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 /** 36 * Abstract base class for reference objects. This class defines the 37 * operations common to all reference objects. Because reference objects are 38 * implemented in close cooperation with the garbage collector, this class may 39 * not be subclassed directly. 40 * @param <T> the type of the referent 41 * 42 * @author Mark Reinhold 43 * @since 1.2 44 * @sealedGraph 45 */ 46 47 public abstract sealed class Reference<T> 48 permits PhantomReference, SoftReference, WeakReference, FinalReference { 49 50 /* The state of a Reference object is characterized by two attributes. It 51 * may be either "active", "pending", or "inactive". It may also be 52 * either "registered", "enqueued", "dequeued", or "unregistered". 53 * 54 * Active: Subject to special treatment by the garbage collector. Some 55 * time after the collector detects that the reachability of the 56 * referent has changed to the appropriate state, the collector 57 * "notifies" the reference, changing the state to either "pending" or 58 * "inactive". 59 * referent != null; discovered = null, or in GC discovered list. 60 * 61 * Pending: An element of the pending-Reference list, waiting to be 62 * processed by the ReferenceHandler thread. The pending-Reference 63 * list is linked through the discovered fields of references in the 64 * list. 65 * referent = null; discovered = next element in pending-Reference list. 66 * 67 * Inactive: Neither Active nor Pending. 68 * referent = null. 69 * 70 * Registered: Associated with a queue when created, and not yet added 71 * to the queue. 72 * queue = the associated queue. 73 * 74 * Enqueued: Added to the associated queue, and not yet removed. 75 * queue = ReferenceQueue.ENQUEUE; next = next entry in list, or this to 76 * indicate end of list. 77 * 78 * Dequeued: Added to the associated queue and then removed. 79 * queue = ReferenceQueue.NULL; next = this. 80 * 81 * Unregistered: Not associated with a queue when created. 82 * queue = ReferenceQueue.NULL. 83 * 84 * The collector only needs to examine the referent field and the 85 * discovered field to determine whether a (non-FinalReference) Reference 86 * object needs special treatment. If the referent is non-null and not 87 * known to be live, then it may need to be discovered for possible later 88 * notification. But if the discovered field is non-null, then it has 89 * already been discovered. 90 * 91 * FinalReference (which exists to support finalization) differs from 92 * other references, because a FinalReference is not cleared when 93 * notified. The referent being null or not cannot be used to distinguish 94 * between the active state and pending or inactive states. However, 95 * FinalReferences do not support enqueue(). Instead, the next field of a 96 * FinalReference object is set to "this" when it is added to the 97 * pending-Reference list. The use of "this" as the value of next in the 98 * enqueued and dequeued states maintains the non-active state. An 99 * additional check that the next field is null is required to determine 100 * that a FinalReference object is active. 101 * 102 * Initial states: 103 * [active/registered] 104 * [active/unregistered] [1] 105 * 106 * Transitions: 107 * clear [2] 108 * [active/registered] -------> [inactive/registered] 109 * | | 110 * | | enqueue 111 * | GC enqueue [2] | 112 * | -----------------| 113 * | | 114 * v | 115 * [pending/registered] --- v 116 * | | ReferenceHandler 117 * | enqueue [2] |---> [inactive/enqueued] 118 * v | | 119 * [pending/enqueued] --- | 120 * | | poll/remove 121 * | poll/remove | + clear [4] 122 * | | 123 * v ReferenceHandler v 124 * [pending/dequeued] ------> [inactive/dequeued] 125 * 126 * 127 * clear/enqueue/GC [3] 128 * [active/unregistered] ------ 129 * | | 130 * | GC | 131 * | |--> [inactive/unregistered] 132 * v | 133 * [pending/unregistered] ------ 134 * ReferenceHandler 135 * 136 * Terminal states: 137 * [inactive/dequeued] 138 * [inactive/unregistered] 139 * 140 * Unreachable states (because enqueue also clears): 141 * [active/enqueued] 142 * [active/dequeued] 143 * 144 * [1] Unregistered is not permitted for FinalReferences. 145 * 146 * [2] These transitions are not possible for FinalReferences, making 147 * [pending/enqueued], [pending/dequeued], and [inactive/registered] 148 * unreachable. 149 * 150 * [3] The garbage collector may directly transition a Reference 151 * from [active/unregistered] to [inactive/unregistered], 152 * bypassing the pending-Reference list. 153 * 154 * [4] The queue handler for FinalReferences also clears the reference. 