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 Reference object needs
80 * special treatment. If the referent is non-null and not known to be
81 * live, then it may need to be discovered for possible later
82 * notification. But if the discovered field is non-null, then either (1)
83 * it has already been discovered, or (2) it is in the pending list.
84 *
85 * FinalReference differs from other references, because a FinalReference
86 * is not cleared when notified. The referent being null or not cannot be
87 * used to distinguish between the active state and pending or inactive
88 * states. However, FinalReferences do not support enqueue(). Instead,
89 * the next field of a FinalReference object is set to the object when it
90 * is added to the pending list, and the use of this as the value of next
91 * in the enqueued and dequeued states maintains the non-active state. An
92 * additional check that the next field is null is required to determine
93 * that a FinalReference object is active.
94 *
95 * Initial states:
96 * active/registered
97 * active/unregistered [1]
98 *
99 * Transitions:
100 * active/registered -> pending/registered - GC
101 * -> inactive/registered - clear
102 * -> inactive/enqueued - enqueue [2]
103 * pending/registered -> pending/enqueued - enqueue [2]
104 * -> inactive/enqueued - pending list processing
105 * pending/enqueued -> inactive/enqueued - pending list processing
106 * -> pending/dequeued - poll/remove
107 * pending/dequeued -> inactive/dequeued - pending list processing
108 * inactive/registered -> inactive/enqueued - enqueue [2]
109 * inactive/enqueued -> inactive/dequeued - poll/remove
110 *
111 * active/unregistered -> pending/unregistered - GC
112 * -> inactive/unregistered - GC, clear, enqueue
113 * pending/unregistered -> inactive/unregistered - pending list processing
114 *
115 * Terminal states:
116 * inactive/dequeued
117 * inactive/unregistered
118 *
119 * Unreachable states (because enqueue also clears):
120 * active/enqeued
121 * active/dequeued
122 *
123 * [1] Unregistered is not permitted for FinalReferences.
124 *
125 * [2] These transitions are not possible for FinalReferences, making
126 * pending/enqueued and pending/dequeued unreachable, and
127 * inactive/registered terminal.
128 */
129
130 private T referent; /* Treated specially by GC */
131
132 volatile ReferenceQueue<? super T> queue;
133
134 /* The link in a ReferenceQueue's list of Reference objects.
135 *
136 * When registered: null
137 * enqueued: next element in queue (or this if last)
138 * dequeued: this (marking FinalReferences as inactive)
139 * unregistered: null
140 */
141 @SuppressWarnings("rawtypes")
142 volatile Reference next;
143
144 /* Used by the garbage collector to accumulate Reference objects that need
145 * to be revisited in order to decide whether they should be notified.
146 * Also used as the link in the pending-Reference list. The discovered
147 * field and the next field are distinct to allow the enqueue() method to
148 * be applied to a Reference object while it is either in the
149 * pending-Reference list or in the garbage collector's discovered set.
150 *
151 * When active: null or next element in a discovered reference list
152 * maintained by the GC (or this if last)
153 * pending: next element in the pending-Reference list (null if last)
154 * inactive: null
155 */
156 private transient Reference<T> discovered;
157
158
159 /* High-priority thread to enqueue pending References
160 */
161 private static class ReferenceHandler extends Thread {
162
163 private static void ensureClassInitialized(Class<?> clazz) {
164 try {
165 Class.forName(clazz.getName(), true, clazz.getClassLoader());
166 } catch (ClassNotFoundException e) {
167 throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
168 }
169 }
170
171 static {
172 // pre-load and initialize Cleaner class so that we don't
173 // get into trouble later in the run loop if there's
174 // memory shortage while loading/initializing it lazily.
175 ensureClassInitialized(Cleaner.class);
176 }
177
178 ReferenceHandler(ThreadGroup g, String name) {
179 super(g, null, name, 0, false);
180 }
181
182 public void run() {
183 while (true) {
184 processPendingReferences();
185 }
186 }
187 }
188
189 /*
190 * Atomically get and clear (set to null) the VM's pending list.
191 */
192 private static native Reference<Object> getAndClearReferencePendingList();
193
194 /*
195 * Test whether the VM's pending list contains any entries.
196 */
197 private static native boolean hasReferencePendingList();
198
199 /*
200 * Wait until the VM's pending list may be non-null.
201 */
202 private static native void waitForReferencePendingList();
203
204 private static final Object processPendingLock = new Object();
205 private static boolean processPendingActive = false;
206
207 private static void processPendingReferences() {
208 // Only the singleton reference processing thread calls
209 // waitForReferencePendingList() and getAndClearReferencePendingList().
210 // These are separate operations to avoid a race with other threads
211 // that are calling waitForReferenceProcessing().
