/* * Copyright (c) 2008, 2011, 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 sun.nio.fs; import java.nio.file.*;import java.util.concurrent.*;import java.io.IOException; /** * Base implementation class for watch services. */ abstract class AbstractWatchService implements WatchService {     // signaled keys waiting to be dequeued    private final LinkedBlockingDeque<WatchKey> pendingKeys =        new LinkedBlockingDeque<WatchKey>();     // special key to indicate that watch service is closed    private final WatchKey CLOSE_KEY =        new AbstractWatchKey(null, null) {            @Override            public boolean isValid() {                return true;            }             @Override            public void cancel() {            }        };     // used when closing watch service    private volatile boolean closed;    private final Object closeLock = new Object();     protected AbstractWatchService() {    }     /**     * Register the given object with this watch service     */    abstract WatchKey register(Path path,                               WatchEvent.Kind<?>[] events,                               WatchEvent.Modifier... modifers)        throws IOException;     // used by AbstractWatchKey to enqueue key    final void enqueueKey(WatchKey key) {        pendingKeys.offer(key);    }     /**     * Throws ClosedWatchServiceException if watch service is closed     */    private void checkOpen() {        if (closed)            throw new ClosedWatchServiceException();    }     /**     * Checks the key isn't the special CLOSE_KEY used to unblock threads when     * the watch service is closed.     */    private void checkKey(WatchKey key) {        if (key == CLOSE_KEY) {            // re-queue in case there are other threads blocked in take/poll            enqueueKey(key);        }        checkOpen();    }     @Override    public final WatchKey poll() {        checkOpen();        WatchKey key = pendingKeys.poll();        checkKey(key);        return key;    }     @Override    public final WatchKey poll(long timeout, TimeUnit unit)        throws InterruptedException    {        checkOpen();        WatchKey key = pendingKeys.poll(timeout, unit);        checkKey(key);        return key;    }     @Override    public final WatchKey take()        throws InterruptedException    {        checkOpen();        WatchKey key = pendingKeys.take();        checkKey(key);        return key;    }     /**     * Tells whether or not this watch service is open.     */    final boolean isOpen() {        return !closed;    }     /**     * Retrieves the object upon which the close method synchronizes.     */    final Object closeLock() {        return closeLock;    }     /**     * Closes this watch service. This method is invoked by the close     * method to perform the actual work of closing the watch service.     */    abstract void implClose() throws IOException;     @Override    public final void close()        throws IOException    {        synchronized (closeLock) {            // nothing to do if already closed            if (closed)                return;            closed = true;             implClose();             // clear pending keys and queue special key to ensure that any            // threads blocked in take/poll wakeup            pendingKeys.clear();            pendingKeys.offer(CLOSE_KEY);        }    }}