1 /*
2 * Copyright (c) 1997, 2018, 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.vm.annotation.ForceInline;
29 import jdk.internal.HotSpotIntrinsicCandidate;
30 import jdk.internal.misc.JavaLangRefAccess;
31 import jdk.internal.misc.SharedSecrets;
32 import jdk.internal.ref.Cleaner;
33
34 /**
35 * Abstract base class for reference objects. This class defines the
36 * operations common to all reference objects. Because reference objects are
37 * implemented in close cooperation with the garbage collector, this class may
38 * not be subclassed directly.
39 *
40 * @author Mark Reinhold
41 * @since 1.2
42 */
43
44 public abstract class Reference<T> {
45
46 /* The state of a Reference object is characterized by two attributes. It
47 * may be either "active", "pending", or "inactive". It may also be
48 * either "registered", "enqueued", "dequeued", or "unregistered".
49 *
50 * Active: Subject to special treatment by the garbage collector. Some
51 * time after the collector detects that the reachability of the referent
52 * has changed to the appropriate state, the collector "notifies" the
53 * reference, changing the state to either "pending" or "inactive".
54 * referent != null; discovered = null, or in GC discovered list.
55 *
56 * Pending: An element of the pending-Reference list, waiting to be
57 * processed by the Reference-handler thread. The pending-Reference list
58 * is linked through the discovered fields of references in the list.
59 * referent = null; discovered = next element in pending-Reference list.
60 *
61 * Inactive: Neither Active nor Pending.
62 * referent = null.
63 *
64 * Registered: Associated with a queue when created, and not yet added to
65 * the queue.
66 * queue = the associated queue.
67 *
68 * Enqueued: Added to the associated queue, and not yet removed.
69 * queue = ReferenceQueue.ENQUEUE; next = next entry in list, or this to
70 * indicate end of list.
71 *
72 * Dequeued: Added to the associated queue and then removed.
73 * queue = ReferenceQueue.NULL; next = this.
74 *
75 * Unregistered: Not associated with a queue when created.
76 * queue = ReferenceQueue.NULL.
77 *
78 * The collector only needs to examine the referent field and the
79 * discovered field to determine whether a normal (non-FinalReference)
80 * Reference object needs special treatment. If the referent is non-null
81 * and not known to be live, then it may need to be discovered for
82 * possible later notification. But if the discovered field is non-null,
83 * then either (1) it has already been discovered, or (2) it is in the
84 * pending list.
85 *
86 * FinalReference (which exists to support finalization, which was
87 * deprecated in JDK 9) differs from normal references, because a
88 * FinalReference is not cleared when notified. The referent being null
89 * or not cannot be used to distinguish between the active state and
90 * pending or inactive states. However, FinalReferences do not support
91 * enqueue(). Instead, the next field of a FinalReference object is set
92 * to "this" when it is added to the pending list. The use of "this"
93 * as the value of next in the enqueued and dequeued states maintains the
94 * non-active state. An additional check that the next field is null is
95 * required to determine that a FinalReference object is active.
96 *
97 * Initial states:
98 * active/registered
99 * active/unregistered [1]
100 *
101 * Transitions:
102 * active/registered -> pending/registered - GC
103 * -> inactive/registered - clear
104 * -> inactive/enqueued - enqueue [2]
105 * pending/registered -> pending/enqueued - enqueue [2]
106 * -> inactive/enqueued - pending list processing
107 * pending/enqueued -> inactive/enqueued - pending list processing
108 * -> pending/dequeued - poll/remove
109 * pending/dequeued -> inactive/dequeued - pending list processing
110 * inactive/registered -> inactive/enqueued - enqueue [2]
111 * inactive/enqueued -> inactive/dequeued - poll/remove
112 *
113 * active/unregistered -> pending/unregistered - GC
114 * -> inactive/unregistered - GC, clear, enqueue
115 * pending/unregistered -> inactive/unregistered - pending list processing
116 *
117 * Terminal states:
118 * inactive/dequeued
119 * inactive/unregistered
120 *
121 * Unreachable states (because enqueue also clears):
122 * active/enqeued
123 * active/dequeued
124 *
125 * [1] Unregistered is not permitted for FinalReferences.
126 *
127 * [2] These transitions are not possible for FinalReferences, making
128 * pending/enqueued and pending/dequeued unreachable, and
129 * inactive/registered terminal.
