1 /*
2 * Copyright (c) 1997, 2017, 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 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 /* A Reference instance is in one of four possible internal states:
47 *
48 * Active: Subject to special treatment by the garbage collector. Some
49 * time after the collector detects that the reachability of the
50 * referent has changed to the appropriate state, it changes the
51 * instance's state to either Pending or Inactive, depending upon
52 * whether or not the instance was registered with a queue when it was
53 * created. In the former case it also adds the instance to the
54 * pending-Reference list. Newly-created instances are Active.
55 *
56 * Pending: An element of the pending-Reference list, waiting to be
57 * enqueued by the Reference-handler thread. Unregistered instances
58 * are never in this state.
59 *
60 * Enqueued: An element of the queue with which the instance was
61 * registered when it was created. When an instance is removed from
62 * its ReferenceQueue, it is made Inactive. Unregistered instances are
63 * never in this state.
64 *
65 * Inactive: Nothing more to do. Once an instance becomes Inactive its
66 * state will never change again.
67 *
68 * The state is encoded in the queue and next fields as follows:
69 *
70 * Active: queue = ReferenceQueue with which instance is registered, or
71 * ReferenceQueue.NULL if it was not registered with a queue; next =
72 * null.
73 *
74 * Pending: queue = ReferenceQueue with which instance is registered;
75 * next = this
76 *
77 * Enqueued: queue = ReferenceQueue.ENQUEUED; next = Following instance
78 * in queue, or this if at end of list.
79 *
80 * Inactive: queue = ReferenceQueue.NULL; next = this.
81 *
82 * With this scheme the collector need only examine the next field in order
83 * to determine whether a Reference instance requires special treatment: If
84 * the next field is null then the instance is active; if it is non-null,
85 * then the collector should treat the instance normally.
86 *
87 * To ensure that a concurrent collector can discover active Reference
88 * objects without interfering with application threads that may apply
89 * the enqueue() method to those objects, collectors should link
90 * discovered objects through the discovered field. The discovered
91 * field is also used for linking Reference objects in the pending list.
92 */
93
94 private T referent; /* Treated specially by GC */
95
96 volatile ReferenceQueue<? super T> queue;
97
98 /* When active: NULL
99 * pending: this
100 * Enqueued: next reference in queue (or this if last)
101 * Inactive: this
102 */
103 @SuppressWarnings("rawtypes")
104 volatile Reference next;
105
106 /* When active: next element in a discovered reference list maintained by GC (or this if last)
107 * pending: next element in the pending list (or null if last)
108 * otherwise: NULL
109 */
110 private transient Reference<T> discovered; /* used by VM */
111
112
113 /* High-priority thread to enqueue pending References
114 */
115 private static class ReferenceHandler extends Thread {
116
117 private static void ensureClassInitialized(Class<?> clazz) {
118 try {
119 Class.forName(clazz.getName(), true, clazz.getClassLoader());
120 } catch (ClassNotFoundException e) {
121 throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
122 }
123 }
124
125 static {
126 // pre-load and initialize Cleaner class so that we don't
127 // get into trouble later in the run loop if there's
128 // memory shortage while loading/initializing it lazily.
129 ensureClassInitialized(Cleaner.class);
130 }
131
132 ReferenceHandler(ThreadGroup g, String name) {
133 super(g, null, name, 0, false);
134 }
135
136 public void run() {
137 while (true) {
138 processPendingReferences();
139 }
140 }
141 }
142
143 /*
144 * Atomically get and clear (set to null) the VM's pending list.
145 */
146 private static native Reference<Object> getAndClearReferencePendingList();
147
148 /*
149 * Test whether the VM's pending list contains any entries.
150 */
151 private static native boolean hasReferencePendingList();
152
153 /*
154 * Wait until the VM's pending list may be non-null.
155 */
156 private static native void waitForReferencePendingList();
157
158 private static final Object processPendingLock = new Object();
159 private static boolean processPendingActive = false;
160
161 private static void processPendingReferences() {
162 // Only the singleton reference processing thread calls
163 // waitForReferencePendingList() and getAndClearReferencePendingList().
164 // These are separate operations to avoid a race with other threads
165 // that are calling waitForReferenceProcessing().
166 waitForReferencePendingList();
167 Reference<Object> pendingList;
168 synchronized (processPendingLock) {
169 pendingList = getAndClearReferencePendingList();
170 processPendingActive = true;
171 }
172 while (pendingList != null) {
173 Reference<Object> ref = pendingList;
174 pendingList = ref.discovered;
175 ref.discovered = null;
176
177 if (ref instanceof Cleaner) {
178 ((Cleaner)ref).clean();
179 // Notify any waiters that progress has been made.
