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
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 /* Object used to synchronize with the garbage collector. The collector
114 * must acquire this lock at the beginning of each collection cycle. It is
115 * therefore critical that any code holding this lock complete as quickly
116 * as possible, allocate no new objects, and avoid calling user code.
117 */
118 private static class Lock { }
119 private static Lock lock = new Lock();
120
121
122 /* List of References waiting to be enqueued. The collector adds
123 * References to this list, while the Reference-handler thread removes
124 * them. This list is protected by the above lock object. The
125 * list uses the discovered field to link its elements.
126 */
127 private static Reference<Object> pending = null;
128
129 /* High-priority thread to enqueue pending References
130 */
131 private static class ReferenceHandler extends Thread {
132
133 private static void ensureClassInitialized(Class<?> clazz) {
134 try {
135 Class.forName(clazz.getName(), true, clazz.getClassLoader());
136 } catch (ClassNotFoundException e) {
137 throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
138 }
139 }
140
141 static {
142 // pre-load and initialize InterruptedException and Cleaner classes
143 // so that we don't get into trouble later in the run loop if there's
144 // memory shortage while loading/initializing them lazily.
145 ensureClassInitialized(InterruptedException.class);
146 ensureClassInitialized(Cleaner.class);
147 }
148
149 ReferenceHandler(ThreadGroup g, String name) {
150 super(g, null, name, 0, false);
151 }
152
153 public void run() {
154 while (true) {
155 tryHandlePending(true);
156 }
157 }
158 }
159
160 /**
161 * Try handle pending {@link Reference} if there is one.<p>
162 * Return {@code true} as a hint that there might be another
163 * {@link Reference} pending or {@code false} when there are no more pending
164 * {@link Reference}s at the moment and the program can do some other
165 * useful work instead of looping.
166 *
167 * @param waitForNotify if {@code true} and there was no pending
168 * {@link Reference}, wait until notified from VM
169 * or interrupted; if {@code false}, return immediately
170 * when there is no pending {@link Reference}.
171 * @return {@code true} if there was a {@link Reference} pending and it
172 * was processed, or we waited for notification and either got it
173 * or thread was interrupted before being notified;
174 * {@code false} otherwise.
175 */
176 static boolean tryHandlePending(boolean waitForNotify) {
177 Reference<Object> r;
178 Cleaner c;
179 try {
180 synchronized (lock) {
181 if (pending != null) {
182 r = pending;
183 // 'instanceof' might throw OutOfMemoryError sometimes
184 // so do this before un-linking 'r' from the 'pending' chain...
185 c = r instanceof Cleaner ? (Cleaner) r : null;
186 // unlink 'r' from 'pending' chain
187 pending = r.discovered;
188 r.discovered = null;
189 } else {
190 // The waiting on the lock may cause an OutOfMemoryError
191 // because it may try to allocate exception objects.
192 if (waitForNotify) {
193 lock.wait();
194 }
195 // retry if waited
196 return waitForNotify;
197 }
198 }
199 } catch (OutOfMemoryError x) {
200 // Give other threads CPU time so they hopefully drop some live references
201 // and GC reclaims some space.
202 // Also prevent CPU intensive spinning in case 'r instanceof Cleaner' above
203 // persistently throws OOME for some time...
204 Thread.yield();
205 // retry
206 return true;
207 } catch (InterruptedException x) {
208 // retry
209 return true;
210 }
211
212 // Fast path for cleaners
213 if (c != null) {
214 c.clean();
215 return true;
216 }
217
218 ReferenceQueue<? super Object> q = r.queue;
219 if (q != ReferenceQueue.NULL) q.enqueue(r);
220 return true;
221 }
222
223 static {
224 ThreadGroup tg = Thread.currentThread().getThreadGroup();
225 for (ThreadGroup tgn = tg;
226 tgn != null;
227 tg = tgn, tgn = tg.getParent());
228 Thread handler = new ReferenceHandler(tg, "Reference Handler");
229 /* If there were a special system-only priority greater than
230 * MAX_PRIORITY, it would be used here
231 */
232 handler.setPriority(Thread.MAX_PRIORITY);
233 handler.setDaemon(true);
234 handler.start();
235
236 // provide access in SharedSecrets
237 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
238 @Override
239 public boolean tryHandlePendingReference() {
240 return tryHandlePending(false);
241 }
242 });
243 }
244
245 /* -- Referent accessor and setters -- */
246
247 /**
248 * Returns this reference object's referent. If this reference object has
249 * been cleared, either by the program or by the garbage collector, then
250 * this method returns <code>null</code>.
251 *
252 * @return The object to which this reference refers, or
253 * <code>null</code> if this reference object has been cleared
254 */
255 @HotSpotIntrinsicCandidate
256 public T get() {
257 return this.referent;
258 }
259
260 /**
|
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.HotSpotIntrinsicCandidate;
29 import jdk.internal.misc.JavaLangRefAccess;
30 import jdk.internal.misc.SharedSecrets;
31 import jdk.internal.vm.annotation.DontInline;
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
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<?> 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 final 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<?> pending;
127
128 /* Discovery phase counter, guarder by above lock.
129 */
130 private static int discoveryPhase;
131
132 /* Enqueue phase, guarded by enqueuePhaseLock object.
133 */
134 private static int enqueuePhase;
135 private static final Object enqueuePhaseLock = new Object();
136
137 /* High-priority thread to enqueue pending References
138 */
139 private static class ReferenceHandler extends Thread {
140
141 private static void ensureClassInitialized(Class<?> clazz) {
142 try {
143 Class.forName(clazz.getName(), true, clazz.getClassLoader());
144 } catch (ClassNotFoundException e) {
145 throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
146 }
147 }
148
149 static {
150 // pre-load and initialize InterruptedException class
151 // so that we don't get into trouble later in the run loop if there's
152 // memory shortage while loading/initializing it lazily.