212 waitForReferencePendingList();
213 Reference<Object> pendingList;
214 synchronized (processPendingLock) {
215 pendingList = getAndClearReferencePendingList();
216 processPendingActive = true;
217 }
218 while (pendingList != null) {
219 Reference<Object> ref = pendingList;
220 pendingList = ref.discovered;
221 ref.discovered = null;
222
223 if (ref instanceof Cleaner) {
224 ((Cleaner)ref).clean();
225 // Notify any waiters that progress has been made.
226 // This improves latency for nio.Bits waiters, which
227 // are the only important ones.
228 synchronized (processPendingLock) {
229 processPendingLock.notifyAll();
230 }
231 } else {
232 ReferenceQueue<? super Object> q = ref.queue;
233 if (q != ReferenceQueue.NULL) q.enqueue(ref);
234 }
235 }
236 // Notify any waiters of completion of current round.
237 synchronized (processPendingLock) {
238 processPendingActive = false;
239 processPendingLock.notifyAll();
240 }
241 }
242
243 // Wait for progress in reference processing.
244 //
245 // Returns true after waiting (for notification from the reference
246 // processing thread) if either (1) the VM has any pending
247 // references, or (2) the reference processing thread is
248 // processing references. Otherwise, returns false immediately.
249 private static boolean waitForReferenceProcessing()
250 throws InterruptedException
251 {
252 synchronized (processPendingLock) {
253 if (processPendingActive || hasReferencePendingList()) {
254 // Wait for progress, not necessarily completion.
255 processPendingLock.wait();
256 return true;
257 } else {
258 return false;
259 }
260 }
261 }
262
263 static {
264 ThreadGroup tg = Thread.currentThread().getThreadGroup();
265 for (ThreadGroup tgn = tg;
266 tgn != null;
267 tg = tgn, tgn = tg.getParent());
268 Thread handler = new ReferenceHandler(tg, "Reference Handler");
269 /* If there were a special system-only priority greater than
270 * MAX_PRIORITY, it would be used here
271 */
272 handler.setPriority(Thread.MAX_PRIORITY);
273 handler.setDaemon(true);
274 handler.start();
275
276 // provide access in SharedSecrets
277 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
278 @Override
279 public boolean waitForReferenceProcessing()
280 throws InterruptedException
281 {
282 return Reference.waitForReferenceProcessing();
283 }
284
285 @Override
286 public void runFinalization() {
287 Finalizer.runFinalization();
288 }
289 });
290 }
291
292 /* -- Referent accessor and setters -- */
293
294 /**
295 * Returns this reference object's referent. If this reference object has
296 * been cleared, either by the program or by the garbage collector, then
297 * this method returns <code>null</code>.
298 *
299 * @return The object to which this reference refers, or
300 * <code>null</code> if this reference object has been cleared
301 */
302 @HotSpotIntrinsicCandidate
303 public T get() {
304 return this.referent;
305 }
306
307 /**
308 * Clears this reference object. Invoking this method will not cause this
309 * object to be enqueued.
310 *
311 * <p> This method is invoked only by Java code; when the garbage collector
312 * clears references it does so directly, without invoking this method.
313 */
314 public void clear() {
315 this.referent = null;
316 }
317
318 /* -- Queue operations -- */
319
320 /**
321 * Tells whether or not this reference object has been enqueued, either by
322 * the program or by the garbage collector. If this reference object was
323 * not registered with a queue when it was created, then this method will
324 * always return <code>false</code>.
325 *
326 * @return <code>true</code> if and only if this reference object has
327 * been enqueued
328 */
329 public boolean isEnqueued() {
330 return (this.queue == ReferenceQueue.ENQUEUED);
331 }
332
333 /**
334 * Clears this reference object and adds it to the queue with which
335 * it is registered, if any.
336 *
337 * <p> This method is invoked only by Java code; when the garbage collector
338 * enqueues references it does so directly, without invoking this method.
339 *
340 * @return <code>true</code> if this reference object was successfully
341 * enqueued; <code>false</code> if it was already enqueued or if
342 * it was not registered with a queue when it was created
343 */
344 public boolean enqueue() {
345 this.referent = null;
346 return this.queue.enqueue(this);
347 }
348
349 /**
350 * Throws {@link CloneNotSupportedException}. A {@code Reference} cannot be
351 * meaningfully cloned. Construct a new {@code Reference} instead.
352 *
353 * @returns never returns normally
354 * @throws CloneNotSupportedException always
355 *
356 * @since 11
357 */
358 @Override
359 protected Object clone() throws CloneNotSupportedException {
360 throw new CloneNotSupportedException();
361 }
362
363 /* -- Constructors -- */
364
365 Reference(T referent) {
366 this(referent, null);
367 }
368
369 Reference(T referent, ReferenceQueue<? super T> queue) {
370 this.referent = referent;
371 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
372 }
373
374 /**
375 * Ensures that the object referenced by the given reference remains
376 * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
377 * regardless of any prior actions of the program that might otherwise cause
378 * the object to become unreachable; thus, the referenced object is not
379 * reclaimable by garbage collection at least until after the invocation of
380 * this method. Invocation of this method does not itself initiate garbage
381 * collection or finalization.