180 // This improves latency for nio.Bits waiters, which
181 // are the only important ones.
182 synchronized (processPendingLock) {
183 processPendingLock.notifyAll();
184 }
185 } else {
186 ReferenceQueue<? super Object> q = ref.queue;
187 if (q != ReferenceQueue.NULL) q.enqueue(ref);
188 }
189 }
190 // Notify any waiters of completion of current round.
191 synchronized (processPendingLock) {
192 processPendingActive = false;
193 processPendingLock.notifyAll();
194 }
195 }
196
197 // Wait for progress in reference processing.
198 //
199 // Returns true after waiting (for notification from the reference
200 // processing thread) if either (1) the VM has any pending
201 // references, or (2) the reference processing thread is
202 // processing references. Otherwise, returns false immediately.
203 private static boolean waitForReferenceProcessing()
204 throws InterruptedException
205 {
206 synchronized (processPendingLock) {
207 if (processPendingActive || hasReferencePendingList()) {
208 // Wait for progress, not necessarily completion.
209 processPendingLock.wait();
210 return true;
211 } else {
212 return false;
213 }
214 }
215 }
216
217 static {
218 ThreadGroup tg = Thread.currentThread().getThreadGroup();
219 for (ThreadGroup tgn = tg;
220 tgn != null;
221 tg = tgn, tgn = tg.getParent());
222 Thread handler = new ReferenceHandler(tg, "Reference Handler");
223 /* If there were a special system-only priority greater than
224 * MAX_PRIORITY, it would be used here
225 */
226 handler.setPriority(Thread.MAX_PRIORITY);
227 handler.setDaemon(true);
228 handler.start();
229
230 // provide access in SharedSecrets
231 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
232 @Override
233 public boolean waitForReferenceProcessing()
234 throws InterruptedException
235 {
236 return Reference.waitForReferenceProcessing();
237 }
238 });
239 }
240
241 /* -- Referent accessor and setters -- */
242
243 /**
244 * Returns this reference object's referent. If this reference object has
245 * been cleared, either by the program or by the garbage collector, then
246 * this method returns <code>null</code>.
247 *
248 * @return The object to which this reference refers, or
249 * <code>null</code> if this reference object has been cleared
250 */
251 @HotSpotIntrinsicCandidate
252 public T get() {
253 return this.referent;
254 }
255
256 /**
257 * Clears this reference object. Invoking this method will not cause this
258 * object to be enqueued.
259 *
260 * <p> This method is invoked only by Java code; when the garbage collector
261 * clears references it does so directly, without invoking this method.
262 */
263 public void clear() {
264 this.referent = null;
265 }
266
267 /* -- Queue operations -- */
268
269 /**
270 * Tells whether or not this reference object has been enqueued, either by
271 * the program or by the garbage collector. If this reference object was
272 * not registered with a queue when it was created, then this method will
273 * always return <code>false</code>.
274 *
275 * @return <code>true</code> if and only if this reference object has
276 * been enqueued
277 */
278 public boolean isEnqueued() {
279 return (this.queue == ReferenceQueue.ENQUEUED);
280 }
281
282 /**
283 * Clears this reference object and adds it to the queue with which
284 * it is registered, if any.
285 *
286 * <p> This method is invoked only by Java code; when the garbage collector
287 * enqueues references it does so directly, without invoking this method.
288 *
289 * @return <code>true</code> if this reference object was successfully
290 * enqueued; <code>false</code> if it was already enqueued or if
291 * it was not registered with a queue when it was created
292 */
293 public boolean enqueue() {
294 this.referent = null;
295 return this.queue.enqueue(this);
296 }
297
298 /* -- Constructors -- */
299
300 Reference(T referent) {
301 this(referent, null);
302 }
303
304 Reference(T referent, ReferenceQueue<? super T> queue) {
305 this.referent = referent;
306 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
307 }
308
309 /**
310 * Ensures that the object referenced by the given reference remains
311 * <a href="package-summary.html#reachability"><em>strongly reachable</em></a>,
312 * regardless of any prior actions of the program that might otherwise cause
313 * the object to become unreachable; thus, the referenced object is not
314 * reclaimable by garbage collection at least until after the invocation of
315 * this method. Invocation of this method does not itself initiate garbage
316 * collection or finalization.
