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 }