1 /*
2 * Copyright (c) 1997, 2015, 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.HotSpotIntrinsicCandidate;
29 import jdk.internal.misc.JavaLangRefAccess;
30 import jdk.internal.misc.SharedSecrets;
31 import jdk.internal.vm.annotation.DontInline;
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 @SuppressWarnings("rawtypes")
103 volatile 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 private transient Reference<?> 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 private static class Lock { }
118 private static final 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<?> pending;
127
128 /* Discovery phase counter, guarder by above lock.
129 */
130 private static int discoveryPhase;
131
132 /* Enqueue phase, guarded by enqueuePhaseLock object.
133 */
134 private static int enqueuePhase;
135 private static final Object enqueuePhaseLock = new Object();
136
137 /* High-priority thread to enqueue pending References
138 */
139 private static class ReferenceHandler extends Thread {
140
141 private static void ensureClassInitialized(Class<?> clazz) {
142 try {
143 Class.forName(clazz.getName(), true, clazz.getClassLoader());
144 } catch (ClassNotFoundException e) {
145 throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
146 }
147 }
148
149 static {
150 // pre-load and initialize InterruptedException class
151 // so that we don't get into trouble later in the run loop if there's
152 // memory shortage while loading/initializing it lazily.
153 ensureClassInitialized(InterruptedException.class);
154 }
155
156 ReferenceHandler(ThreadGroup g, String name) {
157 super(g, null, name, 0, false);
158 }
159
160 public void run() {
161 int[] discoveryPhase = new int[1];
162 while (true) {
163 Reference<?> p = getPendingReferences(discoveryPhase);
164 enqueuePendingReferences(p, discoveryPhase[0]);
165 }
166 }
167 }
168
169 /**
170 * Blocks until GC discovers some pending references, sets the 0-th element
171 * of given {@code discoveryPhaseHolder} to the phase in which they were
172 * discovered and returns the head of the list of discovered references.
173 *
174 * @param discoveryPhaseHolder a 1-element array to hold the returned
175 * discovery phase.
176 * @return the head of a list of pending references linked via
177 * {@link #discovered} field with {@code null} marking the end of list.
178 */
179 static Reference<?> getPendingReferences(int[] discoveryPhaseHolder) {
180 Reference<?> p;
181 synchronized (lock) {
182 while ((p = pending) == null) {
183 try {
184 lock.wait();
185 } catch (OutOfMemoryError x) {
186 // The waiting on the lock may cause an OutOfMemoryError
187 // because it may try to allocate InterruptedException object.
188 // Give other threads CPU time so they hopefully drop some live
189 // references and GC reclaims some space.
190 Thread.yield();
191 } catch (InterruptedException x) {
192 // ignore interrupts
193 }
194 }
195 pending = null;
196 // increment discoveryPhase counter and return it in a holder array
197 discoveryPhaseHolder[0] = ++discoveryPhase;
198 }
199 return p;
200 }
201
202 /**
203 * @return current finished discovery phase.
204 */
205 static int getDiscoveryPhase() {
206 synchronized (lock) {
207 return (pending == null)
208 ? discoveryPhase // already incremented
209 : discoveryPhase + 1; // not yet incremented
210 }
211 }
212
213 /**
214 * Enqueue a list of pending {@link Reference}s linked via {@link #discovered}
215 * field with {@code null} marking the end of list.
216 * <p>
217 * The {@link #enqueuePhase} is set to given {@code discoveryPhase}
218 * after all references from the list have been enqueued and any waiters on
219 * {@link #enqueuePhaseLock} are notified.
220 *
221 * @param p a list of pending references linked via {@link #discovered}
222 * field with {@code null} marking the end of list.
223 * @param discoveryPhase the phase in which given references were discovered.
224 */
225 static void enqueuePendingReferences(Reference<?> p, int discoveryPhase) {
226 try {
227 // distribute unhooked pending references to their respective queues
228 while (p != null) {
229 Reference<?> r = p;
230 p = r.discovered;
231 r.discovered = null;
232 @SuppressWarnings("unchecked")
233 ReferenceQueue<Object> q = (ReferenceQueue) r.queue;
234 if (q != ReferenceQueue.NULL) q.enqueue(r);
235 }
236 } finally {
237 // mark the enqueueing of references discovered in given
238 // discovery phase is finished and notify waiters.
