1 /*
2 * Copyright (c) 1997, 2013, 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 sun.misc.JavaLangRefAccess;
29 import sun.misc.ManagedLocalsThread;
30 import sun.misc.SharedSecrets;
31
32 /**
33 * Abstract base class for reference objects. This class defines the
34 * operations common to all reference objects. Because reference objects are
35 * implemented in close cooperation with the garbage collector, this class may
36 * not be subclassed directly.
37 *
38 * @author Mark Reinhold
39 * @since 1.2
40 */
41
42 public abstract class Reference<T> {
43
44 /* A Reference instance is in one of four possible internal states:
45 *
46 * Active: Subject to special treatment by the garbage collector. Some
47 * time after the collector detects that the reachability of the
48 * referent has changed to the appropriate state, it changes the
49 * instance's state to either Pending or Inactive, depending upon
50 * whether or not the instance was registered with a queue when it was
51 * created. In the former case it also adds the instance to the
52 * pending-Reference list. Newly-created instances are Active.
53 *
54 * Pending: An element of the pending-Reference list, waiting to be
55 * enqueued by the Reference-handler thread. Unregistered instances
56 * are never in this state.
57 *
58 * Enqueued: An element of the queue with which the instance was
59 * registered when it was created. When an instance is removed from
60 * its ReferenceQueue, it is made Inactive. Unregistered instances are
61 * never in this state.
62 *
63 * Inactive: Nothing more to do. Once an instance becomes Inactive its
64 * state will never change again.
65 *
66 * The state is encoded in the queue and next fields as follows:
67 *
68 * Active: queue = ReferenceQueue with which instance is registered, or
69 * ReferenceQueue.NULL if it was not registered with a queue; next =
70 * null.
71 *
72 * Pending: queue = ReferenceQueue with which instance is registered;
73 * next = this
74 *
75 * Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
76 * in queue, or this if at end of list.
77 *
78 * Inactive: queue = ReferenceQueue.NULL; next = this.
79 *
80 * With this scheme the collector need only examine the next field in order
81 * to determine whether a Reference instance requires special treatment: If
82 * the next field is null then the instance is active; if it is non-null,
83 * then the collector should treat the instance normally.
84 *
85 * To ensure that a concurrent collector can discover active Reference
86 * objects without interfering with application threads that may apply
87 * the enqueue() method to those objects, collectors should link
88 * discovered objects through the discovered field. The discovered
89 * field is also used for linking Reference objects in the pending list.
90 */
91
92 private T referent; /* Treated specially by GC */
93
94 volatile ReferenceQueue<? super T> queue;
95
96 /* When active: NULL
97 * pending: this
98 * Enqueued: next reference in queue (or this if last)
99 * Inactive: this
100 */
101 Reference<?> next;
102
103 /* When active: next element in a discovered reference list maintained by GC (or this if last)
104 * pending: next element in the pending list (or null if last)
105 * otherwise: NULL
106 */
107 transient private Reference<?> discovered; /* used by VM */
108
109
110 /* Object used to synchronize with the garbage collector. The collector
111 * must acquire this lock at the beginning of each collection cycle. It is
112 * therefore critical that any code holding this lock complete as quickly
113 * as possible, allocate no new objects, and avoid calling user code.
114 */
115 static private class Lock { }
116 private static final Lock lock = new Lock();
117
118
119 /* List of References waiting to be enqueued. The collector adds
120 * References to this list, while the Reference-handler thread removes
121 * them. This list is protected by the above lock object. The
122 * list uses the discovered field to link its elements.
123 */
124 private static Reference<?> pending;
125
126 /**
127 * Max. number of Reference(s) to unhook from pending chain in one chunk
128 * before releasing the lock, handling them and grabbing the
129 * lock again.
