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 /**
|
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
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 /* 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 Reference<?> p = getPendingReferences();
154 enqueuePendingReferences(p);
155 }
156 }
157 }
158
159 /**
160 * Blocks until GC discovers some pending references and hands them to us.
161 *
162 * @return a list of pending references linked via {@link #discovered} field
163 * with {@code null} marking the end of list.
164 */
165 static Reference<?> getPendingReferences() {
166 Reference<?> p;
167 synchronized (lock) {
168 while ((p = pending) == null) {
169 try {
170 lock.wait();
171 } catch (OutOfMemoryError x) {
172 // The waiting on the lock may cause an OutOfMemoryError
173 // because it may try to allocate InterruptedException object.
174 // Give other threads CPU time so they hopefully drop some live
175 // references and GC reclaims some space.
176 Thread.yield();
177 } catch (InterruptedException x) {
178 // ignore interrupts
179 }
180 }
181 pending = null;
182 }
183 return p;
184 }
185
186 /**
187 * Enqueue a list of pending {@link Reference}s.
188 *
189 * @param p a list of pending references linked via {@link #discovered}
190 * field with {@code null} marking the end of list
191 */
192 static void enqueuePendingReferences(Reference<?> p) {
193 while (p != null) {
194 Reference<?> r = p;
195 p = r.discovered;
196 r.discovered = null;
197 @SuppressWarnings("unchecked")
198 ReferenceQueue<Object> q = (ReferenceQueue) r.queue;
199 if (q != ReferenceQueue.NULL) q.enqueue(r);
200 }
201 }
202
203 static {
204 ThreadGroup tg = Thread.currentThread().getThreadGroup();
205 for (ThreadGroup tgn = tg;
206 tgn != null;
207 tg = tgn, tgn = tg.getParent());
208 Thread handler = new ReferenceHandler(tg, "Reference Handler");
209 /* If there were a special system-only priority greater than
210 * MAX_PRIORITY, it would be used here
211 */
212 handler.setPriority(Thread.MAX_PRIORITY);
213 handler.setDaemon(true);
214 handler.start();
215
216 // provide access in SharedSecrets
217 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
218
219 @Override
220 public boolean cleanNextEnqueuedCleanable(Cleaner cleaner) {
221 return cleaner.cleanNextEnqueued();
222 }
223 });
224 }
225
226 /* -- Referent accessor and setters -- */
227
228 /**
229 * Returns this reference object's referent. If this reference object has
230 * been cleared, either by the program or by the garbage collector, then
231 * this method returns <code>null</code>.
232 *
233 * @return The object to which this reference refers, or
234 * <code>null</code> if this reference object has been cleared
235 */
236 @HotSpotIntrinsicCandidate
237 public T get() {
238 return this.referent;
239 }
240
241 /**
|