317 *
318 * <p> This method establishes an ordering for
319 * <a href="package-summary.html#reachability"><em>strong reachability</em></a>
320 * with respect to garbage collection. It controls relations that are
321 * otherwise only implicit in a program -- the reachability conditions
322 * triggering garbage collection. This method is designed for use in
323 * uncommon situations of premature finalization where using
324 * {@code synchronized} blocks or methods, or using other synchronization
325 * facilities are not possible or do not provide the desired control. This
326 * method is applicable only when reclamation may have visible effects,
327 * which is possible for objects with finalizers (See
328 * <a href="https://docs.oracle.com/javase/specs/jls/se8/html/jls-12.html#jls-12.6">
329 * Section 12.6 17 of <cite>The Java™ Language Specification</cite></a>)
330 * that are implemented in ways that rely on ordering control for correctness.
331 *
332 * @apiNote
333 * Finalization may occur whenever the virtual machine detects that no
334 * reference to an object will ever be stored in the heap: The garbage
335 * collector may reclaim an object even if the fields of that object are
336 * still in use, so long as the object has otherwise become unreachable.
337 * This may have surprising and undesirable effects in cases such as the
338 * following example in which the bookkeeping associated with a class is
339 * managed through array indices. Here, method {@code action} uses a
340 * {@code reachabilityFence} to ensure that the {@code Resource} object is
341 * not reclaimed before bookkeeping on an associated
342 * {@code ExternalResource} has been performed; in particular here, to
343 * ensure that the array slot holding the {@code ExternalResource} is not
344 * nulled out in method {@link Object#finalize}, which may otherwise run
345 * concurrently.
346 *
347 * <pre> {@code
348 * class Resource {
349 * private static ExternalResource[] externalResourceArray = ...
350 *
351 * int myIndex;
352 * Resource(...) {
353 * myIndex = ...
354 * externalResourceArray[myIndex] = ...;
355 * ...
356 * }
357 * protected void finalize() {
358 * externalResourceArray[myIndex] = null;
359 * ...
360 * }
361 * public void action() {
362 * try {
363 * // ...
364 * int i = myIndex;
365 * Resource.update(externalResourceArray[i]);
366 * } finally {
367 * Reference.reachabilityFence(this);
368 * }
369 * }
370 * private static void update(ExternalResource ext) {
371 * ext.status = ...;
372 * }
373 * }}</pre>
374 *
375 * Here, the invocation of {@code reachabilityFence} is nonintuitively
376 * placed <em>after</em> the call to {@code update}, to ensure that the
377 * array slot is not nulled out by {@link Object#finalize} before the
378 * update, even if the call to {@code action} was the last use of this
379 * object. This might be the case if, for example a usage in a user program
380 * had the form {@code new Resource().action();} which retains no other
381 * reference to this {@code Resource}. While probably overkill here,
382 * {@code reachabilityFence} is placed in a {@code finally} block to ensure
383 * that it is invoked across all paths in the method. In a method with more
384 * complex control paths, you might need further precautions to ensure that
385 * {@code reachabilityFence} is encountered along all of them.
386 *
387 * <p> It is sometimes possible to better encapsulate use of
388 * {@code reachabilityFence}. Continuing the above example, if it were
389 * acceptable for the call to method {@code update} to proceed even if the
390 * finalizer had already executed (nulling out slot), then you could
391 * localize use of {@code reachabilityFence}:
392 *
393 * <pre> {@code
394 * public void action2() {
395 * // ...
396 * Resource.update(getExternalResource());
397 * }
398 * private ExternalResource getExternalResource() {
399 * ExternalResource ext = externalResourceArray[myIndex];
400 * Reference.reachabilityFence(this);
401 * return ext;
402 * }}</pre>
403 *
404 * <p> Method {@code reachabilityFence} is not required in constructions
405 * that themselves ensure reachability. For example, because objects that
406 * are locked cannot, in general, be reclaimed, it would suffice if all
407 * accesses of the object, in all methods of class {@code Resource}
408 * (including {@code finalize}) were enclosed in {@code synchronized (this)}
409 * blocks. (Further, such blocks must not include infinite loops, or
410 * themselves be unreachable, which fall into the corner case exceptions to
411 * the "in general" disclaimer.) However, method {@code reachabilityFence}
412 * remains a better option in cases where this approach is not as efficient,
413 * desirable, or possible; for example because it would encounter deadlock.
414 *
415 * @param ref the reference. If {@code null}, this method has no effect.
416 * @since 9
417 */
418 @DontInline
419 public static void reachabilityFence(Object ref) {
420 // Does nothing, because this method is annotated with @DontInline
421 // HotSpot needs to retain the ref and not GC it before a call to this
422 // method
423 }
424 }