1 /* 2 * Copyright (c) 1997, 2022, 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 * <p> 43 * The referent must be an {@linkplain Objects#isIdentityObject(Object) identity object}. 44 * Attempts to create a reference to a {@linkplain Objects#isValueObject value object} 45 * results in an {@link IdentityException}. 46 * @param <T> the type of the referent 47 * 48 * @author Mark Reinhold 49 * @since 1.2 50 * @sealedGraph 51 */ 52 53 public abstract sealed class Reference<T> 54 permits PhantomReference, SoftReference, WeakReference, FinalReference { 55 56 /* The state of a Reference object is characterized by two attributes. It 57 * may be either "active", "pending", or "inactive". It may also be 58 * either "registered", "enqueued", "dequeued", or "unregistered". 59 * 60 * Active: Subject to special treatment by the garbage collector. Some 61 * time after the collector detects that the reachability of the 62 * referent has changed to the appropriate state, the collector 63 * "notifies" the reference, changing the state to either "pending" or 64 * "inactive". 65 * referent != null; discovered = null, or in GC discovered list. 66 * 67 * Pending: An element of the pending-Reference list, waiting to be 68 * processed by the ReferenceHandler thread. The pending-Reference 69 * list is linked through the discovered fields of references in the 70 * list. 71 * referent = null; discovered = next element in pending-Reference list. 72 * 73 * Inactive: Neither Active nor Pending. 74 * referent = null. 75 * 76 * Registered: Associated with a queue when created, and not yet added 77 * to the queue. 78 * queue = the associated queue. 79 * 80 * Enqueued: Added to the associated queue, and not yet removed. 81 * queue = ReferenceQueue.ENQUEUE; next = next entry in list, or this to 82 * indicate end of list. 83 * 84 * Dequeued: Added to the associated queue and then removed. 85 * queue = ReferenceQueue.NULL; next = this. 86 * 87 * Unregistered: Not associated with a queue when created. 88 * queue = ReferenceQueue.NULL. 89 * 90 * The collector only needs to examine the referent field and the 91 * discovered field to determine whether a (non-FinalReference) Reference 92 * object needs special treatment. If the referent is non-null and not 93 * known to be live, then it may need to be discovered for possible later 94 * notification. But if the discovered field is non-null, then it has 95 * already been discovered. 96 * 97 * FinalReference (which exists to support finalization) differs from 98 * other references, because a FinalReference is not cleared when 99 * notified. The referent being null or not cannot be used to distinguish 100 * between the active state and pending or inactive states. However, 101 * FinalReferences do not support enqueue(). Instead, the next field of a 102 * FinalReference object is set to "this" when it is added to the 103 * pending-Reference list. The use of "this" as the value of next in the 104 * enqueued and dequeued states maintains the non-active state. An 105 * additional check that the next field is null is required to determine 106 * that a FinalReference object is active. 107 * 108 * Initial states: 109 * [active/registered] 110 * [active/unregistered] [1] 111 * 112 * Transitions: 113 * clear [2] 114 * [active/registered] -------> [inactive/registered] 115 * | | 116 * | | enqueue 117 * | GC enqueue [2] | 118 * | -----------------| 119 * | | 120 * v | 121 * [pending/registered] --- v 122 * | | ReferenceHandler 123 * | enqueue [2] |---> [inactive/enqueued] 124 * v | | 125 * [pending/enqueued] --- | 126 * | | poll/remove 127 * | poll/remove | + clear [4] 128 * | | 129 * v ReferenceHandler v 130 * [pending/dequeued] ------> [inactive/dequeued] 131 * 132 * 133 * clear/enqueue/GC [3] 134 * [active/unregistered] ------ 135 * | | 136 * | GC | 137 * | |--> [inactive/unregistered] 138 * v | 139 * [pending/unregistered] ------ 140 * ReferenceHandler 141 * 142 * Terminal states: 143 * [inactive/dequeued] 144 * [inactive/unregistered] 145 * 146 * Unreachable states (because enqueue also clears): 147 * [active/enqueued] 148 * [active/dequeued] 149 * 150 * [1] Unregistered is not permitted for FinalReferences. 151 * 152 * [2] These transitions are not possible for FinalReferences, making 153 * [pending/enqueued], [pending/dequeued], and [inactive/registered] 154 * unreachable. 155 * 156 * [3] The garbage collector may directly transition a Reference 157 * from [active/unregistered] to [inactive/unregistered], 158 * bypassing the pending-Reference list. 159 * 160 * [4] The queue handler for FinalReferences also clears the reference. 