130 */
131 private static final int UNHOOK_CHUNK_SIZE = 32768;
132
133 /**
134 * Max. number of j.l.r.Cleaner(s) to execute in one ForkJoinTask
135 */
136 private static final int CLEANER_CHUNK_SIZE = 256;
137
138 /**
139 * Max. number of Reference(s) to enqueue in one chunk
140 */
141 private static final int ENQUEUE_CHUNK_SIZE = 256;
142
143 private static class ReferenceHandler extends ManagedLocalsThread {
144
145 private static void ensureClassInitialized(Class<?> clazz) {
146 try {
147 Class.forName(clazz.getName(), true, clazz.getClassLoader());
148 } catch (ClassNotFoundException e) {
149 throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
150 }
151 }
152
153 static {
154 // pre-load and initialize InterruptedException and Cleaner classes
155 // so that we don't get into trouble later in the run loop if there's
156 // memory shortage while loading/initializing them lazily.
157 ensureClassInitialized(InterruptedException.class);
158 ensureClassInitialized(sun.misc.Cleaner.class);
159 ensureClassInitialized(Cleaner.class);
160 }
161
162 ReferenceHandler(ThreadGroup g, String name) {
163 super(g, name);
164 }
165
166 public void run() {
167 // wait for VM to boot-up before starting reference handling
168 // ForkJoinPool since it needs access to some system properties
169 // and Finalizer needs access to SharedSecrets.
170 while (true) {
171 try {
172 sun.misc.VM.awaitBooted();
173 break;
174 } catch (InterruptedException e) {
175 // ignore;
176 }
177 }
178 // enter endless loop
179 boolean[] morePending = new boolean[1];
180 while (true) {
181 Reference<?> chunk = null;
182 try {
183 synchronized (lock) {
184 chunk = Reference.unhookPendingChunk(UNHOOK_CHUNK_SIZE, morePending);
185 if (chunk == null) {
186 // waiting on notification can throw InterruptedException
187 // if the thread is interrupted, but also OutOfMemoryError
188 // if the InterruptedException can not be allocated.
189 lock.wait();
190 // since we have already re-obtained the lock, we can
191 // re-try poll and will typically get a non-null chunk.
192 chunk = Reference.unhookPendingChunk(UNHOOK_CHUNK_SIZE, morePending);
193 }
194 }
195 } catch (OutOfMemoryError e) {
196 // give other threads some time so they hopefully release some
197 // references and GC reclaims some space, then retry...
198 Thread.yield();
199 } catch (InterruptedException e) {
200 // ignore
201 }
202 if (chunk != null) {
203 if (morePending[0]) {
204 // submit a handling task and return for next chunk
205 new ReferenceHandling.PendingChunkHandler(chunk).submit();
206 } else {
207 // no more pending, so we can handle the chunk directly
208 Reference.handlePendingChunk(chunk);
209 }
210 }
211 }
212 }
213 }
214
215 /**
216 * Try handle a chunk of pending {@link Reference}s if there are any.<p>
217 * Return {@code true} as a hint that there are more
218 * {@link Reference}s pending or {@code false} when there are no more pending
219 * {@link Reference}s at the moment and the program can do some other
220 * useful work instead of looping.
221 *
222 * @return {@code true} if there is be more {@link Reference}s pending.
223 */
224 static boolean tryHandlePending() {
225 Reference<?> r;
226 synchronized (lock) {
227 r = unhookPendingChunk(UNHOOK_CHUNK_SIZE, null);
228 }
229 if (r == null) return false;
230 handlePendingChunk(r);
231 synchronized (lock) {
232 return pending != null;
233 }
234 }
235
236 /**
237 * Unhooks a chunk of max. {@code chunkSize} references from pending chain and
238 * returns the head of the chunk; elements of the chunk can be reached using
239 * {@link #discovered} links; the last in chunk is linked to null.
240 *
241 * @param chunkSize max. number of references to unhook from the pending chain
242 * @param morePending if non null, it should be a boolean array with length 1
243 * to hold the additional result - a flag indicating that
244 * there are more pending references waiting after a chunk
245 * of them has been returned.
246 * @return the head of the chunk of max. {@code chunkSize} pending references or
247 * null if there are none pending.
