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.vm.annotation.DontInline;
29 import jdk.internal.HotSpotIntrinsicCandidate;
30 import jdk.internal.misc.JavaLangRefAccess;
31 import jdk.internal.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 @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<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 private static 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 Thread {
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 class
142 // so that we don't get into trouble later in the run loop if there's
143 // memory shortage while loading/initializing it lazily.
144 ensureClassInitialized(InterruptedException.class);
145 }
146
147 ReferenceHandler(ThreadGroup g, String name) {
148 super(g, null, name, 0, false);
149 }
150
151 public void run() {
152 while (true) {
153 tryHandlePending(true);
154 }
155 }
156 }
157
158 /**
159 * Try handle pending {@link Reference} if there is one.<p>
160 * Return {@code true} as a hint that there might be another
161 * {@link Reference} pending or {@code false} when there are no more pending
162 * {@link Reference}s at the moment and the program can do some other
163 * useful work instead of looping.
164 *
165 * @param waitForNotify if {@code true} and there was no pending
166 * {@link Reference}, wait until notified from VM
167 * or interrupted; if {@code false}, return immediately
168 * when there is no pending {@link Reference}.
169 * @return {@code true} if there was a {@link Reference} pending and it
170 * was processed, or we waited for notification and either got it
171 * or thread was interrupted before being notified;
172 * {@code false} otherwise.
173 */
174 static boolean tryHandlePending(boolean waitForNotify) {
175 Reference<Object> r;
176 try {
177 synchronized (lock) {
178 r = pending;
179 if (r != null) {
180 // unlink 'r' from 'pending' chain
181 pending = r.discovered;
182 r.discovered = null;
183 } else {
184 // The waiting on the lock may cause an OutOfMemoryError
185 // because it may try to allocate exception objects.
186 if (waitForNotify) {
187 lock.wait();
188 }
189 // retry if waited
190 return waitForNotify;
191 }
192 }
193 } catch (OutOfMemoryError x) {
194 // Give other threads CPU time so they hopefully drop some live references
195 // and GC reclaims some space.
196 Thread.yield();
197 // retry
198 return true;
199 } catch (InterruptedException x) {
200 // retry
201 return true;
202 }
203
204 ReferenceQueue<? super Object> q = r.queue;
205 if (q != ReferenceQueue.NULL) q.enqueue(r);
206 return true;
207 }
208
209 static {
210 ThreadGroup tg = Thread.currentThread().getThreadGroup();
211 for (ThreadGroup tgn = tg;
212 tgn != null;
213 tg = tgn, tgn = tg.getParent());
214 Thread handler = new ReferenceHandler(tg, "Reference Handler");
215 /* If there were a special system-only priority greater than
216 * MAX_PRIORITY, it would be used here
217 */
218 handler.setPriority(Thread.MAX_PRIORITY);
219 handler.setDaemon(true);
220 handler.start();
221
222 // provide access in SharedSecrets
223 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
224 @Override
225 public boolean tryHandlePendingReference() {
226 return tryHandlePending(false);
227 }
228 });
229 }
230
231 /* -- Referent accessor and setters -- */
232
233 /**
234 * Returns this reference object's referent. If this reference object has
235 * been cleared, either by the program or by the garbage collector, then
236 * this method returns <code>null</code>.
237 *
238 * @return The object to which this reference refers, or
239 * <code>null</code> if this reference object has been cleared
240 */
241 @HotSpotIntrinsicCandidate
242 public T get() {
243 return this.referent;
244 }
245
246 /**
247 * Clears this reference object. Invoking this method will not cause this
248 * object to be enqueued.
249 *
250 * <p> This method is invoked only by Java code; when the garbage collector
251 * clears references it does so directly, without invoking this method.
252 */
253 public void clear() {
254 this.referent = null;
255 }
256
257
258 /* -- Queue operations -- */
259
260 /**
261 * Tells whether or not this reference object has been enqueued, either by
262 * the program or by the garbage collector. If this reference object was
263 * not registered with a queue when it was created, then this method will
264 * always return <code>false</code>.
265 *
266 * @return <code>true</code> if and only if this reference object has
267 * been enqueued
268 */
269 public boolean isEnqueued() {
270 return (this.queue == ReferenceQueue.ENQUEUED);
271 }
272
273 /**
274 * Adds this reference object to the queue with which it is registered,
275 * if any.
276 *
277 * <p> This method is invoked only by Java code; when the garbage collector
278 * enqueues references it does so directly, without invoking this method.
279 *
280 * @return <code>true</code> if this reference object was successfully
281 * enqueued; <code>false</code> if it was already enqueued or if
282 * it was not registered with a queue when it was created
283 */
284 public boolean enqueue() {
285 return this.queue.enqueue(this);
286 }
287
288
289 /* -- Constructors -- */
290
291 Reference(T referent) {
292 this(referent, null);
293 }
294
295 Reference(T referent, ReferenceQueue<? super T> queue) {
296 this.referent = referent;
297 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
298 }
299
300 /**
301 * Ensures that the object referenced by the given reference remains
302 * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
303 * regardless of any prior actions of the program that might otherwise cause
304 * the object to become unreachable; thus, the referenced object is not
305 * reclaimable by garbage collection at least until after the invocation of
306 * this method. Invocation of this method does not itself initiate garbage
307 * collection or finalization.