130 */
131
132 private T referent; /* Treated specially by GC */
133
134 /* The queue this reference gets enqueued to by GC notification or by
135 * calling enqueue().
136 *
137 * When registered: the queue with which this reference is registered.
138 * enqueued: ReferenceQueue.ENQUEUE
139 * dequeued: ReferenceQueue.NULL
140 * unregistered: ReferenceQueue.NULL
141 */
142 volatile ReferenceQueue<? super T> queue;
143
144 /* The link in a ReferenceQueue's list of Reference objects.
145 *
146 * When registered: null
147 * enqueued: next element in queue (or this if last)
148 * dequeued: this (marking FinalReferences as inactive)
149 * unregistered: null
150 */
151 @SuppressWarnings("rawtypes")
152 volatile Reference next;
153
154 /* Used by the garbage collector to accumulate Reference objects that need
155 * to be revisited in order to decide whether they should be notified.
156 * Also used as the link in the pending-Reference list. The discovered
157 * field and the next field are distinct to allow the enqueue() method to
158 * be applied to a Reference object while it is either in the
159 * pending-Reference list or in the garbage collector's discovered set.
160 *
161 * When active: null or next element in a discovered reference list
162 * maintained by the GC (or this if last)
163 * pending: next element in the pending-Reference list (null if last)
164 * inactive: null
165 */
166 private transient Reference<T> discovered;
167
168
169 /* High-priority thread to enqueue pending References
170 */
171 private static class ReferenceHandler extends Thread {
172
173 private static void ensureClassInitialized(Class<?> clazz) {
174 try {
175 Class.forName(clazz.getName(), true, clazz.getClassLoader());
176 } catch (ClassNotFoundException e) {
177 throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
178 }
179 }
180
181 static {
182 // pre-load and initialize Cleaner class so that we don't
183 // get into trouble later in the run loop if there's
184 // memory shortage while loading/initializing it lazily.
185 ensureClassInitialized(Cleaner.class);
186 }
187
188 ReferenceHandler(ThreadGroup g, String name) {
189 super(g, null, name, 0, false);
190 }
191
192 public void run() {
193 while (true) {
194 processPendingReferences();
195 }
196 }
197 }
198
199 /*
200 * Atomically get and clear (set to null) the VM's pending list.
201 */
202 private static native Reference<Object> getAndClearReferencePendingList();
203
204 /*
205 * Test whether the VM's pending list contains any entries.
206 */
207 private static native boolean hasReferencePendingList();
208
209 /*
210 * Wait until the VM's pending list may be non-null.
211 */
212 private static native void waitForReferencePendingList();
213
214 private static final Object processPendingLock = new Object();
215 private static boolean processPendingActive = false;
216
217 private static void processPendingReferences() {
218 // Only the singleton reference processing thread calls
219 // waitForReferencePendingList() and getAndClearReferencePendingList().
220 // These are separate operations to avoid a race with other threads
221 // that are calling waitForReferenceProcessing().
222 waitForReferencePendingList();
223 Reference<Object> pendingList;
224 synchronized (processPendingLock) {
225 pendingList = getAndClearReferencePendingList();
226 processPendingActive = true;
227 }
228 while (pendingList != null) {
229 Reference<Object> ref = pendingList;
230 pendingList = ref.discovered;
231 ref.discovered = null;
232
233 if (ref instanceof Cleaner) {
234 ((Cleaner)ref).clean();
235 // Notify any waiters that progress has been made.
236 // This improves latency for nio.Bits waiters, which
237 // are the only important ones.
238 synchronized (processPendingLock) {
239 processPendingLock.notifyAll();
240 }
241 } else {
242 ReferenceQueue<? super Object> q = ref.queue;
243 if (q != ReferenceQueue.NULL) q.enqueue(ref);
244 }
245 }
246 // Notify any waiters of completion of current round.
247 synchronized (processPendingLock) {
248 processPendingActive = false;
249 processPendingLock.notifyAll();
250 }
251 }
252
253 // Wait for progress in reference processing.
254 //
255 // Returns true after waiting (for notification from the reference
256 // processing thread) if either (1) the VM has any pending
257 // references, or (2) the reference processing thread is
258 // processing references. Otherwise, returns false immediately.