248 */
249 private static Reference<?> unhookPendingChunk(int chunkSize, boolean[] morePending) {
250 // assert Thread.holdsLock(lock);
251 Reference<?> r;
252 if ((r = pending) != null) {
253 // skip over max. chunkSize references using 'discovered' link
254 Reference<?> p = r;
255 Reference<?> d = p.discovered;
256 for (int i = 0;
257 d != null && ++i < chunkSize;
258 p = d, d = p.discovered) {}
259 pending = d;
260 p.discovered = null; // unlink last in unhooked chunk from the rest
261 if (morePending != null) morePending[0] = (d != null);
262 } else {
263 if (morePending != null) morePending[0] = false;
264 }
265 return r;
266 }
267
268 /**
269 * Takes a non-null chunk of unhooked pending references
270 * (obtained using {@link #unhookPendingChunk}) and handles
271 * them as following:
272 * <ul>
273 * <li>sun.misc.Cleaner(s) are executed immediately</li>
274 * <li>java.lang.ref.Cleaner(s) are submitted in chunks as ForkJoinTask(s)</li>
275 * <li>all other Reference(s) are enqueued in their respective queues</li>
276 * </ul>
277 * The references in chunk are linked using {@link #discovered} link.
278 * Last in chunk is linked to null. As they are handled, their discovered
279 * links are reset to null.
280 *
281 * @param chunk the head of a chunk of pending references
282 */
283 static void handlePendingChunk(Reference<?> chunk) {
284 // batch j.l.r.Cleaner(s)
285 Reference<?> cleaners = null;
286 int cleanersCount = 0;
287 // batch runs of consecutive references having same queue
288 Reference<?> referencesHead = null, referencesTail = null;
289 int referencesCount = 0;
290 ReferenceQueue<?> referenceQueue = null;
291 // dispatch references to appropriate targets
292 for (Reference<?> r = chunk, d = r.discovered;
293 r != null;
294 r = d, d = (r == null) ? null : r.discovered) {
295 // invariant established by GC when marking the reference as not active
296 // assert r.next == r;
297 if (r instanceof sun.misc.Cleaner) { // Fast path for sun.misc.Cleaners
298 // unlink from the rest in chunk
299 r.discovered = null;
300 ((sun.misc.Cleaner) r).clean();
301 } else if (r instanceof Cleaner) { // Submit task(s) for j.l.r.Cleaner(s)
302 // link into the local cleaners list
303 r.discovered = cleaners;
304 cleaners = r;
305 if (++cleanersCount >= CLEANER_CHUNK_SIZE) {
306 // when chunk of finalizers is full, submit a task
307 new ReferenceHandling.CleanersHandler(cleaners).submit();
308 cleaners = null;
309 cleanersCount = 0;
310 }
311 } else { // Enqueue all other references
312 // unlink from the rest in chunk
313 r.discovered = null;
314 ReferenceQueue<?> q = r.queue;
315 if (q != ReferenceQueue.NULL && q.markEnqueued(r)) { // markEnqueued is atomic
316 if (referenceQueue == null || referenceQueue == q) {
317 // no queue or same queue -> hook onto the references[Head|Tail] chain
318 if (referencesHead == null) {
319 // assert referencesTail == null && referenceQueue == null &&
320 // referencesCount == 0 && r.next == r;
321 referenceQueue = q;
322 referencesHead = referencesTail = r;
323 } else {
324 // assert referencesTail != null && referenceQueue == q &&
325 // referencesCount > 0;
326 r.next = referencesHead;
327 referencesHead = r;
328 }
329 if (++referencesCount >= ENQUEUE_CHUNK_SIZE) {
330 // when a chunk of references is full, add them to queue
331 referenceQueue.addChunk(referencesHead, referencesTail);
332 referencesHead = referencesTail = null;
333 referenceQueue = null;
334 referencesCount = 0;
335 }
336 } else {
337 // when a different queue is encountered,
338 // add collected chunk to it's queue and start collecting
339 // new batch for new queue...
