29 * Reference queues, to which registered reference objects are appended by the
30 * garbage collector after the appropriate reachability changes are detected.
31 *
32 * @author Mark Reinhold
33 * @since 1.2
34 */
35
36 public class ReferenceQueue<T> {
37
38 /**
39 * Constructs a new reference-object queue.
40 */
41 public ReferenceQueue() { }
42
43 private static class Null<S> extends ReferenceQueue<S> {
44 boolean enqueue(Reference<? extends S> r) {
45 return false;
46 }
47 }
48
49 static ReferenceQueue<Object> NULL = new Null<>();
50 static ReferenceQueue<Object> ENQUEUED = new Null<>();
51
52 static private class Lock { };
53 private Lock lock = new Lock();
54 private volatile Reference<? extends T> head = null;
55 private long queueLength = 0;
56
57 boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
58 synchronized (lock) {
59 // Check that since getting the lock this reference hasn't already been
60 // enqueued (and even then removed)
61 ReferenceQueue<?> queue = r.queue;
62 if ((queue == NULL) || (queue == ENQUEUED)) {
63 return false;
64 }
65 assert queue == this;
66 r.queue = ENQUEUED;
67 r.next = (head == null) ? r : head;
68 head = r;
69 queueLength++;
70 if (r instanceof FinalReference) {
71 sun.misc.VM.addFinalRefCount(1);
72 }
73 lock.notifyAll();
74 return true;
75 }
76 }
77
78 @SuppressWarnings("unchecked")
79 private Reference<? extends T> reallyPoll() { /* Must hold lock */
80 Reference<? extends T> r = head;
81 if (r != null) {
82 head = (r.next == r) ?
83 null :
84 r.next; // Unchecked due to the next field having a raw type in Reference
85 r.queue = NULL;
86 r.next = r;
87 queueLength--;
88 if (r instanceof FinalReference) {
89 sun.misc.VM.addFinalRefCount(-1);
90 }
91 return r;
92 }
93 return null;
94 }
95
96 /**
97 * Polls this queue to see if a reference object is available. If one is
98 * available without further delay then it is removed from the queue and
99 * returned. Otherwise this method immediately returns <tt>null</tt>.
100 *
101 * @return A reference object, if one was immediately available,
102 * otherwise <code>null</code>
103 */
104 public Reference<? extends T> poll() {
105 if (head == null)
106 return null;
107 synchronized (lock) {
108 return reallyPoll();
109 }
110 }
111
112 /**
113 * Removes the next reference object in this queue, blocking until either
114 * one becomes available or the given timeout period expires.
115 *
116 * <p> This method does not offer real-time guarantees: It schedules the
117 * timeout as if by invoking the {@link Object#wait(long)} method.
118 *
119 * @param timeout If positive, block for up to <code>timeout</code>
120 * milliseconds while waiting for a reference to be
121 * added to this queue. If zero, block indefinitely.
122 *
123 * @return A reference object, if one was available within the specified
124 * timeout period, otherwise <code>null</code>
125 *
126 * @throws IllegalArgumentException
127 * If the value of the timeout argument is negative
128 *
129 * @throws InterruptedException
130 * If the timeout wait is interrupted
131 */
132 public Reference<? extends T> remove(long timeout)
133 throws IllegalArgumentException, InterruptedException
134 {
135 if (timeout < 0) {
136 throw new IllegalArgumentException("Negative timeout value");
137 }
138 synchronized (lock) {
139 Reference<? extends T> r = reallyPoll();
140 if (r != null) return r;
141 long start = (timeout == 0) ? 0 : System.nanoTime();
142 for (;;) {
143 lock.wait(timeout);
144 r = reallyPoll();
145 if (r != null) return r;
146 if (timeout != 0) {
147 long end = System.nanoTime();
148 timeout -= (end - start) / 1000_000;
149 if (timeout <= 0) return null;
150 start = end;
151 }
152 }
153 }
154 }
155
156 /**
157 * Removes the next reference object in this queue, blocking until one
158 * becomes available.
159 *
160 * @return A reference object, blocking until one becomes available
161 * @throws InterruptedException If the wait is interrupted
162 */
163 public Reference<? extends T> remove() throws InterruptedException {
164 return remove(0);
165 }
166
167 }
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29 * Reference queues, to which registered reference objects are appended by the
30 * garbage collector after the appropriate reachability changes are detected.
31 *
32 * @author Mark Reinhold
33 * @since 1.2
34 */
35
36 public class ReferenceQueue<T> {
37
38 /**
39 * Constructs a new reference-object queue.