239 synchronized (enqueuePhaseLock) {
240 enqueuePhase = discoveryPhase;
241 enqueuePhaseLock.notifyAll();
242 }
243 }
244 }
245
246 /**
247 * Triggers discovery of new Reference(s) and returns the phase sequence number
248 * in which they were discovered or previous phase sequence number if no new
249 * Reference(s) were discovered.
250 */
251 static int discoverReferences() {
252 // trigger discovery of new Reference(s)
253 System.gc();
254 // obtain the phase in which they were discovered (if any)
255 return getDiscoveryPhase();
256 }
257
258 /**
259 * Blocks until all Reference(s) that were discovered in given
260 * {@code discoveryPhase} (as returned by {@link #discoverReferences()})
261 * have been enqueued.
262 *
263 * @param discoveryPhase the discovery phase sequence number.
264 * @throws InterruptedException if interrupted while waiting.
265 */
266 static void awaitReferencesEnqueued(int discoveryPhase) throws InterruptedException {
267 // await for them to be enqueued
268 synchronized (enqueuePhaseLock) {
269 while (enqueuePhase - discoveryPhase < 0) {
270 enqueuePhaseLock.wait();
271 }
272 }
273 }
274
275 static {
276 ThreadGroup tg = Thread.currentThread().getThreadGroup();
277 for (ThreadGroup tgn = tg;
278 tgn != null;
279 tg = tgn, tgn = tg.getParent());
280 Thread handler = new ReferenceHandler(tg, "Reference Handler");
281 /* If there were a special system-only priority greater than
282 * MAX_PRIORITY, it would be used here
283 */
284 handler.setPriority(Thread.MAX_PRIORITY);
285 handler.setDaemon(true);
286 handler.start();
287
288 // provide access in SharedSecrets
289 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
290 @Override
291 public int discoverReferences() {
292 return Reference.discoverReferences();
293 }
294
295 @Override
296 public void awaitReferencesEnqueued(
297 int discoveryPhase) throws InterruptedException {
298 Reference.awaitReferencesEnqueued(discoveryPhase);
299 }
300 });
301 }
302
303 /* -- Referent accessor and setters -- */
304
305 /**
306 * Returns this reference object's referent. If this reference object has
307 * been cleared, either by the program or by the garbage collector, then
308 * this method returns <code>null</code>.
309 *
310 * @return The object to which this reference refers, or
311 * <code>null</code> if this reference object has been cleared
312 */
313 @HotSpotIntrinsicCandidate
314 public T get() {
315 return this.referent;
316 }
317
318 /**
319 * Clears this reference object. Invoking this method will not cause this
320 * object to be enqueued.
321 *
322 * <p> This method is invoked only by Java code; when the garbage collector
323 * clears references it does so directly, without invoking this method.
324 */
325 public void clear() {
326 this.referent = null;
327 }
328
329
330 /* -- Queue operations -- */
331
332 /**
333 * Tells whether or not this reference object has been enqueued, either by
334 * the program or by the garbage collector. If this reference object was
335 * not registered with a queue when it was created, then this method will
336 * always return <code>false</code>.
337 *
338 * @return <code>true</code> if and only if this reference object has
339 * been enqueued
340 */
341 public boolean isEnqueued() {
342 return (this.queue == ReferenceQueue.ENQUEUED);
343 }
344
345 /**
346 * Adds this reference object to the queue with which it is registered,
347 * if any.
348 *
349 * <p> This method is invoked only by Java code; when the garbage collector
350 * enqueues references it does so directly, without invoking this method.
351 *
352 * @return <code>true</code> if this reference object was successfully
353 * enqueued; <code>false</code> if it was already enqueued or if
354 * it was not registered with a queue when it was created
355 */
356 public boolean enqueue() {
357 return this.queue.enqueue(this);
358 }
359
360
361 /* -- Constructors -- */
362
363 Reference(T referent) {
364 this(referent, null);
365 }
366
367 Reference(T referent, ReferenceQueue<? super T> queue) {
368 this.referent = referent;
369 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
370 }
371
372 /**
373 * Ensures that the object referenced by the given reference remains
374 * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
375 * regardless of any prior actions of the program that might otherwise cause
376 * the object to become unreachable; thus, the referenced object is not
377 * reclaimable by garbage collection at least until after the invocation of
378 * this method. Invocation of this method does not itself initiate garbage
379 * collection or finalization.