155 */ 156 157 private T referent; /* Treated specially by GC */ 158 159 /* The queue this reference gets enqueued to by GC notification or by 160 * calling enqueue(). 161 * 162 * When registered: the queue with which this reference is registered. 163 * enqueued: ReferenceQueue.ENQUEUE 164 * dequeued: ReferenceQueue.NULL 165 * unregistered: ReferenceQueue.NULL 166 */ 167 volatile ReferenceQueue<? super T> queue; 168 169 /* The link in a ReferenceQueue's list of Reference objects. 170 * 171 * When registered: null 172 * enqueued: next element in queue (or this if last) 173 * dequeued: this (marking FinalReferences as inactive) 174 * unregistered: null 175 */ 176 @SuppressWarnings("rawtypes") 177 volatile Reference next; 178 179 /* Used by the garbage collector to accumulate Reference objects that need 180 * to be revisited in order to decide whether they should be notified. 181 * Also used as the link in the pending-Reference list. The discovered 182 * field and the next field are distinct to allow the enqueue() method to 183 * be applied to a Reference object while it is either in the 184 * pending-Reference list or in the garbage collector's discovered set. 185 * 186 * When active: null or next element in a discovered reference list 187 * maintained by the GC (or this if last) 188 * pending: next element in the pending-Reference list (null if last) 189 * inactive: null 190 */ 191 private transient Reference<?> discovered; 192 193 194 /* High-priority thread to enqueue pending References 195 */ 196 private static class ReferenceHandler extends Thread { 197 ReferenceHandler(ThreadGroup g, String name) { 198 super(g, null, name, 0, false); 199 } 200 201 public void run() { 202 // pre-load and initialize Cleaner class so that we don't 203 // get into trouble later in the run loop if there's 204 // memory shortage while loading/initializing it lazily. 205 Unsafe.getUnsafe().ensureClassInitialized(Cleaner.class); 206 207 while (true) { 208 processPendingReferences(); 209 } 210 } 211 } 212 213 /* 214 * Atomically get and clear (set to null) the VM's pending-Reference list. 215 */ 216 private static native Reference<?> getAndClearReferencePendingList(); 217 218 /* 219 * Test whether the VM's pending-Reference list contains any entries. 220 */ 221 private static native boolean hasReferencePendingList(); 222 223 /* 224 * Wait until the VM's pending-Reference list may be non-null. 225 */ 226 private static native void waitForReferencePendingList(); 227 228 /* 229 * Enqueue a Reference taken from the pending list. Calling this method 230 * takes us from the Reference<?> domain of the pending list elements to 231 * having a Reference<T> with a correspondingly typed queue. 232 */ 233 private void enqueueFromPending() { 234 var q = queue; 235 if (q != ReferenceQueue.NULL) q.enqueue(this); 236 } 237 238 private static final Object processPendingLock = new Object(); 239 private static boolean processPendingActive = false; 240 241 private static void processPendingReferences() { 242 // Only the singleton reference processing thread calls 243 // waitForReferencePendingList() and getAndClearReferencePendingList(). 244 // These are separate operations to avoid a race with other threads 245 // that are calling waitForReferenceProcessing(). 246 waitForReferencePendingList(); 247 Reference<?> pendingList; 248 synchronized (processPendingLock) { 249 pendingList = getAndClearReferencePendingList(); 250 processPendingActive = true; 251 } 252 while (pendingList != null) { 253 Reference<?> ref = pendingList; 254 pendingList = ref.discovered; 255 ref.discovered = null; 256 257 if (ref instanceof Cleaner) { 258 ((Cleaner)ref).clean(); 259 // Notify any waiters that progress has been made. 260 // This improves latency for nio.Bits waiters, which 261 // are the only important ones. 262 synchronized (processPendingLock) { 263 processPendingLock.notifyAll(); 264 } 265 } else { 266 ref.enqueueFromPending(); 267 } 268 } 269 // Notify any waiters of completion of current round. 270 synchronized (processPendingLock) { 271 processPendingActive = false; 272 processPendingLock.notifyAll(); 273 } 274 } 275 276 // Wait for progress in reference processing. 277 // 278 // Returns true after waiting (for notification from the reference 279 // processing thread) if either (1) the VM has any pending 280 // references, or (2) the reference processing thread is 281 // processing references. Otherwise, returns false immediately. 