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 @SuppressWarnings("rawtypes")
103 Reference next;
104
105 /* When active: next element in a discovered reference list maintained by GC (or this if last)
106 * pending: next element in the pending list (or null if last)
107 * otherwise: NULL
108 */
109 transient private Reference<T> discovered; /* used by VM */
110
111
112 /* Object used to synchronize with the garbage collector. The collector
113 * must acquire this lock at the beginning of each collection cycle. It is
114 * therefore critical that any code holding this lock complete as quickly
115 * as possible, allocate no new objects, and avoid calling user code.
116 */
117 static private class Lock { }
118 private static Lock lock = new Lock();
119
120
121 /* List of References waiting to be enqueued. The collector adds
122 * References to this list, while the Reference-handler thread removes
123 * them. This list is protected by the above lock object. The
124 * list uses the discovered field to link its elements.
125 */
126 private static Reference<Object> pending = null;
127
128 /* High-priority thread to enqueue pending References
129 */
130 private static class ReferenceHandler extends ManagedLocalsThread {
131
132 private static void ensureClassInitialized(Class<?> clazz) {
133 try {
134 Class.forName(clazz.getName(), true, clazz.getClassLoader());
135 } catch (ClassNotFoundException e) {
136 throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
137 }
138 }
139
140 static {
141 // pre-load and initialize InterruptedException and Cleaner classes
142 // so that we don't get into trouble later in the run loop if there's
143 // memory shortage while loading/initializing them lazily.
144 ensureClassInitialized(InterruptedException.class);
145 ensureClassInitialized(Cleaner.class);
146 }
147
148 ReferenceHandler(ThreadGroup g, String name) {
149 super(g, name);
150 }
151
152 public void run() {
153 while (true) {
154 tryHandlePending(true);
155 }
156 }
157 }
158
159 /**
160 * Try handle pending {@link Reference} if there is one.<p>
161 * Return {@code true} as a hint that there might be another
162 * {@link Reference} pending or {@code false} when there are no more pending
163 * {@link Reference}s at the moment and the program can do some other
164 * useful work instead of looping.
165 *
166 * @param waitForNotify if {@code true} and there was no pending
167 * {@link Reference}, wait until notified from VM
168 * or interrupted; if {@code false}, return immediately
169 * when there is no pending {@link Reference}.
170 * @return {@code true} if there was a {@link Reference} pending and it
171 * was processed, or we waited for notification and either got it
172 * or thread was interrupted before being notified;
173 * {@code false} otherwise.
174 */
175 static boolean tryHandlePending(boolean waitForNotify) {
176 Reference<Object> r;
177 Cleaner c;
178 try {
179 synchronized (lock) {
180 if (pending != null) {
181 r = pending;
182 // 'instanceof' might throw OutOfMemoryError sometimes
183 // so do this before un-linking 'r' from the 'pending' chain...
184 c = r instanceof Cleaner ? (Cleaner) r : null;
185 // unlink 'r' from 'pending' chain
186 pending = r.discovered;
187 r.discovered = null;
188 } else {
189 // The waiting on the lock may cause an OutOfMemoryError
190 // because it may try to allocate exception objects.
191 if (waitForNotify) {
192 lock.wait();
193 }
194 // retry if waited
195 return waitForNotify;
196 }
197 }
198 } catch (OutOfMemoryError x) {
199 // Give other threads CPU time so they hopefully drop some live references
200 // and GC reclaims some space.
201 // Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above
202 // persistently throws OOME for some time...
203 Thread.yield();
204 // retry
205 return true;
206 } catch (InterruptedException x) {
207 // retry
208 return true;
209 }
210
211 // Fast path for cleaners
212 if (c != null) {
213 c.clean();
214 return true;
215 }
216
217 ReferenceQueue<? super Object> q = r.queue;
218 if (q != ReferenceQueue.NULL) q.enqueue(r);
219 return true;
220 }
221
222 static {
223 ThreadGroup tg = Thread.currentThread().getThreadGroup();
224 for (ThreadGroup tgn = tg;
225 tgn != null;
226 tg = tgn, tgn = tg.getParent());
227 Thread handler = new ReferenceHandler(tg, "Reference Handler");
228 /* If there were a special system-only priority greater than
229 * MAX_PRIORITY, it would be used here
230 */
231 handler.setPriority(Thread.MAX_PRIORITY);
232 handler.setDaemon(true);
233 handler.start();
234
235 // provide access in SharedSecrets
236 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
237 @Override
238 public boolean tryHandlePendingReference() {
239 return tryHandlePending(false);
240 }
241 });
242 }
243
244 /* -- Referent accessor and setters -- */
245
246 /**
247 * Returns this reference object's referent. If this reference object has
248 * been cleared, either by the program or by the garbage collector, then
249 * this method returns <code>null</code>.
250 *
251 * @return The object to which this reference refers, or
252 * <code>null</code> if this reference object has been cleared
253 */
254 public T get() {
255 return this.referent;
256 }
257
258 /**
259 * Clears this reference object. Invoking this method will not cause this
260 * object to be enqueued.
