| /* |
| * Copyright (c) 1997, 2015, Oracle and/or its affiliates. All rights reserved. |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
| * |
| * This code is free software; you can redistribute it and/or modify it |
| * under the terms of the GNU General Public License version 2 only, as |
| * published by the Free Software Foundation. Oracle designates this |
| * particular file as subject to the "Classpath" exception as provided |
| * by Oracle in the LICENSE file that accompanied this code. |
| * |
| * This code is distributed in the hope that it will be useful, but WITHOUT |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
| * version 2 for more details (a copy is included in the LICENSE file that |
| * accompanied this code). |
| * |
| * You should have received a copy of the GNU General Public License version |
| * 2 along with this work; if not, write to the Free Software Foundation, |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
| * |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
| * or visit www.oracle.com if you need additional information or have any |
| * questions. |
| */ |
| |
| package java.lang.ref; |
| |
| import java.util.function.Consumer; |
| import jdk.internal.misc.VM; |
| |
| /** |
| * Reference queues, to which registered reference objects are appended by the |
| * garbage collector after the appropriate reachability changes are detected. |
| * |
| * @author Mark Reinhold |
| * @since 1.2 |
| */ |
| |
| public class ReferenceQueue<T> { |
| |
| /** |
| * Constructs a new reference-object queue. |
| */ |
| public ReferenceQueue() { } |
| |
| private static class Null<S> extends ReferenceQueue<S> { |
| boolean enqueue(Reference<? extends S> r) { |
| return false; |
| } |
| } |
| |
| static ReferenceQueue<Object> NULL = new Null<>(); |
| static ReferenceQueue<Object> ENQUEUED = new Null<>(); |
| |
| private static class Lock { }; |
| private Lock lock = new Lock(); |
| private volatile Reference<? extends T> head; |
| private long queueLength = 0; |
| |
| boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */ |
| synchronized (lock) { |
| // Check that since getting the lock this reference hasn't already been |
| // enqueued (and even then removed) |
| ReferenceQueue<?> queue = r.queue; |
| if ((queue == NULL) || (queue == ENQUEUED)) { |
| return false; |
| } |
| assert queue == this; |
| r.next = (head == null) ? r : head; |
| head = r; |
| queueLength++; |
| // Update r.queue *after* adding to list, to avoid race |
| // with concurrent enqueued checks and fast-path poll(). |
| // Volatiles ensure ordering. |
| r.queue = ENQUEUED; |
| if (r instanceof FinalReference) { |
| VM.addFinalRefCount(1); |
| } |
| lock.notifyAll(); |
| return true; |
| } |
| } |
| |
| private Reference<? extends T> reallyPoll() { /* Must hold lock */ |
| Reference<? extends T> r = head; |
| if (r != null) { |
| r.queue = NULL; |
| // Update r.queue *before* removing from list, to avoid |
| // race with concurrent enqueued checks and fast-path |
| // poll(). Volatiles ensure ordering. |
| @SuppressWarnings("unchecked") |
| Reference<? extends T> rn = r.next; |
| head = (rn == r) ? null : rn; |
| r.next = r; |
| queueLength--; |
| if (r instanceof FinalReference) { |
| VM.addFinalRefCount(-1); |
| } |
| return r; |
| } |
| return null; |
| } |
| |
| /** |
| * Polls this queue to see if a reference object is available. If one is |
| * available without further delay then it is removed from the queue and |
| * returned. Otherwise this method immediately returns {@code null}. |
| * |
| * @return A reference object, if one was immediately available, |
| * otherwise {@code null} |
| */ |
| public Reference<? extends T> poll() { |
| if (head == null) |
| return null; |
| synchronized (lock) { |
| return reallyPoll(); |
| } |
| } |
| |
| /** |
| * Removes the next reference object in this queue, blocking until either |
| * one becomes available or the given timeout period expires. |
| * |
| * <p> This method does not offer real-time guarantees: It schedules the |
| * timeout as if by invoking the {@link Object#wait(long)} method. |
| * |
| * @param timeout If positive, block for up to {@code timeout} |
| * milliseconds while waiting for a reference to be |
| * added to this queue. If zero, block indefinitely. |
| * |
| * @return A reference object, if one was available within the specified |
| * timeout period, otherwise {@code null} |
| * |
| * @throws IllegalArgumentException |
| * If the value of the timeout argument is negative |
| * |
| * @throws InterruptedException |
| * If the timeout wait is interrupted |
| */ |
| public Reference<? extends T> remove(long timeout) |
| throws IllegalArgumentException, InterruptedException |
| { |
| if (timeout < 0) { |
| throw new IllegalArgumentException("Negative timeout value"); |
| } |
| synchronized (lock) { |
| Reference<? extends T> r = reallyPoll(); |
| if (r != null) return r; |
| long start = (timeout == 0) ? 0 : System.nanoTime(); |
| for (;;) { |
| lock.wait(timeout); |
| r = reallyPoll(); |
| if (r != null) return r; |
| if (timeout != 0) { |
| long end = System.nanoTime(); |
| timeout -= (end - start) / 1000_000; |
| if (timeout <= 0) return null; |
| start = end; |
| } |
| } |
| } |
| } |
| |
| /** |
| * Removes the next reference object in this queue, blocking until one |
| * becomes available. |
| * |
| * @return A reference object, blocking until one becomes available |
| * @throws InterruptedException If the wait is interrupted |
| */ |
| public Reference<? extends T> remove() throws InterruptedException { |
| return remove(0); |
| } |
| |
| /** |
| * Iterate queue and invoke given action with each Reference. |
| * Suitable for diagnostic purposes. |
| * WARNING: any use of this method should make sure to not |
| * retain the referents of iterated references (in case of |
| * FinalReference(s)) so that their life is not prolonged more |
| * than necessary. |
| */ |
| void forEach(Consumer<? super Reference<? extends T>> action) { |
| for (Reference<? extends T> r = head; r != null;) { |
| action.accept(r); |
| @SuppressWarnings("unchecked") |
| Reference<? extends T> rn = r.next; |
| if (rn == r) { |
| if (r.queue == ENQUEUED) { |
| // still enqueued -> we reached end of chain |
| r = null; |
| } else { |
| // already dequeued: r.queue == NULL; -> |
| // restart from head when overtaken by queue poller(s) |
| r = head; |
| } |
| } else { |
| // next in chain |
| r = rn; |
| } |
| } |
| } |
| } |