161 */ 162 163 private T referent; /* Treated specially by GC */ 164 165 /* The queue this reference gets enqueued to by GC notification or by 166 * calling enqueue(). 167 * 168 * When registered: the queue with which this reference is registered. 169 * enqueued: ReferenceQueue.ENQUEUE 170 * dequeued: ReferenceQueue.NULL 171 * unregistered: ReferenceQueue.NULL 172 */ 173 volatile ReferenceQueue<? super T> queue; 174 175 /* The link in a ReferenceQueue's list of Reference objects. 176 * 177 * When registered: null 178 * enqueued: next element in queue (or this if last) 179 * dequeued: this (marking FinalReferences as inactive) 180 * unregistered: null 181 */ 182 @SuppressWarnings("rawtypes") 183 volatile Reference next; 184 185 /* Used by the garbage collector to accumulate Reference objects that need 186 * to be revisited in order to decide whether they should be notified. 187 * Also used as the link in the pending-Reference list. The discovered 188 * field and the next field are distinct to allow the enqueue() method to 189 * be applied to a Reference object while it is either in the 190 * pending-Reference list or in the garbage collector's discovered set. 191 * 192 * When active: null or next element in a discovered reference list 193 * maintained by the GC (or this if last) 194 * pending: next element in the pending-Reference list (null if last) 195 * inactive: null 196 */ 197 private transient Reference<?> discovered; 198 199 200 /* High-priority thread to enqueue pending References 201 */ 202 private static class ReferenceHandler extends Thread { 203 ReferenceHandler(ThreadGroup g, String name) { 204 super(g, null, name, 0, false); 205 } 206 207 public void run() { 208 // pre-load and initialize Cleaner class so that we don't 209 // get into trouble later in the run loop if there's 210 // memory shortage while loading/initializing it lazily. 211 Unsafe.getUnsafe().ensureClassInitialized(Cleaner.class); 212 213 while (true) { 214 processPendingReferences(); 215 } 216 } 217 } 218 219 /* 220 * Atomically get and clear (set to null) the VM's pending-Reference list. 221 */ 222 private static native Reference<?> getAndClearReferencePendingList(); 223 224 /* 225 * Test whether the VM's pending-Reference list contains any entries. 226 */ 227 private static native boolean hasReferencePendingList(); 228 229 /* 230 * Wait until the VM's pending-Reference list may be non-null. 231 */ 232 private static native void waitForReferencePendingList(); 233 234 /* 235 * Enqueue a Reference taken from the pending list. Calling this method 236 * takes us from the Reference<?> domain of the pending list elements to 237 * having a Reference<T> with a correspondingly typed queue. 238 */ 239 private void enqueueFromPending() { 240 var q = queue; 241 if (q != ReferenceQueue.NULL) q.enqueue(this); 242 } 243 244 private static final Object processPendingLock = new Object(); 245 private static boolean processPendingActive = false; 246 247 private static void processPendingReferences() { 248 // Only the singleton reference processing thread calls 249 // waitForReferencePendingList() and getAndClearReferencePendingList(). 250 // These are separate operations to avoid a race with other threads 251 // that are calling waitForReferenceProcessing(). 252 waitForReferencePendingList(); 253 Reference<?> pendingList; 254 synchronized (processPendingLock) { 255 pendingList = getAndClearReferencePendingList(); 256 processPendingActive = true; 257 } 258 while (pendingList != null) { 259 Reference<?> ref = pendingList; 260 pendingList = ref.discovered; 261 ref.discovered = null; 262 263 if (ref instanceof Cleaner) { 264 ((Cleaner)ref).clean(); 265 // Notify any waiters that progress has been made. 266 // This improves latency for nio.Bits waiters, which 267 // are the only important ones. 268 synchronized (processPendingLock) { 269 processPendingLock.notifyAll(); 270 } 271 } else { 272 ref.enqueueFromPending(); 273 } 274 } 275 // Notify any waiters of completion of current round. 276 synchronized (processPendingLock) { 277 processPendingActive = false; 278 processPendingLock.notifyAll(); 279 } 280 } 281 282 // Wait for progress in reference processing. 283 // 284 // Returns true after waiting (for notification from the reference 285 // processing thread) if either (1) the VM has any pending 286 // references, or (2) the reference processing thread is 287 // processing references. Otherwise, returns false immediately. 