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