/*
Copyright (C) SYSTAP, LLC DBA Blazegraph 2006-2016. All rights reserved.
Contact:
SYSTAP, LLC DBA Blazegraph
2501 Calvert ST NW #106
Washington, DC 20008
licenses@blazegraph.com
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; version 2 of the License.
This program 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 for more details.
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
*/
/*
* Created on Jul 3, 2008
*/
package com.bigdata.concurrent;
import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.TimeoutException;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;
/**
* A factory for named {@link Lock}s. A simple {@link Lock} manages access to a
* single resource. However, a {@link NamedLock} manages access to the members
* of a set of named resources. This is more efficient when the latency of the
* operation once the lock is acquired is significant, e.g., an RMI call or a
* disk IO.
* <p>
* The locks are stored in a {@link WeakHashMap} so that they will be garbage
* collected if there are no threads waiting in the queue for a given named
* lock.
*
* @author <a href="mailto:thompsonbry@users.sourceforge.net">Bryan Thompson</a>
* @version $Id$
* @param <T>
* The generic type for the "name". This MUST implement hashCode()
* and equals() since the instances of this type will serve as keys
* in a {@link Map}.
*/
public class NamedLock<T> {
final private Map<T, ReentrantLock> locks = new WeakHashMap<T, ReentrantLock>();
/**
* Return the canonical instance of the lock for a named resource.
*
* @param name
* The name.
*
* @return The canonical instance of the lock for that name.
*/
protected Lock lockFactory(final T name) {
if (name == null)
throw new IllegalArgumentException();
ReentrantLock lock;
synchronized (locks) {
lock = locks.get(name);
if (lock == null) {
lock = new ReentrantLock();
locks.put(name, lock);
}
}
return lock;
}
/**
* Block until the {@link Lock} for the named resource is available, then
* {@link Lock#lock()} the {@link Lock} and return the locked {@link Lock}.
*
* @param name
* The name of the resource whose {@link Lock} is desired.
*
* @return The {@link Lock}. It will have already been {@link Lock#lock()}ed.
*/
public Lock acquireLock(final T name) {
final Lock lock = lockFactory(name);
lock.lock();
return lock;
}
public Lock acquireLock(final T name, final long timeout, final TimeUnit unit)
throws InterruptedException, TimeoutException {
final Lock lock = lockFactory(name);
if(!lock.tryLock(timeout, unit))
throw new TimeoutException();
return lock;
}
}