Java tutorial
/* * Copyright (c) 1997, 2018, 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 extends ReferenceQueue<Object> { boolean enqueue(Reference<?> r) { return false; } } static final ReferenceQueue<Object> NULL = new Null(); static final ReferenceQueue<Object> ENQUEUED = new Null(); private static class Lock { }; private final 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; // Self-loop end, so if a FinalReference it remains inactive. 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; // Handle self-looped next as end of list designator. head = (rn == r) ? null : rn; // Self-loop next rather than setting to null, so if a // FinalReference it remains inactive. 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; } } } }