282 private static boolean waitForReferenceProcessing() 283 throws InterruptedException 284 { 285 synchronized (processPendingLock) { 286 if (processPendingActive || hasReferencePendingList()) { 287 // Wait for progress, not necessarily completion. 288 processPendingLock.wait(); 289 return true; 290 } else { 291 return false; 292 } 293 } 294 } 295 296 /** 297 * Start the Reference Handler thread as a daemon thread. 298 */ 299 static void startReferenceHandlerThread(ThreadGroup tg) { 300 Thread handler = new ReferenceHandler(tg, "Reference Handler"); 301 /* If there were a special system-only priority greater than 302 * MAX_PRIORITY, it would be used here 303 */ 304 handler.setPriority(Thread.MAX_PRIORITY); 305 handler.setDaemon(true); 306 handler.start(); 307 } 308 309 static { 310 // provide access in SharedSecrets 311 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() { 312 @Override 313 public void startThreads() { 314 ThreadGroup tg = Thread.currentThread().getThreadGroup(); 315 for (ThreadGroup tgn = tg; 316 tgn != null; 317 tg = tgn, tgn = tg.getParent()); 318 Reference.startReferenceHandlerThread(tg); 319 Finalizer.startFinalizerThread(tg); 320 } 321 322 @Override 323 public boolean waitForReferenceProcessing() 324 throws InterruptedException 325 { 326 return Reference.waitForReferenceProcessing(); 327 } 328 329 @Override 330 public void runFinalization() { 331 Finalizer.runFinalization(); 332 } 333 334 @Override 335 public <T> ReferenceQueue<T> newNativeReferenceQueue() { 336 return new NativeReferenceQueue<T>(); 337 } 338 }); 339 } 340 341 /* -- Referent accessor and setters -- */ 342 343 /** 344 * Returns this reference object's referent. If this reference object has 345 * been cleared, either by the program or by the garbage collector, then 346 * this method returns {@code null}. 347 * 348 * @apiNote 349 * This method returns a strong reference to the referent. This may cause 350 * the garbage collector to treat it as strongly reachable until some later 351 * collection cycle. The {@link #refersTo(Object) refersTo} method can be 352 * used to avoid such strengthening when testing whether some object is 353 * the referent of a reference object; that is, use {@code ref.refersTo(obj)} 354 * rather than {@code ref.get() == obj}. 355 * 356 * @return The object to which this reference refers, or 357 * {@code null} if this reference object has been cleared 358 * @see #refersTo 359 */ 360 @IntrinsicCandidate 361 public T get() { 362 return this.referent; 363 } 364 365 /** 366 * Tests if the referent of this reference object is {@code obj}. 367 * Using a {@code null} {@code obj} returns {@code true} if the 368 * reference object has been cleared. 369 * 370 * @param obj the object to compare with this reference object's referent 371 * @return {@code true} if {@code obj} is the referent of this reference object 372 * @since 16 373 */ 374 public final boolean refersTo(T obj) { 375 return refersToImpl(obj); 376 } 377 378 /* Implementation of refersTo(), overridden for phantom references. 379 * This method exists only to avoid making refersTo0() virtual. Making 380 * refersTo0() virtual has the undesirable effect of C2 often preferring 381 * to call the native implementation over the intrinsic. 382 */ 383 boolean refersToImpl(T obj) { 384 return refersTo0(obj); 385 } 386 387 @IntrinsicCandidate 388 private native boolean refersTo0(Object o); 389 390 /** 391 * Clears this reference object. Invoking this method will not cause this 392 * object to be enqueued. 393 * 394 * <p> This method is invoked only by Java code; when the garbage collector 395 * clears references it does so directly, without invoking this method. 396 */ 397 public void clear() { 398 clear0(); 399 } 400 401 /* Implementation of clear(), also used by enqueue(). A simple 402 * assignment of the referent field won't do for some garbage 403 * collectors. 404 */ 405 private native void clear0(); 406 407 /* -- Operations on inactive FinalReferences -- */ 408 409 /* These functions are only used by FinalReference, and must only be 410 * called after the reference becomes inactive. While active, a 411 * FinalReference is considered weak but the referent is not normally 412 * accessed. Once a FinalReference becomes inactive it is considered a 413 * strong reference. These functions are used to bypass the 414 * corresponding weak implementations, directly accessing the referent 415 * field with strong semantics. 416 */ 417 418 /** 419 * Load referent with strong semantics. 