/*
* Copyright (c) 1997, 2005, 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;
/**
* 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 {
boolean enqueue(Reference r) {
return false;
}
}
static ReferenceQueue NULL = new Null();
static ReferenceQueue ENQUEUED = new Null();
static private class Lock { };
private Lock lock = new Lock();
private Reference<? extends T> head = null;
private long queueLength = 0;
boolean enqueue(Reference<? extends T> r) { /* Called only by Reference class */
synchronized (r) {
if (r.queue == ENQUEUED) return false;
synchronized (lock) {
r.queue = ENQUEUED;
r.next = (head == null) ? r : head;
head = r;
queueLength++;
if (r instanceof FinalReference) {
sun.misc.VM.addFinalRefCount(1);
}
lock.notifyAll();
return true;
}
}
}
private Reference<? extends T> reallyPoll() { /* Must hold lock */
if (head != null) {
Reference<? extends T> r = head;
head = (r.next == r) ? null : r.next;
r.queue = NULL;
r.next = r;
queueLength--;
if (r instanceof FinalReference) {
sun.misc.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 <tt>null</tt>.
*
* @return A reference object, if one was immediately available,
* otherwise <code>null</code>
*/
public Reference<? extends T> poll() {
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</code>
* 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</code>
*
* @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;
for (;;) {
lock.wait(timeout);
r = reallyPoll();
if (r != null) return r;
if (timeout != 0) return null;
}
}
}
/**
* 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);
}
}