308 *
309 * <p> This method establishes an ordering for
310 * <a href="package-summary.html#reachability"><em>strong reachability</em></a>
311 * with respect to garbage collection. It controls relations that are
312 * otherwise only implicit in a program -- the reachability conditions
313 * triggering garbage collection. This method is designed for use in
314 * uncommon situations of premature finalization where using
315 * {@code synchronized} blocks or methods, or using other synchronization
316 * facilities are not possible or do not provide the desired control. This
317 * method is applicable only when reclamation may have visible effects,
318 * which is possible for objects with finalizers (See
319 * <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6">
320 * Section 12.6 17 of <cite>The Java™ Language Specification</cite></a>)
321 * that are implemented in ways that rely on ordering control for correctness.
322 *
323 * @apiNote
324 * Finalization may occur whenever the virtual machine detects that no
325 * reference to an object will ever be stored in the heap: The garbage
326 * collector may reclaim an object even if the fields of that object are
327 * still in use, so long as the object has otherwise become unreachable.
328 * This may have surprising and undesirable effects in cases such as the
329 * following example in which the bookkeeping associated with a class is
330 * managed through array indices. Here, method {@code action} uses a
331 * {@code reachabilityFence} to ensure that the {@code Resource} object is
332 * not reclaimed before bookkeeping on an associated
333 * {@code ExternalResource} has been performed; in particular here, to
334 * ensure that the array slot holding the {@code ExternalResource} is not
335 * nulled out in method {@link Object#finalize}, which may otherwise run
336 * concurrently.
337 *
338 * <pre> {@code
339 * class Resource {
340 * private static ExternalResource[] externalResourceArray = ...
341 *
342 * int myIndex;
343 * Resource(...) {
344 * myIndex = ...
345 * externalResourceArray[myIndex] = ...;
346 * ...
347 * }
348 * protected void finalize() {
349 * externalResourceArray[myIndex] = null;
350 * ...
351 * }
352 * public void action() {
353 * try {
354 * // ...
355 * int i = myIndex;
356 * Resource.update(externalResourceArray[i]);
357 * } finally {
358 * Reference.reachabilityFence(this);
359 * }
360 * }
361 * private static void update(ExternalResource ext) {
362 * ext.status = ...;
363 * }
364 * }}</pre>
365 *
366 * Here, the invocation of {@code reachabilityFence} is nonintuitively
367 * placed <em>after</em> the call to {@code update}, to ensure that the
368 * array slot is not nulled out by {@link Object#finalize} before the
369 * update, even if the call to {@code action} was the last use of this
370 * object. This might be the case if, for example a usage in a user program
371 * had the form {@code new Resource().action();} which retains no other
372 * reference to this {@code Resource}. While probably overkill here,
373 * {@code reachabilityFence} is placed in a {@code finally} block to ensure
374 * that it is invoked across all paths in the method. In a method with more
375 * complex control paths, you might need further precautions to ensure that
376 * {@code reachabilityFence} is encountered along all of them.
377 *
378 * <p> It is sometimes possible to better encapsulate use of
379 * {@code reachabilityFence}. Continuing the above example, if it were
380 * acceptable for the call to method {@code update} to proceed even if the
381 * finalizer had already executed (nulling out slot), then you could
382 * localize use of {@code reachabilityFence}:
383 *
384 * <pre> {@code
385 * public void action2() {
386 * // ...
387 * Resource.update(getExternalResource());
388 * }
389 * private ExternalResource getExternalResource() {
390 * ExternalResource ext = externalResourceArray[myIndex];
391 * Reference.reachabilityFence(this);
392 * return ext;
393 * }}</pre>
394 *
395 * <p> Method {@code reachabilityFence} is not required in constructions
396 * that themselves ensure reachability. For example, because objects that
397 * are locked cannot, in general, be reclaimed, it would suffice if all
398 * accesses of the object, in all methods of class {@code Resource}
399 * (including {@code finalize}) were enclosed in {@code synchronized (this)}
400 * blocks. (Further, such blocks must not include infinite loops, or
401 * themselves be unreachable, which fall into the corner case exceptions to
402 * the "in general" disclaimer.) However, method {@code reachabilityFence}
403 * remains a better option in cases where this approach is not as efficient,
404 * desirable, or possible; for example because it would encounter deadlock.
405 *
406 * @param ref the reference. If {@code null}, this method has no effect.
407 * @since 9
408 */
409 @DontInline
410 public static void reachabilityFence(Object ref) {
411 // Does nothing, because this method is annotated with @DontInline
412 // HotSpot needs to retain the ref and not GC it before a call to this
413 // method
414 }
415
416 }