1 /*
2 * Copyright (c) 1997, 2013, 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 sun.misc.Cleaner;
29 import sun.misc.JavaLangRefAccess;
30 import sun.misc.ManagedLocalsThread;
31 import sun.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 a Reference is de-queued from ReferenceQueue, Reference.next
98 * points to this object.
99 */
100 private static class Dequeued extends Reference<Void> { Dequeued() { super(null); } }
101 static final Reference DEQUEUED = new Dequeued();
102
103 /* When active: NULL
104 * pending: this
105 * Enqueued: next reference in queue (or this if last)
106 * Inactive: DEQUEUED
107 */
108 @SuppressWarnings("rawtypes")
109 Reference next;
110
111 /* When active: next element in a discovered reference list maintained by GC (or this if last)
112 * pending: next element in the pending list (or null if last)
113 * otherwise: NULL
114 */
115 transient private Reference<T> discovered; /* used by VM */
116
117
118 /* Object used to synchronize with the garbage collector. The collector
119 * must acquire this lock at the beginning of each collection cycle. It is
120 * therefore critical that any code holding this lock complete as quickly
121 * as possible, allocate no new objects, and avoid calling user code.
122 */
123 static private class Lock { }
124 private static Lock lock = new Lock();
125
126
127 /* List of References waiting to be enqueued. The collector adds
128 * References to this list, while the Reference-handler thread removes
129 * them. This list is protected by the above lock object. The
130 * list uses the discovered field to link its elements.
131 */
132 private static Reference<Object> pending = null;
133
134 /* High-priority thread to enqueue pending References
135 */
136 private static class ReferenceHandler extends ManagedLocalsThread {
137
138 private static void ensureClassInitialized(Class<?> clazz) {
139 try {
140 Class.forName(clazz.getName(), true, clazz.getClassLoader());
141 } catch (ClassNotFoundException e) {
142 throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
143 }
144 }
145
146 static {
147 // pre-load and initialize InterruptedException and Cleaner classes
148 // so that we don't get into trouble later in the run loop if there's
149 // memory shortage while loading/initializing them lazily.
150 ensureClassInitialized(InterruptedException.class);
151 ensureClassInitialized(Cleaner.class);
152 }
153
154 ReferenceHandler(ThreadGroup g, String name) {
155 super(g, name);
156 }
157
158 public void run() {
159 while (true) {
160 tryHandlePending(true);
161 }
162 }
163 }
164
165 /**
166 * Try handle pending {@link Reference} if there is one.<p>
167 * Return {@code true} as a hint that there might be another
168 * {@link Reference} pending or {@code false} when there are no more pending
169 * {@link Reference}s at the moment and the program can do some other
170 * useful work instead of looping.
171 *
172 * @param waitForNotify if {@code true} and there was no pending
173 * {@link Reference}, wait until notified from VM
174 * or interrupted; if {@code false}, return immediately
175 * when there is no pending {@link Reference}.
176 * @return {@code true} if there was a {@link Reference} pending and it
177 * was processed, or we waited for notification and either got it
178 * or thread was interrupted before being notified;
179 * {@code false} otherwise.
180 */
181 static boolean tryHandlePending(boolean waitForNotify) {
182 Reference<Object> r;
183 Cleaner c;
184 try {
185 synchronized (lock) {
186 if (pending != null) {
187 r = pending;
188 // 'instanceof' might throw OutOfMemoryError sometimes
189 // so do this before un-linking 'r' from the 'pending' chain...
190 c = r instanceof Cleaner ? (Cleaner) r : null;
191 // unlink 'r' from 'pending' chain
192 pending = r.discovered;
193 r.discovered = null;
194 } else {
195 // The waiting on the lock may cause an OutOfMemoryError
196 // because it may try to allocate exception objects.
197 if (waitForNotify) {
198 lock.wait();
199 }
200 // retry if waited
201 return waitForNotify;
202 }
203 }
204 } catch (OutOfMemoryError x) {
205 // Give other threads CPU time so they hopefully drop some live references
206 // and GC reclaims some space.
207 // Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above
208 // persistently throws OOME for some time...
209 Thread.yield();
210 // retry
211 return true;
212 } catch (InterruptedException x) {
213 // retry
214 return true;
215 }
216
217 // Fast path for cleaners
218 if (c != null) {
219 c.clean();
220 return true;
221 }
222
223 ReferenceQueue<? super Object> q = r.queue;
224 if (q != ReferenceQueue.NULL) q.enqueue(r);
225 return true;
226 }
227
228 static {
229 ThreadGroup tg = Thread.currentThread().getThreadGroup();
230 for (ThreadGroup tgn = tg;
231 tgn != null;
232 tg = tgn, tgn = tg.getParent());
233 Thread handler = new ReferenceHandler(tg, "Reference Handler");
234 /* If there were a special system-only priority greater than
235 * MAX_PRIORITY, it would be used here
236 */
237 handler.setPriority(Thread.MAX_PRIORITY);
238 handler.setDaemon(true);
239 handler.start();
240
241 // provide access in SharedSecrets
242 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
243 @Override
244 public boolean tryHandlePendingReference() {
245 return tryHandlePending(false);
246 }
247 });
248 }
249
250 /* -- Referent accessor and setters -- */
251
252 /**
253 * Returns this reference object's referent. If this reference object has
254 * been cleared, either by the program or by the garbage collector, then
255 * this method returns <code>null</code>.
256 *
257 * @return The object to which this reference refers, or
258 * <code>null</code> if this reference object has been cleared
259 */
260 public T get() {
261 return this.referent;
262 }
263
264 /**
265 * Clears this reference object. Invoking this method will not cause this
266 * object to be enqueued.
267 *
268 * <p> This method is invoked only by Java code; when the garbage collector
269 * clears references it does so directly, without invoking this method.
270 */
271 public void clear() {
272 this.referent = null;
273 }
274
275
276 /* -- Queue operations -- */
277
278 /**
279 * Tells whether or not this reference object has been enqueued, either by
280 * the program or by the garbage collector. If this reference object was
281 * not registered with a queue when it was created, then this method will
282 * always return <code>false</code>.
283 *
284 * @return <code>true</code> if and only if this reference object has
285 * been enqueued
286 */
287 public boolean isEnqueued() {
288 return (this.queue == ReferenceQueue.ENQUEUED);
289 }
290
291 /**
292 * Adds this reference object to the queue with which it is registered,
293 * if any.
294 *
295 * <p> This method is invoked only by Java code; when the garbage collector
296 * enqueues references it does so directly, without invoking this method.
297 *
298 * @return <code>true</code> if this reference object was successfully
299 * enqueued; <code>false</code> if it was already enqueued or if
300 * it was not registered with a queue when it was created
301 */
302 public boolean enqueue() {
303 return this.queue.enqueue(this);
304 }
305
306
307 /* -- Constructors -- */
308
309 Reference(T referent) {
310 this(referent, null);
311 }
312
313 Reference(T referent, ReferenceQueue<? super T> queue) {
314 this.referent = referent;
315 this.queue = (queue == null) ? ReferenceQueue.NULL : queue;
316 }
317
318 }