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 /**
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 does not enqueue this
392 * object, and the garbage collector will not clear or enqueue this object.
393 *
394 * <p>When the garbage collector or the {@link #enqueue()} method clear
395 * references they do so directly, without invoking this method.
396 *
397 * @apiNote
398 * There is a potential race condition with the garbage collector. When this
399 * method is called, the garbage collector may already be in the process of
400 * (or already completed) clearing and/or enqueueing this reference.
401 * Avoid this race by ensuring the referent remains strongly reachable until
402 * after the call to clear(), using {@link #reachabilityFence(Object)} if
403 * necessary.
404 */
405 public void clear() {
406 clearImpl();
407 }
408
409 /* Implementation of clear(). A simple assignment of the referent field
410 * won't do for some garbage collectors. There is the override for phantom
411 * references, which requires different semantics. This method is also
412 * used by enqueue().
413 *
414 * <p>This method exists only to avoid making clear0() virtual. Making
415 * clear0() virtual has the undesirable effect of C2 often preferring
416 * to call the native implementation over the intrinsic.
417 */
418 void clearImpl() {
419 clear0();
420 }
421
422 @IntrinsicCandidate
423 private native void clear0();
424
425 /* -- Operations on inactive FinalReferences -- */
426
427 /* These functions are only used by FinalReference, and must only be
428 * called after the reference becomes inactive. While active, a
429 * FinalReference is considered weak but the referent is not normally
430 * accessed. Once a FinalReference becomes inactive it is considered a
431 * strong reference. These functions are used to bypass the
432 * corresponding weak implementations, directly accessing the referent
433 * field with strong semantics.
434 */
435
436 /**
437 * Load referent with strong semantics.
438 */
439 T getFromInactiveFinalReference() {
440 assert this instanceof FinalReference;
441 assert next != null; // I.e. FinalReference is inactive
442 return this.referent;
443 }
444
445 /**
446 * Clear referent with strong semantics.
447 */
448 void clearInactiveFinalReference() {
449 assert this instanceof FinalReference;
450 assert next != null; // I.e. FinalReference is inactive
451 this.referent = null;
452 }
453
454 /* -- Queue operations -- */
455
456 /**
457 * Tests if this reference object is in its associated queue, if any.
458 * This method returns {@code true} only if all of the following conditions
459 * are met:
460 * <ul>
461 * <li>this reference object was registered with a queue when it was created; and
462 * <li>the garbage collector has added this reference object to the queue
463 * or {@link #enqueue()} is called; and
464 * <li>this reference object is not yet removed from the queue.
465 * </ul>
466 * Otherwise, this method returns {@code false}.
467 * This method may return {@code false} if this reference object has been cleared
468 * but not enqueued due to the race condition.
469 *
470 * @deprecated
471 * This method was originally specified to test if a reference object has
472 * been cleared and enqueued but was never implemented to do this test.
473 * This method could be misused due to the inherent race condition
474 * or without an associated {@code ReferenceQueue}.
475 * An application relying on this method to release critical resources
476 * could cause serious performance issue.
477 * An application should use {@link ReferenceQueue} to reliably determine
478 * what reference objects that have been enqueued or
479 * {@link #refersTo(Object) refersTo(null)} to determine if this reference
480 * object has been cleared.
481 *
482 * @return {@code true} if and only if this reference object is
483 * in its associated queue (if any).
484 */
485 @Deprecated(since="16")
486 public boolean isEnqueued() {
487 return (this.queue == ReferenceQueue.ENQUEUED);
488 }
489
490 /**
491 * Clears this reference object, then attempts to add it to the queue with
492 * which it is registered, if any.
493 *
494 * <p>If this reference is registered with a queue but not yet enqueued,
495 * the reference is added to the queue; this method is
496 * <b><i>successful</i></b> and returns true.
497 * If this reference is not registered with a queue, or was already enqueued
498 * (by the garbage collector, or a previous call to {@code enqueue}), this
499 * method is <b><i>unsuccessful</i></b> and returns false.
500 *
501 * <p>{@linkplain java.lang.ref##MemoryConsistency Memory consistency effects}:
502 * Actions in a thread prior to a <b><i>successful</i></b> call to {@code enqueue}
503 * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happen-before</i></a>
504 * the reference is removed from the queue by {@link ReferenceQueue#poll}
505 * or {@link ReferenceQueue#remove}. <b><i>Unsuccessful</i></b> calls to
506 * {@code enqueue} have no specified memory consistency effects.
507 *
508 * <p> When this method clears references it does so directly, without
509 * invoking the {@link #clear()} method. When the garbage collector clears
510 * and enqueues references it does so directly, without invoking the
511 * {@link #clear()} method or this method.
512 *
513 * @apiNote
514 * Use of this method allows the registered queue's
515 * {@link ReferenceQueue#poll} and {@link ReferenceQueue#remove} methods
516 * to return this reference even though the referent may still be strongly
517 * reachable.
518 *
519 * @return {@code true} if this reference object was successfully
520 * enqueued; {@code false} if it was already enqueued or if
521 * it was not registered with a queue when it was created
522 */
523 public boolean enqueue() {
524 clearImpl(); // Intentionally clearImpl() to dispatch to overridden method, if needed
525 return this.queue.enqueue(this);
526 }
527
528 /**
529 * Throws {@link CloneNotSupportedException}. A {@code Reference} cannot be
530 * meaningfully cloned. Construct a new {@code Reference} instead.
