AbstractWatchKey.java

/*
 * Copyright 2008-2009 Sun Microsystems, Inc.  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.  Sun designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
 * CA 95054 USA or visit www.sun.com if you need additional information or
 * have any questions.
 */

package com.barbarysoftware.watchservice;

import java.util.ArrayList;
import java.util.List;

/**
 * Base implementation class for watch keys.
 */

abstract class AbstractWatchKey extends WatchKey {

    /**
     * Maximum size of event list (in the future this may be tunable)
     */
    static final int MAX_EVENT_LIST_SIZE = 512;

    /**
     * Special event to signal overflow
     */
    static final Event<Void> OVERFLOW_EVENT =
            new Event<Void>(StandardWatchEventKind.OVERFLOW, null);

    /**
     * Possible key states
     */
    private static enum State {
        READY, SIGNALLED
    }

    // reference to watcher
    private final AbstractWatchService watcher;

    // key state
    private State state;

    // pending events
    private List<WatchEvent<?>> events;

    protected AbstractWatchKey(AbstractWatchService watcher) {
        this.watcher = watcher;
        this.state = State.READY;
        this.events = new ArrayList<WatchEvent<?>>();
    }

    final AbstractWatchService watcher() {
        return watcher;
    }

    /**
     * Enqueues this key to the watch service
     */
    final void signal() {
        synchronized (this) {
            if (state == State.READY) {
                state = State.SIGNALLED;
                watcher.enqueueKey(this);
            }
        }
    }

    /**
     * Adds the event to this key and signals it.
     * @param kind   event kind
     * @param context context
     */
    @SuppressWarnings("unchecked")
    final void signalEvent(WatchEvent.Kind<?> kind, Object context) {
        synchronized (this) {
            int size = events.size();
            if (size > 1) {
                // don't let list get too big
                if (size >= MAX_EVENT_LIST_SIZE) {
                    kind = StandardWatchEventKind.OVERFLOW;
                    context = null;
                }

                // repeated event
                WatchEvent<?> prev = events.get(size - 1);
                if (kind == prev.kind()) {
                    boolean isRepeat;
                    if (context == null) {
                        isRepeat = (prev.context() == null);
                    } else {
                        isRepeat = context.equals(prev.context());
                    }
                    if (isRepeat) {
                        ((Event<?>) prev).increment();
                        return;
                    }
                }
            }

            // non-repeated event
            events.add(new Event<Object>((WatchEvent.Kind<Object>) kind, context));
            signal();
        }
    }

    @Override
    public final List<WatchEvent<?>> pollEvents() {
        synchronized (this) {
            List<WatchEvent<?>> result = events;
            events = new ArrayList<WatchEvent<?>>();
            return result;
        }
    }

    @Override
    public final boolean reset() {
        synchronized (this) {
            if (state == State.SIGNALLED && isValid()) {
                if (events.isEmpty()) {
                    state = State.READY;
                } else {
                    // pending events so re-queue key
                    watcher.enqueueKey(this);
                }
            }
            return isValid();
        }
    }

    /**
     * WatchEvent implementation
     */
    private static class Event<T> extends WatchEvent<T> {
        private final WatchEvent.Kind<T> kind;
        private final T context;

        // synchronize on watch key to access/increment count
        private int count;

        Event(WatchEvent.Kind<T> type, T context) {
            this.kind = type;
            this.context = context;
            this.count = 1;
        }

        @Override
        public WatchEvent.Kind<T> kind() {
            return kind;
        }

        @Override
        public T context() {
            return context;
        }

        @Override
        public int count() {
            return count;
        }

        // for repeated events
        void increment() {
            count++;
        }
    }
}