288 private static boolean waitForReferenceProcessing() 289 throws InterruptedException 290 { 291 synchronized (processPendingLock) { 292 if (processPendingActive || hasReferencePendingList()) { 293 // Wait for progress, not necessarily completion. 294 processPendingLock.wait(); 295 return true; 296 } else { 297 return false; 298 } 299 } 300 } 301 302 /** 303 * Start the Reference Handler thread as a daemon thread. 304 */ 305 static void startReferenceHandlerThread(ThreadGroup tg) { 306 Thread handler = new ReferenceHandler(tg, "Reference Handler"); 307 /* If there were a special system-only priority greater than 308 * MAX_PRIORITY, it would be used here 309 */ 310 handler.setPriority(Thread.MAX_PRIORITY); 311 handler.setDaemon(true); 312 handler.start(); 313 } 314 315 static { 316 // provide access in SharedSecrets 317 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() { 318 @Override 319 public void startThreads() { 320 ThreadGroup tg = Thread.currentThread().getThreadGroup(); 321 for (ThreadGroup tgn = tg; 322 tgn != null; 323 tg = tgn, tgn = tg.getParent()); 324 Reference.startReferenceHandlerThread(tg); 325 Finalizer.startFinalizerThread(tg); 326 } 327 328 @Override 329 public boolean waitForReferenceProcessing() 330 throws InterruptedException 331 { 332 return Reference.waitForReferenceProcessing(); 333 } 334 335 @Override 336 public void runFinalization() { 337 Finalizer.runFinalization(); 338 } 339 340 @Override 341 public <T> ReferenceQueue<T> newNativeReferenceQueue() { 342 return new NativeReferenceQueue<T>(); 343 } 344 }); 345 } 346 347 /* -- Referent accessor and setters -- */ 348 349 /** 350 * Returns this reference object's referent. If this reference object has 351 * been cleared, either by the program or by the garbage collector, then 352 * this method returns {@code null}. 353 * 354 * @apiNote 355 * This method returns a strong reference to the referent. This may cause 356 * the garbage collector to treat it as strongly reachable until some later 357 * collection cycle. The {@link #refersTo(Object) refersTo} method can be 358 * used to avoid such strengthening when testing whether some object is 359 * the referent of a reference object; that is, use {@code ref.refersTo(obj)} 360 * rather than {@code ref.get() == obj}. 361 * 362 * @return The object to which this reference refers, or 363 * {@code null} if this reference object has been cleared 364 * @see #refersTo 365 */ 366 @IntrinsicCandidate 367 public T get() { 368 return this.referent; 369 } 370 371 /** 372 * Tests if the referent of this reference object is {@code obj}. 373 * Using a {@code null} {@code obj} returns {@code true} if the 374 * reference object has been cleared. 375 * 376 * @param obj the object to compare with this reference object's referent 377 * @return {@code true} if {@code obj} is the referent of this reference object 378 * @since 16 379 */ 380 public final boolean refersTo(T obj) { 381 return refersToImpl(obj); 382 } 383 384 /* Implementation of refersTo(), overridden for phantom references. 385 * This method exists only to avoid making refersTo0() virtual. Making 386 * refersTo0() virtual has the undesirable effect of C2 often preferring 387 * to call the native implementation over the intrinsic. 388 */ 389 boolean refersToImpl(T obj) { 390 return refersTo0(obj); 391 } 392 393 @IntrinsicCandidate 394 private native boolean refersTo0(Object o); 395 396 /** 397 * Clears this reference object. Invoking this method will not cause this 398 * object to be enqueued. 399 * 400 * <p> This method is invoked only by Java code; when the garbage collector 401 * clears references it does so directly, without invoking this method. 402 */ 403 public void clear() { 404 clear0(); 405 } 406 407 /* Implementation of clear(), also used by enqueue(). A simple 408 * assignment of the referent field won't do for some garbage 409 * collectors. 410 */ 411 private native void clear0(); 412 413 /* -- Operations on inactive FinalReferences -- */ 414 415 /* These functions are only used by FinalReference, and must only be 416 * called after the reference becomes inactive. While active, a 417 * FinalReference is considered weak but the referent is not normally 418 * accessed. Once a FinalReference becomes inactive it is considered a 419 * strong reference. These functions are used to bypass the 420 * corresponding weak implementations, directly accessing the referent 421 * field with strong semantics. 422 */ 423 424 /** 425 * Load referent with strong semantics. 