Pool.java

/*
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
 *
 * Copyright (c) 1997-2010 Oracle and/or its affiliates. All rights reserved.
 *
 * The contents of this file are subject to the terms of either the GNU
 * General Public License Version 2 only ("GPL") or the Common Development
 * and Distribution License("CDDL") (collectively, the "License").  You
 * may not use this file except in compliance with the License.  You can
 * obtain a copy of the License at
 * https://glassfish.dev.java.net/public/CDDL+GPL_1_1.html
 * or packager/legal/LICENSE.txt.  See the License for the specific
 * language governing permissions and limitations under the License.
 *
 * When distributing the software, include this License Header Notice in each
 * file and include the License file at packager/legal/LICENSE.txt.
 *
 * GPL Classpath Exception:
 * Oracle designates this particular file as subject to the "Classpath"
 * exception as provided by Oracle in the GPL Version 2 section of the License
 * file that accompanied this code.
 *
 * Modifications:
 * If applicable, add the following below the License Header, with the fields
 * enclosed by brackets [] replaced by your own identifying information:
 * "Portions Copyright [year] [name of copyright owner]"
 *
 * Contributor(s):
 * If you wish your version of this file to be governed by only the CDDL or
 * only the GPL Version 2, indicate your decision by adding "[Contributor]
 * elects to include this software in this distribution under the [CDDL or GPL
 * Version 2] license."  If you don't indicate a single choice of license, a
 * recipient has the option to distribute your version of this file under
 * either the CDDL, the GPL Version 2 or to extend the choice of license to
 * its licensees as provided above.  However, if you add GPL Version 2 code
 * and therefore, elected the GPL Version 2 license, then the option applies
 * only if the new code is made subject to such option by the copyright
 * holder.
 */

package com.sun.istack;

import java.util.concurrent.ConcurrentLinkedQueue;
import java.lang.ref.WeakReference;

/**
 * Pool of reusable objects that are indistinguishable from each other,
 * such as JAXB marshallers.
 *
 * @author Kohsuke Kawaguchi
 */
public interface Pool<T> {

    /**
     * Gets a new object from the pool.
     *
     * <p>
     * If no object is available in the pool, this method creates a new one.
     */
    @NotNull T take();

    /**
     * Returns an object back to the pool.
     */
    void recycle(@NotNull T t);

    /**
     * Default implementation that uses {@link ConcurrentLinkedQueue}
     * as the data store.
     *
     * <h2>Note for Implementors</h2>
     * <p>
     * Don't rely on the fact that this class extends from {@link ConcurrentLinkedQueue}.
     */
    public abstract class Impl<T> implements Pool<T> {

        private volatile WeakReference<ConcurrentLinkedQueue<T>> queue;

        /**
         * Gets a new object from the pool.
         *
         * <p>
         * If no object is available in the pool, this method creates a new one.
         *
         * @return
         *      always non-null.
         */
        public final @NotNull T take() {
            T t = getQueue().poll();
            if(t==null) {
                return create();
            }
            return t;
        }

        /**
         * Returns an object back to the pool.
         */
        public final void recycle(T t) {
            getQueue().offer(t);
        }

        private ConcurrentLinkedQueue<T> getQueue() {
            WeakReference<ConcurrentLinkedQueue<T>> q = queue;
            if (q != null) {
                ConcurrentLinkedQueue<T> d = q.get();
                if (d != null) {
                    return d;
                }
            }
            // overwrite the queue
            ConcurrentLinkedQueue<T> d = new ConcurrentLinkedQueue<T>();
            queue = new WeakReference<ConcurrentLinkedQueue<T>>(d);

            return d;
        }

        /**
         * Creates a new instance of object.
         *
         * <p>
         * This method is used when someone wants to
         * {@link #take() take} an object from an empty pool.
         *
         * <p>
         * Also note that multiple threads may call this method
         * concurrently.
         */
        protected abstract @NotNull T create();
    }
}