153 ensureClassInitialized(InterruptedException.class);
154 }
155
156 ReferenceHandler(ThreadGroup g, String name) {
157 super(g, null, name, 0, false);
158 }
159
160 public void run() {
161 int[] discoveryPhase = new int[1];
162 while (true) {
163 Reference<?> p = getPendingReferences(discoveryPhase);
164 enqueuePendingReferences(p, discoveryPhase[0]);
165 }
166 }
167 }
168
169 /**
170 * Blocks until GC discovers some pending references, sets the 0-th element
171 * of given {@code discoveryPhaseHolder} to the phase in which they were
172 * discovered and returns the head of the list of discovered references.
173 *
174 * @param discoveryPhaseHolder a 1-element array to hold the returned
175 * discovery phase.
176 * @return the head of a list of pending references linked via
177 * {@link #discovered} field with {@code null} marking the end of list.
178 */
179 static Reference<?> getPendingReferences(int[] discoveryPhaseHolder) {
180 Reference<?> p;
181 synchronized (lock) {
182 while ((p = pending) == null) {
183 try {
184 lock.wait();
185 } catch (OutOfMemoryError x) {
186 // The waiting on the lock may cause an OutOfMemoryError
187 // because it may try to allocate InterruptedException object.
188 // Give other threads CPU time so they hopefully drop some live
189 // references and GC reclaims some space.
190 Thread.yield();
191 } catch (InterruptedException x) {
192 // ignore interrupts
193 }
194 }
195 pending = null;
196 // increment discoveryPhase counter and return it in a holder array
197 discoveryPhaseHolder[0] = ++discoveryPhase;
198 }
199 return p;
200 }
201
202 /**
203 * @return current finished discovery phase.
204 */
205 static int getDiscoveryPhase() {
206 synchronized (lock) {
207 return (pending == null)
208 ? discoveryPhase // already incremented
209 : discoveryPhase + 1; // not yet incremented
210 }
211 }
212
213 /**
214 * Enqueue a list of pending {@link Reference}s linked via {@link #discovered}
215 * field with {@code null} marking the end of list.
216 * <p>
217 * The {@link #enqueuePhase} is set to given {@code discoveryPhase}
218 * after all references from the list have been enqueued and any waiters on
219 * {@link #enqueuePhaseLock} are notified.
220 *
221 * @param p a list of pending references linked via {@link #discovered}
222 * field with {@code null} marking the end of list.
223 * @param discoveryPhase the phase in which given references were discovered.
224 */
225 static void enqueuePendingReferences(Reference<?> p, int discoveryPhase) {
226 try {
227 // distribute unhooked pending references to their respective queues
228 while (p != null) {
229 Reference<?> r = p;
230 p = r.discovered;
231 r.discovered = null;
232 @SuppressWarnings("unchecked")
233 ReferenceQueue<Object> q = (ReferenceQueue) r.queue;
234 if (q != ReferenceQueue.NULL) q.enqueue(r);
235 }
236 } finally {
237 // mark the enqueueing of references discovered in given
238 // discovery phase is finished and notify waiters.
239 synchronized (enqueuePhaseLock) {
240 enqueuePhase = discoveryPhase;
241 enqueuePhaseLock.notifyAll();
242 }
243 }
244 }
245
246 /**
247 * Triggers discovery of new Reference(s) and returns the phase sequence number
248 * in which they were discovered or previous phase sequence number if no new
249 * Reference(s) were discovered.
250 */
251 static int discoverReferences() {
252 // trigger discovery of new Reference(s)
253 System.gc();
254 // obtain the phase in which they were discovered (if any)
255 return getDiscoveryPhase();
256 }
257
258 /**
259 * Blocks until all Reference(s) that were discovered in given
260 * {@code discoveryPhase} (as returned by {@link #discoverReferences()})
261 * have been enqueued.
262 *
263 * @param discoveryPhase the discovery phase sequence number.
264 * @throws InterruptedException if interrupted while waiting.
265 */
266 static void awaitReferencesEnqueued(int discoveryPhase) throws InterruptedException {
267 // await for them to be enqueued
268 synchronized (enqueuePhaseLock) {
269 while (enqueuePhase - discoveryPhase < 0) {
270 enqueuePhaseLock.wait();
271 }
272 }
273 }
274
275 static {
276 ThreadGroup tg = Thread.currentThread().getThreadGroup();
277 for (ThreadGroup tgn = tg;
278 tgn != null;
279 tg = tgn, tgn = tg.getParent());
280 Thread handler = new ReferenceHandler(tg, "Reference Handler");
281 /* If there were a special system-only priority greater than
282 * MAX_PRIORITY, it would be used here
283 */
284 handler.setPriority(Thread.MAX_PRIORITY);
285 handler.setDaemon(true);
286 handler.start();
287
288 // provide access in SharedSecrets
289 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
290 @Override
291 public int discoverReferences() {
292 return Reference.discoverReferences();
293 }
294
295 @Override
296 public void awaitReferencesEnqueued(
297 int discoveryPhase) throws InterruptedException {
298 Reference.awaitReferencesEnqueued(discoveryPhase);
299 }
300 });
301 }
302
303 /* -- Referent accessor and setters -- */
304
305 /**
306 * Returns this reference object's referent. If this reference object has
307 * been cleared, either by the program or by the garbage collector, then
308 * this method returns <code>null</code>.
309 *
310 * @return The object to which this reference refers, or
311 * <code>null</code> if this reference object has been cleared
312 */
313 @HotSpotIntrinsicCandidate
314 public T get() {
315 return this.referent;
316 }
317
318 /**
|