259 private static boolean waitForReferenceProcessing()
260 throws InterruptedException
261 {
262 synchronized (processPendingLock) {
263 if (processPendingActive || hasReferencePendingList()) {
264 // Wait for progress, not necessarily completion.
265 processPendingLock.wait();
266 return true;
267 } else {
268 return false;
269 }
270 }
271 }
272
273 static {
274 ThreadGroup tg = Thread.currentThread().getThreadGroup();
275 for (ThreadGroup tgn = tg;
276 tgn != null;
277 tg = tgn, tgn = tg.getParent());
278 Thread handler = new ReferenceHandler(tg, "Reference Handler");
279 /* If there were a special system-only priority greater than
280 * MAX_PRIORITY, it would be used here
281 */
282 handler.setPriority(Thread.MAX_PRIORITY);
283 handler.setDaemon(true);
284 handler.start();
285
286 // provide access in SharedSecrets
287 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
288 @Override
289 public boolean waitForReferenceProcessing()
290 throws InterruptedException
291 {
292 return Reference.waitForReferenceProcessing();
293 }
294
295 @Override
296 public void runFinalization() {
297 Finalizer.runFinalization();
298 }
299 });
300 }
301
302 /* -- Referent accessor and setters -- */
303
304 /**
305 * Returns this reference object's referent. If this reference object has
306 * been cleared, either by the program or by the garbage collector, then
307 * this method returns <code>null</code>.
308 *
309 * @return The object to which this reference refers, or
310 * <code>null</code> if this reference object has been cleared
311 */
312 @HotSpotIntrinsicCandidate
313 public T get() {
314 return this.referent;
315 }
316
317 /**
318 * Clears this reference object. Invoking this method will not cause this
319 * object to be enqueued.
320 *
321 * <p> This method is invoked only by Java code; when the garbage collector
322 * clears references it does so directly, without invoking this method.
323 */
324 public void clear() {
325 this.referent = null;
326 }
327
328 /* -- Queue operations -- */
329
330 /**
331 * Tells whether or not this reference object has been enqueued, either by
332 * the program or by the garbage collector. If this reference object was
333 * not registered with a queue when it was created, then this method will
334 * always return <code>false</code>.
335 *
336 * @return <code>true</code> if and only if this reference object has
337 * been enqueued
338 */
339 public boolean isEnqueued() {
340 return (this.queue == ReferenceQueue.ENQUEUED);
341 }
342
343 /**
344 * Clears this reference object and adds it to the queue with which
345 * it is registered, if any.
346 *
347 * <p> This method is invoked only by Java code; when the garbage collector
348 * enqueues references it does so directly, without invoking this method.
349 *
350 * @return <code>true</code> if this reference object was successfully
351 * enqueued; <code>false</code> if it was already enqueued or if
352 * it was not registered with a queue when it was created
353 */
354 public boolean enqueue() {
355 this.referent = null;
356 return this.queue.enqueue(this);
357 }
358
359 /**
360 * Throws {@link CloneNotSupportedException}. A {@code Reference} cannot be
361 * meaningfully cloned. Construct a new {@code Reference} instead.
362 *
363 * @returns never returns normally
364 * @throws CloneNotSupportedException always
365 *
366 * @since 11
367 */
368 @Override
369 protected Object clone() throws CloneNotSupportedException {
370 throw new CloneNotSupportedException();
371 }
372
373 /* -- Constructors -- */
374
375 Reference(T referent) {
376 this(referent, null);
377 }
378
379 Reference(T referent, ReferenceQueue<? super T> queue) {
380 this.referent = referent;
381 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
382 }
383
384 /**
385 * Ensures that the object referenced by the given reference remains
386 * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
387 * regardless of any prior actions of the program that might otherwise cause
388 * the object to become unreachable; thus, the referenced object is not
389 * reclaimable by garbage collection at least until after the invocation of
390 * this method. Invocation of this method does not itself initiate garbage
391 * collection or finalization.
392 *
393 * <p> This method establishes an ordering for
394 * <a href="package-summary.html#reachability"><em>strong reachability</em></a>
395 * with respect to garbage collection. It controls relations that are
396 * otherwise only implicit in a program -- the reachability conditions
397 * triggering garbage collection. This method is designed for use in
398 * uncommon situations of premature finalization where using
399 * {@code synchronized} blocks or methods, or using other synchronization
400 * facilities are not possible or do not provide the desired control. This
401 * method is applicable only when reclamation may have visible effects,
402 * which is possible for objects with finalizers (See
403 * <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6">
404 * Section 12.6 17 of <cite>The Java™ Language Specification</cite></a>)
405 * that are implemented in ways that rely on ordering control for correctness.