380 *
381 * <p> This method establishes an ordering for
382 * <a href="package-summary.html#reachability"><em>strong reachability</em></a>
383 * with respect to garbage collection. It controls relations that are
384 * otherwise only implicit in a program -- the reachability conditions
385 * triggering garbage collection. This method is designed for use in
386 * uncommon situations of premature finalization where using
387 * {@code synchronized} blocks or methods, or using other synchronization
388 * facilities are not possible or do not provide the desired control. This
389 * method is applicable only when reclamation may have visible effects,
390 * which is possible for objects with finalizers (See
391 * <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6">
392 * Section 12.6 17 of <cite>The Java™ Language Specification</cite></a>)
393 * that are implemented in ways that rely on ordering control for correctness.
394 *
395 * @apiNote
396 * Finalization may occur whenever the virtual machine detects that no
397 * reference to an object will ever be stored in the heap: The garbage
398 * collector may reclaim an object even if the fields of that object are
399 * still in use, so long as the object has otherwise become unreachable.
400 * This may have surprising and undesirable effects in cases such as the
401 * following example in which the bookkeeping associated with a class is
402 * managed through array indices. Here, method {@code action} uses a
403 * {@code reachabilityFence} to ensure that the {@code Resource} object is
404 * not reclaimed before bookkeeping on an associated
405 * {@code ExternalResource} has been performed; in particular here, to
406 * ensure that the array slot holding the {@code ExternalResource} is not
407 * nulled out in method {@link Object#finalize}, which may otherwise run
408 * concurrently.
409 *
410 * <pre> {@code
411 * class Resource {
412 * private static ExternalResource[] externalResourceArray = ...
413 *
414 * int myIndex;
415 * Resource(...) {
416 * myIndex = ...
417 * externalResourceArray[myIndex] = ...;
418 * ...
419 * }
420 * protected void finalize() {
421 * externalResourceArray[myIndex] = null;
422 * ...
423 * }
424 * public void action() {
425 * try {
426 * // ...
427 * int i = myIndex;
428 * Resource.update(externalResourceArray[i]);
429 * } finally {
430 * Reference.reachabilityFence(this);
431 * }
432 * }
433 * private static void update(ExternalResource ext) {
434 * ext.status = ...;
435 * }
436 * }}</pre>
437 *
438 * Here, the invocation of {@code reachabilityFence} is nonintuitively
439 * placed <em>after</em> the call to {@code update}, to ensure that the
440 * array slot is not nulled out by {@link Object#finalize} before the
441 * update, even if the call to {@code action} was the last use of this
442 * object. This might be the case if, for example a usage in a user program
443 * had the form {@code new Resource().action();} which retains no other
444 * reference to this {@code Resource}. While probably overkill here,
445 * {@code reachabilityFence} is placed in a {@code finally} block to ensure
446 * that it is invoked across all paths in the method. In a method with more
447 * complex control paths, you might need further precautions to ensure that
448 * {@code reachabilityFence} is encountered along all of them.
449 *
450 * <p> It is sometimes possible to better encapsulate use of
451 * {@code reachabilityFence}. Continuing the above example, if it were
452 * acceptable for the call to method {@code update} to proceed even if the
453 * finalizer had already executed (nulling out slot), then you could
454 * localize use of {@code reachabilityFence}:
455 *
456 * <pre> {@code
457 * public void action2() {
458 * // ...
459 * Resource.update(getExternalResource());
460 * }
461 * private ExternalResource getExternalResource() {
462 * ExternalResource ext = externalResourceArray[myIndex];
463 * Reference.reachabilityFence(this);
464 * return ext;
465 * }}</pre>
466 *
467 * <p> Method {@code reachabilityFence} is not required in constructions
468 * that themselves ensure reachability. For example, because objects that
469 * are locked cannot, in general, be reclaimed, it would suffice if all
470 * accesses of the object, in all methods of class {@code Resource}
471 * (including {@code finalize}) were enclosed in {@code synchronized (this)}
472 * blocks. (Further, such blocks must not include infinite loops, or
473 * themselves be unreachable, which fall into the corner case exceptions to
474 * the "in general" disclaimer.) However, method {@code reachabilityFence}
475 * remains a better option in cases where this approach is not as efficient,
476 * desirable, or possible; for example because it would encounter deadlock.
477 *
478 * @param ref the reference. If {@code null}, this method has no effect.
479 * @since 9
480 */
481 @DontInline
482 public static void reachabilityFence(Object ref) {
483 // Does nothing, because this method is annotated with @DontInline
484 // HotSpot needs to retain the ref and not GC it before a call to this
485 // method
486 }
487
488 }