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() {
|
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 /* Blocks until the reference pending list becomes non-empty and
114 * returns the list.
115 */
116 private static native Reference<Object> getReferencePendingList();
117
118
119 /* High-priority thread to enqueue pending References
120 */
121 private static class ReferenceHandler extends Thread {
122
123 private static void ensureClassInitialized(Class<?> clazz) {
124 try {
125 Class.forName(clazz.getName(), true, clazz.getClassLoader());
126 } catch (ClassNotFoundException e) {
127 throw (Error) new NoClassDefFoundError(e.getMessage()).initCause(e);
128 }
129 }
130
131 static {
132 // pre-load and initialize Cleaner class so that we don't get into trouble later in
133 // the run loop if there's memory shortage while loading/initializing them lazily.
134 ensureClassInitialized(Cleaner.class);
135 }
136
137 ReferenceHandler(ThreadGroup g, String name) {
138 super(g, null, name, 0, false);
139 }
140
141 public void run() {
142 while (true) {
143 tryHandlePending(true);
144 }
145 }
146 }
147
148 /**
149 * Try handle pending {@link Reference} if there is one.<p>
150 * Return {@code true} as a hint that there might be another
151 * {@link Reference} pending or {@code false} when there are no more pending
152 * {@link Reference}s at the moment and the program can do some other
153 * useful work instead of looping.
154 *
155 * @param waitForNotify if {@code true} and there was no pending
156 * {@link Reference}, wait until notified from VM
157 * or interrupted; if {@code false}, return immediately
158 * when there is no pending {@link Reference}.
159 * @return {@code true} if there was a {@link Reference} pending and it
160 * was processed, or we waited for notification and either got it
161 * or thread was interrupted before being notified;
162 * {@code false} otherwise.
163 */
164 static boolean tryHandlePending(boolean waitForNotify) {
165 if (!waitForNotify) {
166 // FIXME! Calls from non-blocking helper threads not yet supported.
167 return false;
168 }
169
170 // Get pending references from the VM
171 Reference<Object> pending_list = getReferencePendingList();
172
173 // Enqueue references
174 while (pending_list != null) {
175 // Unlink
176 Reference<Object> r = pending_list;
177 pending_list = r.discovered;
178 r.discovered = null;
179
180 // Fast path for cleaners
181 if (r instanceof Cleaner) {
182 try {
183 ((Cleaner)r).clean();
184 } catch (Throwable x) {
185 // Ignored
186 }
187
188 // Don't enqueue
189 continue;
190 }
191
192 // Enqueue
193 ReferenceQueue<? super Object> q = r.queue;
194 if (q != ReferenceQueue.NULL) {
195 q.enqueue(r);
196 }
197 }
198
199 return true;
200 }
201
202 static {
203 ThreadGroup tg = Thread.currentThread().getThreadGroup();
204 for (ThreadGroup tgn = tg;
205 tgn != null;
206 tg = tgn, tgn = tg.getParent());
207 Thread handler = new ReferenceHandler(tg, "Reference Handler");
208 /* If there were a special system-only priority greater than
209 * MAX_PRIORITY, it would be used here
210 */
211 handler.setPriority(Thread.MAX_PRIORITY);
212 handler.setDaemon(true);
213 handler.start();
214
215 // provide access in SharedSecrets
216 SharedSecrets.setJavaLangRefAccess(new JavaLangRefAccess() {
217 @Override
218 public boolean tryHandlePendingReference() {
|