406 *
407 * @apiNote
408 * Finalization may occur whenever the virtual machine detects that no
409 * reference to an object will ever be stored in the heap: The garbage
410 * collector may reclaim an object even if the fields of that object are
411 * still in use, so long as the object has otherwise become unreachable.
412 * This may have surprising and undesirable effects in cases such as the
413 * following example in which the bookkeeping associated with a class is
414 * managed through array indices. Here, method {@code action} uses a
415 * {@code reachabilityFence} to ensure that the {@code Resource} object is
416 * not reclaimed before bookkeeping on an associated
417 * {@code ExternalResource} has been performed; in particular here, to
418 * ensure that the array slot holding the {@code ExternalResource} is not
419 * nulled out in method {@link Object#finalize}, which may otherwise run
420 * concurrently.
421 *
422 * <pre> {@code
423 * class Resource {
424 * private static ExternalResource[] externalResourceArray = ...
425 *
426 * int myIndex;
427 * Resource(...) {
428 * myIndex = ...
429 * externalResourceArray[myIndex] = ...;
430 * ...
431 * }
432 * protected void finalize() {
433 * externalResourceArray[myIndex] = null;
434 * ...
435 * }
436 * public void action() {
437 * try {
438 * // ...
439 * int i = myIndex;
440 * Resource.update(externalResourceArray[i]);
441 * } finally {
442 * Reference.reachabilityFence(this);
443 * }
444 * }
445 * private static void update(ExternalResource ext) {
446 * ext.status = ...;
447 * }
448 * }}</pre>
449 *
450 * Here, the invocation of {@code reachabilityFence} is nonintuitively
451 * placed <em>after</em> the call to {@code update}, to ensure that the
452 * array slot is not nulled out by {@link Object#finalize} before the
453 * update, even if the call to {@code action} was the last use of this
454 * object. This might be the case if, for example a usage in a user program
455 * had the form {@code new Resource().action();} which retains no other
456 * reference to this {@code Resource}. While probably overkill here,
457 * {@code reachabilityFence} is placed in a {@code finally} block to ensure
458 * that it is invoked across all paths in the method. In a method with more
459 * complex control paths, you might need further precautions to ensure that
460 * {@code reachabilityFence} is encountered along all of them.
461 *
462 * <p> It is sometimes possible to better encapsulate use of
463 * {@code reachabilityFence}. Continuing the above example, if it were
464 * acceptable for the call to method {@code update} to proceed even if the
465 * finalizer had already executed (nulling out slot), then you could
466 * localize use of {@code reachabilityFence}:
467 *
468 * <pre> {@code
469 * public void action2() {
470 * // ...
471 * Resource.update(getExternalResource());
472 * }
473 * private ExternalResource getExternalResource() {
474 * ExternalResource ext = externalResourceArray[myIndex];
475 * Reference.reachabilityFence(this);
476 * return ext;
477 * }}</pre>
478 *
479 * <p> Method {@code reachabilityFence} is not required in constructions
480 * that themselves ensure reachability. For example, because objects that
481 * are locked cannot, in general, be reclaimed, it would suffice if all
482 * accesses of the object, in all methods of class {@code Resource}
483 * (including {@code finalize}) were enclosed in {@code synchronized (this)}
484 * blocks. (Further, such blocks must not include infinite loops, or
485 * themselves be unreachable, which fall into the corner case exceptions to
486 * the "in general" disclaimer.) However, method {@code reachabilityFence}
487 * remains a better option in cases where this approach is not as efficient,
488 * desirable, or possible; for example because it would encounter deadlock.
489 *
490 * @param ref the reference. If {@code null}, this method has no effect.
491 * @since 9
492 */
493 @ForceInline
494 public static void reachabilityFence(Object ref) {
495 // Does nothing. This method is annotated with @ForceInline to eliminate
496 // most of the overhead that using @DontInline would cause with the
497 // HotSpot JVM, when this fence is used in a wide variety of situations.
498 // HotSpot JVM retains the ref and does not GC it before a call to
499 // this method, because the JIT-compilers do not have GC-only safepoints.
500 }
501 }