420 */ 421 T getFromInactiveFinalReference() { 422 assert this instanceof FinalReference; 423 assert next != null; // I.e. FinalReference is inactive 424 return this.referent; 425 } 426 427 /** 428 * Clear referent with strong semantics. 429 */ 430 void clearInactiveFinalReference() { 431 assert this instanceof FinalReference; 432 assert next != null; // I.e. FinalReference is inactive 433 this.referent = null; 434 } 435 436 /* -- Queue operations -- */ 437 438 /** 439 * Tests if this reference object is in its associated queue, if any. 440 * This method returns {@code true} only if all of the following conditions 441 * are met: 442 * <ul> 443 * <li>this reference object was registered with a queue when it was created; and 444 * <li>the garbage collector has added this reference object to the queue 445 * or {@link #enqueue()} is called; and 446 * <li>this reference object is not yet removed from the queue. 447 * </ul> 448 * Otherwise, this method returns {@code false}. 449 * This method may return {@code false} if this reference object has been cleared 450 * but not enqueued due to the race condition. 451 * 452 * @deprecated 453 * This method was originally specified to test if a reference object has 454 * been cleared and enqueued but was never implemented to do this test. 455 * This method could be misused due to the inherent race condition 456 * or without an associated {@code ReferenceQueue}. 457 * An application relying on this method to release critical resources 458 * could cause serious performance issue. 459 * An application should use {@link ReferenceQueue} to reliably determine 460 * what reference objects that have been enqueued or 461 * {@link #refersTo(Object) refersTo(null)} to determine if this reference 462 * object has been cleared. 463 * 464 * @return {@code true} if and only if this reference object is 465 * in its associated queue (if any). 466 */ 467 @Deprecated(since="16") 468 public boolean isEnqueued() { 469 return (this.queue == ReferenceQueue.ENQUEUED); 470 } 471 472 /** 473 * Clears this reference object and adds it to the queue with which 474 * it is registered, if any. 475 * 476 * <p> This method is invoked only by Java code; when the garbage collector 477 * enqueues references it does so directly, without invoking this method. 478 * 479 * @return {@code true} if this reference object was successfully 480 * enqueued; {@code false} if it was already enqueued or if 481 * it was not registered with a queue when it was created 482 */ 483 public boolean enqueue() { 484 clear0(); // Intentionally clear0() rather than clear() 485 return this.queue.enqueue(this); 486 } 487 488 /** 489 * Throws {@link CloneNotSupportedException}. A {@code Reference} cannot be 490 * meaningfully cloned. Construct a new {@code Reference} instead. 491 * 492 * @return never returns normally 493 * @throws CloneNotSupportedException always 494 * 495 * @since 11 496 */ 497 @Override 498 protected Object clone() throws CloneNotSupportedException { 499 throw new CloneNotSupportedException(); 500 } 501 502 /* -- Constructors -- */ 503 504 Reference(T referent) { 505 this(referent, null); 506 } 507 508 Reference(T referent, ReferenceQueue<? super T> queue) { 509 this.referent = referent; 510 this.queue = (queue == null) ? ReferenceQueue.NULL : queue; 511 } 512 513 /** 514 * Ensures that the object referenced by the given reference remains 515 * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>, 516 * regardless of any prior actions of the program that might otherwise cause 517 * the object to become unreachable; thus, the referenced object is not 518 * reclaimable by garbage collection at least until after the invocation of 519 * this method. Invocation of this method does not itself initiate garbage 520 * collection or finalization. 521 * 522 * <p> This method establishes an ordering for <em>strong reachability</em> 523 * with respect to garbage collection. It controls relations that are 524 * otherwise only implicit in a program -- the reachability conditions 525 * triggering garbage collection. This method is designed for use in 526 * uncommon situations of premature finalization where using 527 * {@code synchronized} blocks or methods, or using other synchronization 528 * facilities are not possible or do not provide the desired control. This 529 * method is applicable only when reclamation may have visible effects, 530 * which is possible for objects with finalizers (See Section {@jls 12.6} 531 * of <cite>The Java Language Specification</cite>) that 532 * are implemented in ways that rely on ordering control for 533 * correctness. 