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 }