simpleselector.java

jxta_src_2.41b jxta 2.41b 最新版源码 from www.jxta.org

/*
 *
 * $Id: SimpleSelector.java,v 1.4 2005/09/01 18:00:32 bondolo Exp $
 *
 * Copyright (c) 2004 Sun Microsystems, Inc.  All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 *
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in
 *    the documentation and/or other materials provided with the
 *    distribution.
 *
 * 3. The end-user documentation included with the redistribution,
 *    if any, must include the following acknowledgment:
 *       "This product includes software developed by the
 *       Sun Microsystems, Inc. for Project JXTA."
 *    Alternately, this acknowledgment may appear in the software itself,
 *    if and wherever such third-party acknowledgments normally appear.
 *
 * 4. The names "Sun", "Sun Microsystems, Inc.", "JXTA" and "Project JXTA"
 *    must not be used to endorse or promote products derived from this
 *    software without prior written permission. For written
 *    permission, please contact Project JXTA at http://www.jxta.org.
 *
 * 5. Products derived from this software may not be called "JXTA",
 *    nor may "JXTA" appear in their name, without prior written
 *    permission of Sun.
 *
 * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESSED OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
 * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * DISCLAIMED.  IN NO EVENT SHALL SUN MICROSYSTEMS OR
 * ITS CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
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 * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF
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 * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
 * OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 * ====================================================================
 *
 * This software consists of voluntary contributions made by many
 * individuals on behalf of Project JXTA.  For more
 * information on Project JXTA, please see
 * <http://www.jxta.org/>.
 *
 * This license is based on the BSD license adopted by the Apache Foundation.
 */

package net.jxta.util;

import java.util.Set;
import java.util.List;
import java.util.Arrays;
import java.util.HashSet;

/**
 * A very primitive version of NIO's select mechanism.  Applications should not code to that API yet. This is subject to change
 * and for use only by internal mechanisms. This is only good at implementing efficient polling. Users must wait for a batch of
 * state changes and then figure out which item's state changed to something interresting. The batch returned by simpleSelect is
 * a set of the items which state did change since the previous time select returned.  Since a batch is returned as soon as there
 * is at least one item in it, most batches are very small, they will contain the first item that was added, plus whatever few
 * could sneak in between then and when select wakes-up and grabs that batch.
 **/

public final class SimpleSelector extends AbstractSimpleSelectable {

    /*
     * A simpleSelector It is not very usuable by multiple threads at once, unless they all do the same thing. Items are removed
     * from the current batch when select returns it. As a result, any state change occuring in-between two calls to select is
     * guaranteed to be reported by the next call. However, the new state may be observed before the next select call to select
     * returns the corresponding event, which may then be viewed as redundant. So events are reported too much rather than too
     * little.
     */

    /**
     * The small set of items that changed since the last batch was returned
     * by select.
     **/
    private final Set currentBatch = new HashSet(2);

    /**
     * Let it be newed for now. We need to find a place for a factory.
     **/
    public SimpleSelector() {
    }

    /**
     * This is invoked by registered items when their state changes. Records changes for the benefit of {@link #select()},
     * and performs notifyChange(), which will cause notification of cascaded selectors, if any. It is thus possible to register
     * selectors with a selector and come to a particular one only when it has something to report.
     **/
    public final void itemChanged(SimpleSelectable item) {
        synchronized(currentBatch) {
            currentBatch.add(item.getIdentityReference());
            currentBatch.notifyAll();
        }
        notifyChange();
    }

    /**
     * This blocks unless and until at least one of the selected items reports to have changed. Then the list of such items
     * is returned. More than one item may be added to the list by the time it is returned to the invoker.
     *
     * The name is deliberately not "select", so that we easily catch spots where retrofitting is required
     * when moving to NIO (if ever). Note: the result cannot be a set, otherwise we would be prevented
     * from returning objects that overload hashCode/equals. It is thus a List. However, an item will never be found twice
     * in that list.
     *
     * The invoker should <b>never</b> assume that all items in the result have indeed changed in any expected manner or even
     * changed at all. The simple action of registering a selector may and usually does cause the selectable object to report
     * a change. In some cases a selectable object may just be unable to prove that it has not changed, and thus report a
     * change. It is up to the invoker to inspect the relevant item's state every time.
     **/
    public List select() throws InterruptedException {

        Object[] result = null;
        int resLen = 0;

        synchronized(currentBatch) {
            while ((resLen = currentBatch.size()) == 0) {
                currentBatch.wait();
            }
            result = new Object[resLen];
            currentBatch.toArray(result);
            currentBatch.clear();
        }

        // Now we have to retrieve the real objects behind the identity references.
        // Costly, but unavoidable.
        int i = resLen;
        while (i-->0) {
            result[i] = ((IdentityReference) result[i]).getObject();
        }
        return Arrays.asList(result);
    }
}