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