534 * 535 * @apiNote 536 * Finalization may occur whenever the virtual machine detects that no 537 * reference to an object will ever be stored in the heap: The garbage 538 * collector may reclaim an object even if the fields of that object are 539 * still in use, so long as the object has otherwise become unreachable. 540 * This may have surprising and undesirable effects in cases such as the 541 * following example in which the bookkeeping associated with a class is 542 * managed through array indices. Here, method {@code action} uses a 543 * {@code reachabilityFence} to ensure that the {@code Resource} object is 544 * not reclaimed before bookkeeping on an associated 545 * {@code ExternalResource} has been performed; in particular here, to 546 * ensure that the array slot holding the {@code ExternalResource} is not 547 * nulled out in method {@link Object#finalize}, which may otherwise run 548 * concurrently. 549 * 550 * <pre> {@code 551 * class Resource { 552 * private static ExternalResource[] externalResourceArray = ... 553 * 554 * int myIndex; 555 * Resource(...) { 556 * myIndex = ... 557 * externalResourceArray[myIndex] = ...; 558 * ... 559 * } 560 * protected void finalize() { 561 * externalResourceArray[myIndex] = null; 562 * ... 563 * } 564 * public void action() { 565 * try { 566 * // ... 567 * int i = myIndex; 568 * Resource.update(externalResourceArray[i]); 569 * } finally { 570 * Reference.reachabilityFence(this); 571 * } 572 * } 573 * private static void update(ExternalResource ext) { 574 * ext.status = ...; 575 * } 576 * }}</pre> 577 * 578 * Here, the invocation of {@code reachabilityFence} is nonintuitively 579 * placed <em>after</em> the call to {@code update}, to ensure that the 580 * array slot is not nulled out by {@link Object#finalize} before the 581 * update, even if the call to {@code action} was the last use of this 582 * object. This might be the case if, for example a usage in a user program 583 * had the form {@code new Resource().action();} which retains no other 584 * reference to this {@code Resource}. While probably overkill here, 585 * {@code reachabilityFence} is placed in a {@code finally} block to ensure 586 * that it is invoked across all paths in the method. In a method with more 587 * complex control paths, you might need further precautions to ensure that 588 * {@code reachabilityFence} is encountered along all of them. 589 * 590 * <p> It is sometimes possible to better encapsulate use of 591 * {@code reachabilityFence}. Continuing the above example, if it were 592 * acceptable for the call to method {@code update} to proceed even if the 593 * finalizer had already executed (nulling out slot), then you could 594 * localize use of {@code reachabilityFence}: 595 * 596 * <pre> {@code 597 * public void action2() { 598 * // ... 599 * Resource.update(getExternalResource()); 600 * } 601 * private ExternalResource getExternalResource() { 602 * ExternalResource ext = externalResourceArray[myIndex]; 603 * Reference.reachabilityFence(this); 604 * return ext; 605 * }}</pre> 606 * 607 * <p> Method {@code reachabilityFence} is not required in constructions 608 * that themselves ensure reachability. For example, because objects that 609 * are locked cannot, in general, be reclaimed, it would suffice if all 610 * accesses of the object, in all methods of class {@code Resource} 611 * (including {@code finalize}) were enclosed in {@code synchronized (this)} 612 * blocks. (Further, such blocks must not include infinite loops, or 613 * themselves be unreachable, which fall into the corner case exceptions to 614 * the "in general" disclaimer.) However, method {@code reachabilityFence} 615 * remains a better option in cases where this approach is not as efficient, 616 * desirable, or possible; for example because it would encounter deadlock. 617 * 618 * @param ref the reference. If {@code null}, this method has no effect. 619 * @since 9 620 */ 621 @ForceInline 622 public static void reachabilityFence(Object ref) { 623 // Does nothing. This method is annotated with @ForceInline to eliminate 624 // most of the overhead that using @DontInline would cause with the 625 // HotSpot JVM, when this fence is used in a wide variety of situations. 626 // HotSpot JVM retains the ref and does not GC it before a call to 627 // this method, because the JIT-compilers do not have GC-only safepoints. 628 } 629 }