases where synchronization is not as efficient,
664 * desirable, or possible; for example because it would encounter deadlock.
665 *
666 * @param ref the reference to the object to keep strongly reachable. If
667 * {@code null}, this method has no effect.
668 * @since 9
669 */
670 @ForceInline
671 public static void reachabilityFence(Object ref) {
672 // Does nothing. This method is annotated with @ForceInline to eliminate
673 // most of the overhead that using @DontInline would cause with the
674 // HotSpot JVM, when this fence is used in a wide variety of situations.
675 // HotSpot JVM retains the ref and does not GC it before a call to
676 // this method, because the JIT-compilers do not have GC-only safepoints.
677 }
678 }