531 *
532 * @return never returns normally
533 * @throws CloneNotSupportedException always
534 */
535 @Override
536 protected Object clone() throws CloneNotSupportedException {
537 throw new CloneNotSupportedException();
538 }
539
540 /* -- Constructors -- */
541
542 Reference(T referent) {
543 this(referent, null);
544 }
545
546 Reference(T referent, ReferenceQueue<? super T> queue) {
547 this.referent = referent;
548 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
549 }
550
551 /**
552 * Ensures that the given object remains
553 * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>.
554 * This reachability is assured regardless of any optimizing transformations
555 * the virtual machine may perform that might otherwise allow the object to
556 * become unreachable (see JLS {@jls 12.6.1}). Thus, the given object is not
557 * reclaimable by garbage collection at least until after the invocation of
558 * this method. References to the given object will not be cleared (or
559 * enqueued, if applicable) by the garbage collector until after invocation
560 * of this method.
561 * Invocation of this method does not itself initiate reference processing,
562 * garbage collection, or finalization.
563 *
564 * <p> This method establishes an ordering for <em>strong reachability</em>
565 * with respect to garbage collection. It controls relations that are
566 * otherwise only implicit in a program -- the reachability conditions
567 * triggering garbage collection. This method is applicable only
568 * when reclamation may have visible effects,
569 * such as for objects that use finalizers or {@link Cleaner}, or code that
570 * performs {@linkplain java.lang.ref reference processing}.
571 *
572 * <p>{@linkplain java.lang.ref##MemoryConsistency Memory consistency effects}:
573 * Actions in a thread prior to calling {@code reachabilityFence(x)}
574 * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happen-before</i></a>
575 * the garbage collector clears any reference to {@code x}.
576 *
577 * @apiNote
578 * Reference processing or finalization can occur after an object becomes
579 * unreachable. An object can become unreachable when the virtual machine
580 * detects that there is no further need for the object (other than for
581 * running a finalizer). In the course of optimization, the virtual machine
582 * can reorder operations of an object's methods such that the object
583 * becomes unneeded earlier than might naively be expected —
584 * including while a method of the object is still running. For instance,
585 * the VM can move the loading of <em>values</em> from the object's fields
586 * to occur earlier. The object itself is then no longer needed and becomes
587 * unreachable, and the method can continue running using the obtained values.
588 * This may have surprising and undesirable effects when using a Cleaner or
589 * finalizer for cleanup: there is a race between the
590 * program thread running the method, and the cleanup thread running the
591 * Cleaner or finalizer. The cleanup thread could free a
592 * resource, followed by the program thread (still running the method)
593 * attempting to access the now-already-freed resource.
594 * Use of {@code reachabilityFence} can prevent this race by ensuring that the
595 * object remains strongly reachable.
596 * <p>
597 * The following is an example in which the bookkeeping associated with a class is
598 * managed through array indices. Here, method {@code action} uses a
599 * {@code reachabilityFence} to ensure that the {@code Resource} object is
600 * not reclaimed before bookkeeping on an associated
601 * {@code ExternalResource} has been performed; specifically, to
602 * ensure that the array slot holding the {@code ExternalResource} is not
603 * nulled out in method {@link Object#finalize}, which may otherwise run
604 * concurrently.
605 *
606 * {@snippet :
607 * class Resource {
608 * private static ExternalResource[] externalResourceArray = ...
609 *
610 * int myIndex;
611 * Resource(...) {
612 * this.myIndex = ...
613 * externalResourceArray[myIndex] = ...;
614 * ...
615 * }
616 * protected void finalize() {
617 * externalResourceArray[this.myIndex] = null;
618 * ...
619 * }
620 * public void action() {
621 * try {
622 * // ...
623 * int i = this.myIndex; // last use of 'this' Resource in action()
624 * Resource.update(externalResourceArray[i]);
625 * } finally {
626 * Reference.reachabilityFence(this);
627 * }
628 * }
629 * private static void update(ExternalResource ext) {
630 * ext.status = ...;
631 * }
632 * }
633 * }
634 *
635 * The invocation of {@code reachabilityFence} is
636 * placed <em>after</em> the call to {@code update}, to ensure that the
637 * array slot is not nulled out by {@link Object#finalize} before the
638 * update, even if the call to {@code action} was the last use of this
639 * object. This might be the case if, for example, a usage in a user program
640 * had the form {@code new Resource().action();} which retains no other
641 * reference to this {@code Resource}.
642 * The {@code reachabilityFence} call is placed in a {@code finally} block to
643 * ensure that it is invoked across all paths in the method. A more complex
644 * method might need further precautions to ensure that
645 * {@code reachabilityFence} is encountered along all code paths.
646 *
647 * <p> Method {@code reachabilityFence} is not required in constructions
648 * that themselves ensure reachability. For example, because objects that
649 * are locked cannot, in general, be reclaimed, it would suffice if all
650 * accesses of the object, in all methods of class {@code Resource}
651 * (including {@code finalize}) were enclosed in {@code synchronized (this)}
652 * blocks. (Further, such blocks must not include infinite loops, or
653 * themselves be unreachable, which fall into the corner case exceptions to
654 * the "in general" disclaimer.) However, method {@code reachabilityFence}
655 * remains a better option in cases where synchronization is not as efficient,
656 * desirable, or possible; for example because it would encounter deadlock.
657 *
658 * @param ref the reference to the object to keep strongly reachable. If
659 * {@code null}, this method has no effect.
660 * @since 9
661 */
662 @ForceInline
663 public static void reachabilityFence(Object ref) {
664 // Does nothing. This method is annotated with @ForceInline to eliminate
665 // most of the overhead that using @DontInline would cause with the
666 // HotSpot JVM, when this fence is used in a wide variety of situations.
667 // HotSpot JVM retains the ref and does not GC it before a call to
668 // this method, because the JIT-compilers do not have GC-only safepoints.
669 }
670 }