40 */
41 public ReferenceQueue() { }
42
43 private static class Null<S> extends ReferenceQueue<S> {
44 boolean enqueue(Reference<? extends S> r) {
45 return false;
46 }
47 }
48
49 static final ReferenceQueue<Object> NULL = new Null<>();
50 static final ReferenceQueue<Object> ENQUEUED = new Null<>();
51
52 static private class Lock { }
53 private final Lock lock = new Lock();
54 private volatile int waiters;
55
56 @SuppressWarnings("unused")
57 private volatile Reference<? extends T> head; // we assign using Unsafe CAS
58
59 boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
60 if (markEnqueued(r)) { // markEnqueued is atomic
61 addChunk(r, r);
62 return true;
63 } else {
64 return false;
65 }
66 }
67
68 boolean markEnqueued(Reference<?> r) {
69 ReferenceQueue<?> queue;
70 do {
71 // Check that this reference hasn't already been
72 // enqueued (and even then removed)
73 queue = r.queue;
74 if ((queue == NULL) || (queue == ENQUEUED)) {
75 return false;
76 }
77 } while (!r.casQueue(queue, ENQUEUED));
78 assert queue == this;
79 return true;
80 }
81
82 @SuppressWarnings("unchecked")
83 void addChunk(Reference<?> chunkHead, Reference<?> chunkTail) {
84 Reference<? extends T> h;
85 do {
86 h = head;
87 chunkTail.next = (h == null) ? chunkTail : h;
88 } while (!casHead(h, (Reference) chunkHead));
89 // notify waiters
90 if (waiters > 0) {
91 synchronized (lock) {
92 if (waiters > 0) {
93 lock.notifyAll();
94 }
95 }
96 }
97 }
98
99 private Reference<? extends T> reallyPoll() {
100 Reference<? extends T> r;
101 while ((r = head) != null) {
102 @SuppressWarnings("unchecked") // due to cast to raw type
103 Reference<? extends T> nh = (r.next == r)
104 ? null : (Reference) r.next;
105 if (casHead(r, nh)) {
106 r.lazySetQueue(NULL);
107 UNSAFE.storeFence();
108 r.next = r;
109 break;
110 }
111 }
112 return r;
113 }
114
115 /**
116 * Polls this queue to see if a reference object is available. If one is
117 * available without further delay then it is removed from the queue and
118 * returned. Otherwise this method immediately returns <tt>null</tt>.
119 *
120 * @return A reference object, if one was immediately available,
121 * otherwise <code>null</code>
122 */
123 public Reference<? extends T> poll() {
124 return reallyPoll();
125 }
126
127 /**
128 * Removes the next reference object in this queue, blocking until either
129 * one becomes available or the given timeout period expires.
130 *
131 * <p> This method does not offer real-time guarantees: It schedules the
132 * timeout as if by invoking the {@link Object#wait(long)} method.
133 *
134 * @param timeout If positive, block for up to <code>timeout</code>
135 * milliseconds while waiting for a reference to be
136 * added to this queue. If zero, block indefinitely.
137 *
138 * @return A reference object, if one was available within the specified
139 * timeout period, otherwise <code>null</code>
140 *
141 * @throws IllegalArgumentException
142 * If the value of the timeout argument is negative
143 *
144 * @throws InterruptedException
145 * If the timeout wait is interrupted
146 */
147 public Reference<? extends T> remove(long timeout)
148 throws IllegalArgumentException, InterruptedException
149 {
150 if (timeout < 0) {
151 throw new IllegalArgumentException("Negative timeout value");
152 }
153 Reference<? extends T> r = reallyPoll();
154 if (r != null) return r;
155 return reallyRemove(timeout);
156 }
157
158 private Reference<? extends T> reallyRemove(long timeout) throws InterruptedException {
159 long deadline = (timeout == 0)
160 ? 0 : System.nanoTime() + timeout * 1000_000L;
161
162 synchronized (lock) {
163 ++waiters;
164 try {
165 for (; ; ) {
166 Reference<? extends T> r = reallyPoll();
167 if (r != null) return r;
168 if (timeout == 0) {
169 lock.wait(0);
170 } else {
171 long timeoutNanos = deadline - System.nanoTime();
172 if (timeoutNanos <= 0) return null;
173 lock.wait(timeoutNanos / 1000_000L, (int) (timeoutNanos % 1000_000L));
174 }
175 }
176 } finally {
177 --waiters;
178 }
179 }
180 }
181
182 /**
183 * Removes the next reference object in this queue, blocking until one
184 * becomes available.
185 *
186 * @return A reference object, blocking until one becomes available
187 * @throws InterruptedException If the wait is interrupted
188 */
189 public Reference<? extends T> remove() throws InterruptedException {
190 return remove(0);
191 }
192
193 // Unsafe machinery
194
195 private boolean casHead(Reference<?> cmp, Reference<? extends T> val) {
196 return UNSAFE.compareAndSwapObject(this, headOffset, cmp, val);
197 }
198
199 private static final sun.misc.Unsafe UNSAFE;
200 private static final long headOffset;
201
202 static {
203 try {
204 UNSAFE = sun.misc.Unsafe.getUnsafe();
205 Class<ReferenceQueue> rqc = ReferenceQueue.class;
206 headOffset = UNSAFE.objectFieldOffset(rqc.getDeclaredField("head"));
207 } catch (Exception e) {
208 throw new Error(e);
209 }
210 }
211 }
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