java.lang.ref.ReferenceQueue Maven / Gradle / Ivy
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* 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).
*
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* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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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 {
/**
* 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 volatile Reference head = null;
private long queueLength = 0;
boolean enqueue(Reference 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++;
lock.notifyAll();
return true;
}
}
}
private Reference reallyPoll() { /* Must hold lock */
if (head != null) {
Reference r = head;
head = (r.next == r) ? null : r.next;
r.queue = NULL;
r.next = r;
queueLength--;
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 null.
*
* @return A reference object, if one was immediately available,
* otherwise null
*/
public Reference 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.
*
* 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 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 null
*
* @throws IllegalArgumentException
* If the value of the timeout argument is negative
*
* @throws InterruptedException
* If the timeout wait is interrupted
*/
public Reference remove(long timeout)
throws IllegalArgumentException, InterruptedException
{
if (timeout < 0) {
throw new IllegalArgumentException("Negative timeout value");
}
synchronized (lock) {
Reference 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 remove() throws InterruptedException {
return remove(0);
}
}