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26 package java.lang.ref;
29 * Reference queues, to which registered reference objects are appended by the
30 * garbage collector after the appropriate reachability changes are detected.
32 * @author Mark Reinhold
36 public class ReferenceQueue<T> {
39 * Constructs a new reference-object queue.
41 public ReferenceQueue() { }
43 private static class Null extends ReferenceQueue {
44 boolean enqueue(Reference r) {
49 static ReferenceQueue NULL = new Null();
50 static ReferenceQueue ENQUEUED = new Null();
52 static private class Lock { };
53 private Lock lock = new Lock();
54 private volatile Reference<? extends T> head = null;
55 private long queueLength = 0;
57 boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
59 if (r.queue == ENQUEUED) return false;
62 r.next = (head == null) ? r : head;
71 private Reference<? extends T> reallyPoll() { /* Must hold lock */
73 Reference<? extends T> r = head;
74 head = (r.next == r) ? null : r.next;
84 * Polls this queue to see if a reference object is available. If one is
85 * available without further delay then it is removed from the queue and
86 * returned. Otherwise this method immediately returns <tt>null</tt>.
88 * @return A reference object, if one was immediately available,
89 * otherwise <code>null</code>
91 public Reference<? extends T> poll() {
100 * Removes the next reference object in this queue, blocking until either
101 * one becomes available or the given timeout period expires.
103 * <p> This method does not offer real-time guarantees: It schedules the
104 * timeout as if by invoking the {@link Object#wait(long)} method.
106 * @param timeout If positive, block for up to <code>timeout</code>
107 * milliseconds while waiting for a reference to be
108 * added to this queue. If zero, block indefinitely.
110 * @return A reference object, if one was available within the specified
111 * timeout period, otherwise <code>null</code>
113 * @throws IllegalArgumentException
114 * If the value of the timeout argument is negative
116 * @throws InterruptedException
117 * If the timeout wait is interrupted
119 public Reference<? extends T> remove(long timeout)
120 throws IllegalArgumentException, InterruptedException
123 throw new IllegalArgumentException("Negative timeout value");
125 synchronized (lock) {
126 Reference<? extends T> r = reallyPoll();
127 if (r != null) return r;
131 if (r != null) return r;
132 if (timeout != 0) return null;
138 * Removes the next reference object in this queue, blocking until one
141 * @return A reference object, blocking until one becomes available
142 * @throws InterruptedException If the wait is interrupted
144 public Reference<? extends T> remove() throws InterruptedException {