340 // assert referenceQueue != null && referenceQueue != q &&
341 // referencesHead != null && referencesTail != null &&
342 // referencesCount > 0 && r.next == r;
343 referenceQueue.addChunk(referencesHead, referencesTail);
344 referenceQueue = q;
345 referencesHead = referencesTail = r;
346 referencesCount = 1;
347 }
348 }
349 // else just drop it on the flor
350 }
351 }
352 // any j.l.r.Cleaner(s) left?
353 if (cleaners != null) {
354 new ReferenceHandling.CleanersHandler(cleaners).submit();
355 cleaners = null;
356 cleanersCount = 0;
357 }
358 // any references left to enqueue?
359 if (referenceQueue != null) {
360 // assert referencesHead != null && referencesTail != null && referencesCount > 0;
361 referenceQueue.addChunk(referencesHead, referencesTail);
362 referencesHead = referencesTail = null;
363 referenceQueue = null;
364 referencesCount = 0;
365 }
366 }
367
368 /**
369 * Handles a non-null chunk of j.l.r.Cleaner(s)
370 *
371 * @param cleaners the head of a chunk of Reference(s) implementing j.l.r.Cleaner
372 * linked using {@link #discovered} link. Last in chunk is
373 * linked to null.
374 */
375 static void handleCleaners(Reference<?> cleaners) {
376 for (Reference<?> c = cleaners, d = c.discovered;
377 c != null;
378 c = d, d = (c == null) ? null : c.discovered) {
379 // unlink it from the rest in chunk
380 c.discovered = null;
381 // Handle it
382 try {
383 ((Cleaner) c).clean();
384 } catch (Throwable t) {
385 // ignore
386 }
387 }
388 }
389
390 /* -- Referent accessor and setters -- */
391
392 /**
393 * Returns this reference object's referent. If this reference object has
394 * been cleared, either by the program or by the garbage collector, then
395 * this method returns <code>null</code>.
396 *
397 * @return The object to which this reference refers, or
398 * <code>null</code> if this reference object has been cleared
399 */
400 public T get() {
401 return this.referent;
402 }
403
404 /**
405 * Clears this reference object. Invoking this method will not cause this
406 * object to be enqueued.
407 *
408 * <p> This method is invoked only by Java code; when the garbage collector
409 * clears references it does so directly, without invoking this method.
410 */
411 public void clear() {
412 this.referent = null;
413 }
414
415
416 /* -- Queue operations -- */
417
418 /**
419 * Tells whether or not this reference object has been enqueued, either by
420 * the program or by the garbage collector. If this reference object was
421 * not registered with a queue when it was created, then this method will
422 * always return <code>false</code>.
423 *
424 * @return <code>true</code> if and only if this reference object has
425 * been enqueued
426 */
427 public boolean isEnqueued() {
428 return (this.queue == ReferenceQueue.ENQUEUED);
429 }
430
431 /**
432 * Adds this reference object to the queue with which it is registered,
433 * if any.
434 *
435 * <p> This method is invoked only by Java code; when the garbage collector
436 * enqueues references it does so directly, without invoking this method.
437 *
438 * @return <code>true</code> if this reference object was successfully
439 * enqueued; <code>false</code> if it was already enqueued or if
440 * it was not registered with a queue when it was created
441 */
442 public boolean enqueue() {
443 return this.queue.enqueue(this);
444 }
445
446
447 /* -- Constructors -- */
448
449 Reference(T referent) {
450 this(referent, null);
451 }
452
453 Reference(T referent, ReferenceQueue<? super T> queue) {
454 this.referent = referent;
455 if (this instanceof Cleaner && queue != null) {
456 throw new IllegalArgumentException(
457 "Reference implementing Cleaner can't be registered with a reference queue");
458 }
459 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
460 }
461
462 // Methods that enable selected Reference subclasses to be elements of a DLList
463
464 /**
465 * Some Cleaner's (such as Finalizer) wish to be registered in a doubly-linked
466 * list so that they are kept alive until discovered by VM, processed by
467 * reference handling threads and then unlinked. There are several lists to
468 * distribute them randomly into to reduce contention among concurrent threads
469 * trying to link/unlink them.
