--- /dev/null Thu Jan 01 00:00:00 1970 +0000
+++ b/src/java.base/share/classes/java/lang/ref/ReferenceQueue.java Tue Sep 12 19:03:39 2017 +0200
@@ -0,0 +1,203 @@
+/*
+ * 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;
+ }
+ }
+ }
+}