1 /*
2 * Copyright (c) 1997, 2025, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
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 java.util.function.Consumer;
29 import jdk.internal.misc.VM;
30 import jdk.internal.vm.Continuation;
31 import jdk.internal.vm.ContinuationSupport;
32
33 /**
34 * Reference queues, to which registered reference objects are appended by the
35 * garbage collector after the appropriate reachability changes are detected.
36 *
37 * <p>{@linkplain java.lang.ref##MemoryConsistency Memory consistency effects}:
38 * The enqueueing of a reference to a queue (by the garbage collector, or by a
39 * successful call to {@link Reference#enqueue})
40 * <a href="{@docRoot}/java.base/java/util/concurrent/package-summary.html#MemoryVisibility"><i>happens-before</i></a>
41 * the reference is removed from the queue by {@link ReferenceQueue#poll} or
42 * {@link ReferenceQueue#remove}.
43 *
44 * @param <T> the type of the reference object
45 *
46 * @author Mark Reinhold
47 * @since 1.2
48 */
49
50 public class ReferenceQueue<T> {
51 private static class Null extends ReferenceQueue<Object> {
52 @Override
53 boolean enqueue(Reference<?> r) {
54 return false;
55 }
56 }
57
58 static final ReferenceQueue<Object> NULL = new Null();
59 static final ReferenceQueue<Object> ENQUEUED = new Null();
60
61 private volatile Reference<? extends T> head;
62 private long queueLength = 0;
63
64 private static class Lock { };
65 private final Lock lock = new Lock();
66
67 /**
68 * Constructs a new reference-object queue.
69 */
70 public ReferenceQueue() {
71 }
72
73 private boolean enqueue0(Reference<? extends T> r) { // must hold lock
74 // Check that since getting the lock this reference hasn't already been
75 // enqueued (and even then removed)
76 ReferenceQueue<?> queue = r.queue;
77 if ((queue == NULL) || (queue == ENQUEUED)) {
78 return false;
79 }
80 assert queue == this;
81 // Self-loop end, so if a FinalReference it remains inactive.
82 r.next = (head == null) ? r : head;
83 head = r;
84 queueLength++;
85 // Update r.queue *after* adding to list, to avoid race
86 // with concurrent enqueued checks and fast-path poll().
87 // Volatiles ensure ordering.
88 r.queue = ENQUEUED;
89 if (r instanceof FinalReference) {
90 VM.addFinalRefCount(1);
91 }
92 lock.notifyAll();
93 return true;
94 }
95
96 private Reference<? extends T> poll0() { // must hold lock
97 Reference<? extends T> r = head;
98 if (r != null) {
99 r.queue = NULL;
100 // Update r.queue *before* removing from list, to avoid
101 // race with concurrent enqueued checks and fast-path
102 // poll(). Volatiles ensure ordering.
103 @SuppressWarnings("unchecked")
104 Reference<? extends T> rn = r.next;
105 // Handle self-looped next as end of list designator.
106 head = (rn == r) ? null : rn;
107 // Self-loop next rather than setting to null, so if a
108 // FinalReference it remains inactive.
109 r.next = r;
110 queueLength--;
111 if (r instanceof FinalReference) {
112 VM.addFinalRefCount(-1);
113 }
114 return r;
115 }
116 return null;
117 }
118
119 private Reference<? extends T> remove0(long timeout) throws InterruptedException { // must hold lock
120 Reference<? extends T> r = poll0();
121 if (r != null) return r;
122 long start = System.nanoTime();
123 for (;;) {
124 lock.wait(timeout);
125 r = poll0();
126 if (r != null) return r;
127 long end = System.nanoTime();
128 timeout -= (end - start) / 1000_000;
129 if (timeout <= 0) return null;
130 start = end;
131 }
132 }
133
134 private Reference<? extends T> remove0() throws InterruptedException { // must hold lock
135 for (;;) {
136 var r = poll0();
137 if (r != null) return r;
138 lock.wait();
139 }
140 }
141
142 boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
143 synchronized (lock) {
144 return enqueue0(r);
145 }
146 }
147
148 /**
149 * Polls this queue to see if a reference object is available. If one is
150 * available without further delay then it is removed from the queue and
151 * returned. Otherwise this method immediately returns {@code null}.
152 *
153 * @return A reference object, if one was immediately available,
154 * otherwise {@code null}
155 * @see java.lang.ref.Reference#enqueue()
156 */
157 public Reference<? extends T> poll() {
158 if (head == null)
159 return null;
160
161 // Prevent a virtual thread from being preempted as this could potentially
162 // deadlock with a carrier that is polling the same reference queue.
163 boolean pinned = Thread.currentThread().isVirtual() && ContinuationSupport.pinIfSupported();
164 try {
165 synchronized (lock) {
166 return poll0();
167 }
168 } finally {
169 if (pinned) Continuation.unpin();
170 }
171 }
172
173 /**
174 * Removes the next reference object in this queue, blocking until either
175 * one becomes available or the given timeout period expires.
176 *
177 * <p> This method does not offer real-time guarantees: It schedules the
178 * timeout as if by invoking the {@link Object#wait(long)} method.
179 *
180 * @param timeout If positive, block for up to {@code timeout}
181 * milliseconds while waiting for a reference to be
182 * added to this queue. If zero, block indefinitely.
183 *
184 * @return A reference object, if one was available within the specified
185 * timeout period, otherwise {@code null}
186 *
187 * @throws IllegalArgumentException
188 * If the value of the timeout argument is negative
189 *
190 * @throws InterruptedException
191 * If the timeout wait is interrupted
192 *
193 * @see java.lang.ref.Reference#enqueue()
194 */
195 public Reference<? extends T> remove(long timeout) throws InterruptedException {
196 if (timeout < 0)
197 throw new IllegalArgumentException("Negative timeout value");
198 if (timeout == 0)
199 return remove();
200
201 synchronized (lock) {
202 return remove0(timeout);
203 }
204 }
205
206 /**
207 * Removes the next reference object in this queue, blocking until one
208 * becomes available.
209 *
210 * @return A reference object, blocking until one becomes available
211 * @throws InterruptedException If the wait is interrupted
212 * @see java.lang.ref.Reference#enqueue()
213 */
214 public Reference<? extends T> remove() throws InterruptedException {
215 synchronized (lock) {
216 return remove0();
217 }
218 }
219
220 /**
221 * Iterate queue and invoke given action with each Reference.
222 * Suitable for diagnostic purposes.
223 * WARNING: any use of this method should make sure to not
224 * retain the referents of iterated references (in case of
225 * FinalReference(s)) so that their life is not prolonged more
226 * than necessary.
227 */
228 void forEach(Consumer<? super Reference<? extends T>> action) {
229 for (Reference<? extends T> r = head; r != null;) {
230 action.accept(r);
231 @SuppressWarnings("unchecked")
232 Reference<? extends T> rn = r.next;
233 if (rn == r) {
234 if (r.queue == ENQUEUED) {
235 // still enqueued -> we reached end of chain
236 r = null;
237 } else {
238 // already dequeued: r.queue == NULL; ->
239 // restart from head when overtaken by queue poller(s)
240 r = head;
241 }
242 } else {
243 // next in chain
244 r = rn;
245 }
246 }
247 }
248 }