261 *
292 * @return <code>true</code> if this reference object was successfully
293 * enqueued; <code>false</code> if it was already enqueued or if
294 * it was not registered with a queue when it was created
295 */
296 public boolean enqueue() {
297 return this.queue.enqueue(this);
298 }
299
300
301 /* -- Constructors -- */
302
303 Reference(T referent) {
304 this(referent, null);
305 }
306
307 Reference(T referent, ReferenceQueue<? super T> queue) {
308 this.referent = referent;
309 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
310 }
311
312 }
|
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 finaliz(ato|e)rs and Finalizators
286 Reference<?> finalizrs = null;
287 int finalizrsCount = 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 ||
299 r instanceof Finalizator) { // Submit task(s) for finaliz(ato|e)rs
300 // hook onto the finalizers chain
301 r.next = (finalizrs == null) ? r : finalizrs;
302 finalizrs = r;
303 if (++finalizrsCount >= FINALIZE_CHUNK_SIZE) {
304 // when chunk of finaliz(ato|e)rs is full, submit a task
305 new ReferenceHandling.FinalizrHandler(finalizrs).submit();
306 finalizrs = null;
307 finalizrsCount = 0;
308 }
309 } else { // Enqueue all other references
310 // take 'r' off the chain
311 r.next = r;
312 ReferenceQueue<?> q = r.queue;
313 if (q != ReferenceQueue.NULL && q.markEnqueued(r)) { // markEnqueued is atomic
314 if (referenceQueue == null || referenceQueue == q) {
315 // no queue or same queue -> hook onto the references[Head|Tail] chain
316 if (referencesHead == null) {
317 // assert referencesTail == null && referenceQueue == null &&
318 // referencesCount == 0 && r.next == r;
319 referenceQueue = q;
320 referencesHead = referencesTail = r;
321 } else {
322 // assert referencesTail != null && referenceQueue == q &&
323 // referencesCount > 0;
324 r.next = referencesHead;
325 referencesHead = r;
326 }
327 if (++referencesCount >= ENQUEUE_CHUNK_SIZE) {
328 // when a chunk of references is full, add them to queue
329 referenceQueue.addChunk(referencesHead, referencesTail);
330 referencesHead = referencesTail = null;
331 referenceQueue = null;
332 referencesCount = 0;
333 }
334 } else {
335 // when a different queue is encountered,
336 // add collected chunk to it's queue and start collecting
337 // into new queue...
338 // assert referenceQueue != null && referenceQueue != q &&
339 // referencesHead != null && referencesTail != null &&
340 // referencesCount > 0 && r.next == r;
341 referenceQueue.addChunk(referencesHead, referencesTail);
342 referenceQueue = q;
343 referencesHead = referencesTail = r;
344 referencesCount = 1;
345 }
346 }
347 }
348 if (rn == r) { // last in chain
349 break;
350 }
351 }
352 // any finalizers left?
353 if (finalizrs != null) {
354 new ReferenceHandling.FinalizrHandler(finalizrs).submit();
355 finalizrs = null;
356 finalizrsCount = 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 /* -- Referent accessor and setters -- */
369
370 /**
371 * Returns this reference object's referent. If this reference object has
372 * been cleared, either by the program or by the garbage collector, then
373 * this method returns <code>null</code>.
374 *
375 * @return The object to which this reference refers, or
376 * <code>null</code> if this reference object has been cleared
377 */
378 public T get() {
379 return this.referent;
380 }
381
382 /**
383 * Clears this reference object. Invoking this method will not cause this
384 * object to be enqueued.
385 *
416 * @return <code>true</code> if this reference object was successfully
417 * enqueued; <code>false</code> if it was already enqueued or if
418 * it was not registered with a queue when it was created
419 */
420 public boolean enqueue() {
421 return this.queue.enqueue(this);
422 }
423
424
425 /* -- Constructors -- */
426
427 Reference(T referent) {
428 this(referent, null);
429 }
430
431 Reference(T referent, ReferenceQueue<? super T> queue) {
432 this.referent = referent;
433 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
434 }
435
436 // Unsafe machinery
437
438 @SuppressWarnings("unchecked")
439 T getReferentVolatile() {
440 return (T) UNSAFE.getObjectVolatile(this, referentOffset);
441 }
442
443 boolean casReferent(T cmp, T val) {
444 return UNSAFE.compareAndSwapObject(this, referentOffset, cmp, val);
445 }
446
447 void lazySetQueue(ReferenceQueue<? super T> val) {
448 UNSAFE.putOrderedObject(this, queueOffset, val);
449 }
450
451 boolean casQueue(ReferenceQueue<?> cmp, ReferenceQueue<? super T> val) {
452 return UNSAFE.compareAndSwapObject(this, queueOffset, cmp, val);
453 }
454
455 private static final sun.misc.Unsafe UNSAFE;
456 private static final long referentOffset;
457 private static final long queueOffset;
458
459 static {
460 try {
461 UNSAFE = sun.misc.Unsafe.getUnsafe();
462 Class<Reference> rc = Reference.class;
463 referentOffset = UNSAFE.objectFieldOffset(rc.getDeclaredField("referent"));
464 queueOffset = UNSAFE.objectFieldOffset(rc.getDeclaredField("queue"));
465 } catch (Exception e) {
466 throw new Error(e);
467 }
468
469 ThreadGroup tg = Thread.currentThread().getThreadGroup();
470 for (ThreadGroup tgn = tg;
471 tgn != null;
472 tg = tgn, tgn = tg.getParent());
473 Thread handler = new ReferenceHandler(tg, "Reference Handler");
474 /* If there were a special system-only priority greater than
475 * MAX_PRIORITY, it would be used here
476 */
477 handler.setPriority(Thread.MAX_PRIORITY);
478 handler.setDaemon(true);
479 handler.start();
480
481 // provide access in SharedSecrets
482 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
483 @Override
484 public boolean tryHandlePendingReference() {
485 return tryHandlePending();
486 }
487 });
488 }
489 }
|