426 */ 427 T getFromInactiveFinalReference() { 428 assert this instanceof FinalReference; 429 assert next != null; // I.e. FinalReference is inactive 430 return this.referent; 431 } 432 433 /** 434 * Clear referent with strong semantics. 435 */ 436 void clearInactiveFinalReference() { 437 assert this instanceof FinalReference; 438 assert next != null; // I.e. FinalReference is inactive 439 this.referent = null; 440 } 441 442 /* -- Queue operations -- */ 443 444 /** 445 * Tests if this reference object is in its associated queue, if any. 446 * This method returns {@code true} only if all of the following conditions 447 * are met: 448 * <ul> 449 * <li>this reference object was registered with a queue when it was created; and 450 * <li>the garbage collector has added this reference object to the queue 451 * or {@link #enqueue()} is called; and 452 * <li>this reference object is not yet removed from the queue. 453 * </ul> 454 * Otherwise, this method returns {@code false}. 455 * This method may return {@code false} if this reference object has been cleared 456 * but not enqueued due to the race condition. 457 * 458 * @deprecated 459 * This method was originally specified to test if a reference object has 460 * been cleared and enqueued but was never implemented to do this test. 461 * This method could be misused due to the inherent race condition 462 * or without an associated {@code ReferenceQueue}. 463 * An application relying on this method to release critical resources 464 * could cause serious performance issue. 465 * An application should use {@link ReferenceQueue} to reliably determine 466 * what reference objects that have been enqueued or 467 * {@link #refersTo(Object) refersTo(null)} to determine if this reference 468 * object has been cleared. 469 * 470 * @return {@code true} if and only if this reference object is 471 * in its associated queue (if any). 472 */ 473 @Deprecated(since="16") 474 public boolean isEnqueued() { 475 return (this.queue == ReferenceQueue.ENQUEUED); 476 } 477 478 /** 479 * Clears this reference object and adds it to the queue with which 480 * it is registered, if any. 481 * 482 * <p> This method is invoked only by Java code; when the garbage collector 483 * enqueues references it does so directly, without invoking this method. 484 * 485 * @return {@code true} if this reference object was successfully 486 * enqueued; {@code false} if it was already enqueued or if 487 * it was not registered with a queue when it was created 488 */ 489 public boolean enqueue() { 490 clear0(); // Intentionally clear0() rather than clear() 491 return this.queue.enqueue(this); 492 } 493 494 /** 495 * Throws {@link CloneNotSupportedException}. A {@code Reference} cannot be 496 * meaningfully cloned. Construct a new {@code Reference} instead. 497 * 498 * @return never returns normally 499 * @throws CloneNotSupportedException always 500 * 501 * @since 11 502 */ 503 @Override 504 protected Object clone() throws CloneNotSupportedException { 505 throw new CloneNotSupportedException(); 506 } 507 508 /* -- Constructors -- */ 509 510 Reference(T referent) { 511 this(referent, null); 512 } 513 514 Reference(T referent, ReferenceQueue<? super T> queue) { 515 if (referent != null) { 516 Objects.requireIdentity(referent); 517 } 518 this.referent = referent; 519 this.queue = (queue == null) ? ReferenceQueue.NULL : queue; 520 } 521 522 /** 523 * Ensures that the object referenced by the given reference remains 524 * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>, 525 * regardless of any prior actions of the program that might otherwise cause 526 * the object to become unreachable; thus, the referenced object is not 527 * reclaimable by garbage collection at least until after the invocation of 528 * this method. Invocation of this method does not itself initiate garbage 529 * collection or finalization. 530 * 531 * <p> This method establishes an ordering for <em>strong reachability</em> 532 * with respect to garbage collection. It controls relations that are 533 * otherwise only implicit in a program -- the reachability conditions 534 * triggering garbage collection. This method is designed for use in 535 * uncommon situations of premature finalization where using 536 * {@code synchronized} blocks or methods, or using other synchronization 537 * facilities are not possible or do not provide the desired control. This 538 * method is applicable only when reclamation may have visible effects, 539 * which is possible for objects with finalizers (See Section {@jls 12.6} 540 * of <cite>The Java Language Specification</cite>) that 541 * are implemented in ways that rely on ordering control for 542 * correctness. 