470 */
471 static final DLList[] uncleanedLists;
472
473 /**
474 * Register this reference into the associated doubly-linked list.
475 */
476 final void link() {
477 int h = System.identityHashCode(this);
478 int i = (h >>> 1) & (uncleanedLists.length - 1);
479 uncleanedLists[i].link(this, (h & 1) == 0);
480 }
481
482 /**
483 * De-register this reference from the associated doubly-linked list.
484 */
485 final void unlink() {
486 int h = System.identityHashCode(this);
487 int i = (h >>> 1) & (uncleanedLists.length - 1);
488 uncleanedLists[i].unlink(this);
489 }
490
491 boolean isDeletedDll() {
492 throw new UnsupportedOperationException();
493 }
494
495 Reference<?> getPrevDll() {
496 throw new UnsupportedOperationException();
497 }
498
499 void lazySetPrevDll(Reference<?> val) {
500 throw new UnsupportedOperationException();
501 }
502
503 boolean casPrevDll(Reference<?> cmp, Reference<?> val) {
504 throw new UnsupportedOperationException();
505 }
506
507 Reference<?> getNextDll() {
508 throw new UnsupportedOperationException();
509 }
510
511 void lazySetNextDll(Reference<?> val) {
512 throw new UnsupportedOperationException();
513 }
514
515 boolean casNextDll(Reference<?> cmp, Reference<?> val) {
516 throw new UnsupportedOperationException();
517 }
518
519 // Unsafe machinery
520
521 @SuppressWarnings("unchecked")
522 T getReferentVolatile() {
523 return (T) UNSAFE.getObjectVolatile(this, referentOffset);
524 }
525
526 boolean casReferent(T cmp, T val) {
527 return UNSAFE.compareAndSwapObject(this, referentOffset, cmp, val);
528 }
529
530 void lazySetQueue(ReferenceQueue<? super T> val) {
531 UNSAFE.putOrderedObject(this, queueOffset, val);
532 }
533
534 boolean casQueue(ReferenceQueue<?> cmp, ReferenceQueue<? super T> val) {
535 return UNSAFE.compareAndSwapObject(this, queueOffset, cmp, val);
536 }
537
538 private static final sun.misc.Unsafe UNSAFE;
539 private static final long referentOffset;
540 private static final long queueOffset;
541
542 static {
543 try {
544 UNSAFE = sun.misc.Unsafe.getUnsafe();
545 Class<Reference> rc = Reference.class;
546 referentOffset = UNSAFE.objectFieldOffset(rc.getDeclaredField("referent"));
547 queueOffset = UNSAFE.objectFieldOffset(rc.getDeclaredField("queue"));
548 } catch (Exception e) {
549 throw new Error(e);
550 }
551
552 // DLList(s) for selected uncleaned Cleaner(s)
553 int cpus = Runtime.getRuntime().availableProcessors();
554 // smallest power of two equal or greater than 2 * # of CPUs
555 int lists = (cpus <= 1) ? 2 : Integer.highestOneBit(cpus - 1) << 2;
556 uncleanedLists = new DLList[lists];
557 for (int i = 0; i < uncleanedLists.length; i++) {
558 uncleanedLists[i] = new DLList();
559 }
560
561 ThreadGroup tg = Thread.currentThread().getThreadGroup();
562 for (ThreadGroup tgn = tg;
563 tgn != null;
564 tg = tgn, tgn = tg.getParent());
565 Thread handler = new ReferenceHandler(tg, "Reference Handler");
566 /* If there were a special system-only priority greater than
567 * MAX_PRIORITY, it would be used here
568 */
569 handler.setPriority(Thread.MAX_PRIORITY);
570 handler.setDaemon(true);
571 handler.start();
572
573 // provide access in SharedSecrets
574 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
575 @Override
576 public boolean tryHandlePendingReference() {
577 return tryHandlePending();
578 }
579 });
580 }
581 }