382 *
383 * <p> This method establishes an ordering for
384 * <a href="package-summary.html#reachability"><em>strong reachability</em></a>
385 * with respect to garbage collection. It controls relations that are
386 * otherwise only implicit in a program -- the reachability conditions
387 * triggering garbage collection. This method is designed for use in
388 * uncommon situations of premature finalization where using
389 * {@code synchronized} blocks or methods, or using other synchronization
390 * facilities are not possible or do not provide the desired control. This
391 * method is applicable only when reclamation may have visible effects,
392 * which is possible for objects with finalizers (See
393 * <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6">
394 * Section 12.6 17 of <cite>The Java™ Language Specification</cite></a>)
395 * that are implemented in ways that rely on ordering control for correctness.
396 *
397 * @apiNote
398 * Finalization may occur whenever the virtual machine detects that no
399 * reference to an object will ever be stored in the heap: The garbage
400 * collector may reclaim an object even if the fields of that object are
401 * still in use, so long as the object has otherwise become unreachable.
402 * This may have surprising and undesirable effects in cases such as the
403 * following example in which the bookkeeping associated with a class is
404 * managed through array indices. Here, method {@code action} uses a
405 * {@code reachabilityFence} to ensure that the {@code Resource} object is
406 * not reclaimed before bookkeeping on an associated
407 * {@code ExternalResource} has been performed; in particular here, to
408 * ensure that the array slot holding the {@code ExternalResource} is not
409 * nulled out in method {@link Object#finalize}, which may otherwise run
410 * concurrently.
411 *
412 * <pre> {@code
413 * class Resource {
414 * private static ExternalResource[] externalResourceArray = ...
415 *
416 * int myIndex;
417 * Resource(...) {
418 * myIndex = ...
419 * externalResourceArray[myIndex] = ...;
420 * ...
421 * }
422 * protected void finalize() {
423 * externalResourceArray[myIndex] = null;
424 * ...
425 * }
426 * public void action() {
427 * try {
428 * // ...
429 * int i = myIndex;
430 * Resource.update(externalResourceArray[i]);
431 * } finally {
432 * Reference.reachabilityFence(this);
433 * }
434 * }
435 * private static void update(ExternalResource ext) {
436 * ext.status = ...;
437 * }
438 * }}</pre>
439 *
440 * Here, the invocation of {@code reachabilityFence} is nonintuitively
441 * placed <em>after</em> the call to {@code update}, to ensure that the
442 * array slot is not nulled out by {@link Object#finalize} before the
443 * update, even if the call to {@code action} was the last use of this
444 * object. This might be the case if, for example a usage in a user program
445 * had the form {@code new Resource().action();} which retains no other
446 * reference to this {@code Resource}. While probably overkill here,
447 * {@code reachabilityFence} is placed in a {@code finally} block to ensure
448 * that it is invoked across all paths in the method. In a method with more
449 * complex control paths, you might need further precautions to ensure that
450 * {@code reachabilityFence} is encountered along all of them.
451 *
452 * <p> It is sometimes possible to better encapsulate use of
453 * {@code reachabilityFence}. Continuing the above example, if it were
454 * acceptable for the call to method {@code update} to proceed even if the
455 * finalizer had already executed (nulling out slot), then you could
456 * localize use of {@code reachabilityFence}:
457 *
458 * <pre> {@code
459 * public void action2() {
460 * // ...
461 * Resource.update(getExternalResource());
462 * }
463 * private ExternalResource getExternalResource() {
464 * ExternalResource ext = externalResourceArray[myIndex];
465 * Reference.reachabilityFence(this);
466 * return ext;
467 * }}</pre>
468 *
469 * <p> Method {@code reachabilityFence} is not required in constructions
470 * that themselves ensure reachability. For example, because objects that
471 * are locked cannot, in general, be reclaimed, it would suffice if all
472 * accesses of the object, in all methods of class {@code Resource}
473 * (including {@code finalize}) were enclosed in {@code synchronized (this)}
474 * blocks. (Further, such blocks must not include infinite loops, or
475 * themselves be unreachable, which fall into the corner case exceptions to
476 * the "in general" disclaimer.) However, method {@code reachabilityFence}
477 * remains a better option in cases where this approach is not as efficient,
478 * desirable, or possible; for example because it would encounter deadlock.
479 *
480 * @param ref the reference. If {@code null}, this method has no effect.
481 * @since 9
482 */
483 @ForceInline
484 public static void reachabilityFence(Object ref) {
485 // Does nothing. This method is annotated with @ForceInline to eliminate
486 // most of the overhead that using @DontInline would cause with the
487 // HotSpot JVM, when this fence is used in a wide variety of situations.
488 // HotSpot JVM retains the ref and does not GC it before a call to
489 // this method, because the JIT-compilers do not have GC-only safepoints.
490 }
491 }