/**
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 Oct 19, 2010
*/
package com.bigdata.relation.accesspath;
import java.util.NoSuchElementException;
import java.util.Queue;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.locks.ReentrantLock;
import org.apache.log4j.Logger;
import cutthecrap.utils.striterators.ICloseableIterator;
/**
* Class allows new sources to be attached dynamically. If the existing sources
* are drained then the iterator will {@link #close()} itself so that new
* sources can no longer be attached.
*
* @author <a href="mailto:thompsonbry@users.sourceforge.net">Bryan Thompson</a>
* @version $Id$
*/
public class MultiSourceSequentialCloseableIterator<E> implements
IMultiSourceCloseableIterator<E> {
private final static Logger log = Logger.getLogger(MultiSourceSequentialCloseableIterator.class);
private final static boolean INFO = log.isInfoEnabled();
private final ReentrantLock lock = new ReentrantLock();
private final Queue<ICloseableIterator<E>> sources = new LinkedBlockingQueue<ICloseableIterator<E>>();
/**
* The current inner iterator. When <code>null</code> the outer iterator has
* been closed and will not deliver any more results and will not accept any
* new sources.
* <p>
* Note: This can be asynchronously closed if the application invokes
* {@link #close()}. Methods which test on this can not assume that it will
* be non-<code>null</code> the next time they check unless they are holding
* the {@link #lock}. Methods which do not obtain the lock can offer a
* weaker atomicity by copying the reference to a local variable and then
* testing that variable.
*/
private volatile ICloseableIterator<E> current;
public MultiSourceSequentialCloseableIterator(final ICloseableIterator<E> src) {
current = src;
}
@Override
public void close() {
lock.lock();
try {
/*
* Ensure that all sources are eventually closed.
*/
// close the current source (if any).
final ICloseableIterator<E> current = this.current;
this.current = null;
if (current != null) {
if (INFO)
log.info("Closing source: " + current);
current.close();
}
// Close any sources still in the queue.
for(ICloseableIterator<E> t : sources) {
if (INFO)
log.info("Closing source: " + t);
t.close();
}
// Clear the queue.
sources.clear();
} finally {
lock.unlock();
}
}
@Override
public boolean add(final ICloseableIterator<E> src) {
if (src == null)
throw new IllegalArgumentException();
lock.lock();
try {
if (current == null)
return false;
sources.add(src);
return true;
} finally {
lock.unlock();
}
}
/**
* If the current source is not exhausted, then return it immediately.
* Otherwise, return the next source which is not exhausted. If no such
* sources are available, then {@link #close()} the iterator. The decision
* to accept another source or to close the iterator is made atomic by the
* use of the {@link #lock} in this method and in {@link #close()}.
*
* @return The next source -or- <code>null</code> if there are no sources
* available.
*/
private ICloseableIterator<E> nextSource() {
final ICloseableIterator<E> tmp = current;
if (tmp == null)
return null;
if (tmp.hasNext())
return current; // Note: MAY be asynchronously cleared!
// current is known to be [null].
lock.lock();
try {
/**
* Close iterator which has been consumed.
*
* @see <a
* href="https://sourceforge.net/apps/trac/bigdata/ticket/706"
* > MultiSourceSequentialCloseableIterator.nextSource() can
* throw NPE </a>
*/
ICloseableIterator<E> t = this.current;
{
if (t != null) {
if (INFO)
log.info("Closing source: " + t);
t.close();
}
}
// remove the head of the queue (non-blocking)
while ((t = current = sources.poll()) != null) {
if (t.hasNext()) {
return t;
} else {
// Note: should already be closed since exhausted.
t.close();
}
}
// no more sources with data, close while holding lock.
close();
return null;
} finally {
lock.unlock();
}
}
@Override
public boolean hasNext() {
while (true) {
final ICloseableIterator<E> tmp = nextSource();
if (tmp == null)
return false;
if (tmp.hasNext())
return true;
}
}
/**
* {@inheritDoc}
*
* @todo Due to the inherent non-atomicity of the while(hasNext()) next()
* idiom, it is possible for {@link #hasNext()} to report true and for
* {@link #next()} to throw {@link NoSuchElementException} if the
* iterator has been concurrently closed.
*/
@Override
public E next() {
while (true) {
final ICloseableIterator<E> tmp = nextSource();
if (tmp == null)
throw new NoSuchElementException();
if (tmp.hasNext())
return tmp.next();
}
}
@Override
public void remove() {
throw new UnsupportedOperationException();
}
}