543 * 544 * @apiNote 545 * Finalization may occur whenever the virtual machine detects that no 546 * reference to an object will ever be stored in the heap: The garbage 547 * collector may reclaim an object even if the fields of that object are 548 * still in use, so long as the object has otherwise become unreachable. 549 * This may have surprising and undesirable effects in cases such as the 550 * following example in which the bookkeeping associated with a class is 551 * managed through array indices. Here, method {@code action} uses a 552 * {@code reachabilityFence} to ensure that the {@code Resource} object is 553 * not reclaimed before bookkeeping on an associated 554 * {@code ExternalResource} has been performed; in particular here, to 555 * ensure that the array slot holding the {@code ExternalResource} is not 556 * nulled out in method {@link Object#finalize}, which may otherwise run 557 * concurrently. 558 * 559 * <pre> {@code 560 * class Resource { 561 * private static ExternalResource[] externalResourceArray = ... 562 * 563 * int myIndex; 564 * Resource(...) { 565 * myIndex = ... 566 * externalResourceArray[myIndex] = ...; 567 * ... 568 * } 569 * protected void finalize() { 570 * externalResourceArray[myIndex] = null; 571 * ... 572 * } 573 * public void action() { 574 * try { 575 * // ... 576 * int i = myIndex; 577 * Resource.update(externalResourceArray[i]); 578 * } finally { 579 * Reference.reachabilityFence(this); 580 * } 581 * } 582 * private static void update(ExternalResource ext) { 583 * ext.status = ...; 584 * } 585 * }}</pre> 586 * 587 * Here, the invocation of {@code reachabilityFence} is nonintuitively 588 * placed <em>after</em> the call to {@code update}, to ensure that the 589 * array slot is not nulled out by {@link Object#finalize} before the 590 * update, even if the call to {@code action} was the last use of this 591 * object. This might be the case if, for example a usage in a user program 592 * had the form {@code new Resource().action();} which retains no other 593 * reference to this {@code Resource}. While probably overkill here, 594 * {@code reachabilityFence} is placed in a {@code finally} block to ensure 595 * that it is invoked across all paths in the method. In a method with more 596 * complex control paths, you might need further precautions to ensure that 597 * {@code reachabilityFence} is encountered along all of them. 598 * 599 * <p> It is sometimes possible to better encapsulate use of 600 * {@code reachabilityFence}. Continuing the above example, if it were 601 * acceptable for the call to method {@code update} to proceed even if the 602 * finalizer had already executed (nulling out slot), then you could 603 * localize use of {@code reachabilityFence}: 604 * 605 * <pre> {@code 606 * public void action2() { 607 * // ... 608 * Resource.update(getExternalResource()); 609 * } 610 * private ExternalResource getExternalResource() { 611 * ExternalResource ext = externalResourceArray[myIndex]; 612 * Reference.reachabilityFence(this); 613 * return ext; 614 * }}</pre> 615 * 616 * <p> Method {@code reachabilityFence} is not required in constructions 617 * that themselves ensure reachability. For example, because objects that 618 * are locked cannot, in general, be reclaimed, it would suffice if all 619 * accesses of the object, in all methods of class {@code Resource} 620 * (including {@code finalize}) were enclosed in {@code synchronized (this)} 621 * blocks. (Further, such blocks must not include infinite loops, or 622 * themselves be unreachable, which fall into the corner case exceptions to 623 * the "in general" disclaimer.) However, method {@code reachabilityFence} 624 * remains a better option in cases where this approach is not as efficient, 625 * desirable, or possible; for example because it would encounter deadlock. 626 * 627 * @param ref the reference. If {@code null}, this method has no effect. 628 * @since 9 629 */ 630 @ForceInline 631 public static void reachabilityFence(Object ref) { 632 // Does nothing. This method is annotated with @ForceInline to eliminate 633 // most of the overhead that using @DontInline would cause with the 634 // HotSpot JVM, when this fence is used in a wide variety of situations. 635 // HotSpot JVM retains the ref and does not GC it before a call to 636 // this method, because the JIT-compilers do not have GC-only safepoints. 637 } 638 }