    001    /*
    002     * Licensed to the Apache Software Foundation (ASF) under one or more
    003     * contributor license agreements.  See the NOTICE file distributed with
    004     * this work for additional information regarding copyright ownership.
    005     * The ASF licenses this file to You under the Apache License, Version 2.0
    006     * (the "License"); you may not use this file except in compliance with
    007     * the License.  You may obtain a copy of the License at
    008     *
    009     *      http://www.apache.org/licenses/LICENSE-2.0
    010     *
    011     * Unless required by applicable law or agreed to in writing, software
    012     * distributed under the License is distributed on an "AS IS" BASIS,
    013     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    014     * See the License for the specific language governing permissions and
    015     * limitations under the License.
    016     */
    017    package org.apache.commons.lang3.concurrent;
    018
    019    import java.util.concurrent.atomic.AtomicReference;
    020
    021    /**
    022     * <p>
    023     * A specialized {@code ConcurrentInitializer} implementation which is similar
    024     * to {@link AtomicInitializer}, but ensures that the {@link #initialize()}
    025     * method is called only once.
    026     * </p>
    027     * <p>
    028     * As {@link AtomicInitializer} this class is based on atomic variables, so it
    029     * can create an object under concurrent access without synchronization.
    030     * However, it implements an additional check to guarantee that the
    031     * {@link #initialize()} method which actually creates the object cannot be
    032     * called multiple times.
    033     * </p>
    034     * <p>
    035     * Because of this additional check this implementation is slightly less
    036     * efficient than {@link AtomicInitializer}, but if the object creation in the
    037     * {@code initialize()} method is expensive or if multiple invocations of
    038     * {@code initialize()} are problematic, it is the better alternative.
    039     * </p>
    040     * <p>
    041     * From its semantics this class has the same properties as
    042     * {@link LazyInitializer}. It is a &quot;save&quot; implementation of the lazy
    043     * initializer pattern. Comparing both classes in terms of efficiency is
    044     * difficult because which one is faster depends on multiple factors. Because
    045     * {@code AtomicSafeInitializer} does not use synchronization at all it probably
    046     * outruns {@link LazyInitializer}, at least under low or moderate concurrent
    047     * access. Developers should run their own benchmarks on the expected target
    048     * platform to decide which implementation is suitable for their specific use
    049     * case.
    050     * </p>
    051     *
    052     * @since 3.0
    053     * @version $Id: AtomicSafeInitializer.java 1088899 2011-04-05 05:31:27Z bayard $
    054     * @param <T> the type of the object managed by this initializer class
    055     */
    056    public abstract class AtomicSafeInitializer<T> implements
    057            ConcurrentInitializer<T> {
    058        /** A guard which ensures that initialize() is called only once. */
    059        private final AtomicReference<AtomicSafeInitializer<T>> factory =
    060                new AtomicReference<AtomicSafeInitializer<T>>();
    061
    062        /** Holds the reference to the managed object. */
    063        private final AtomicReference<T> reference = new AtomicReference<T>();
    064
    065        /**
    066         * Get (and initialize, if not initialized yet) the required object
    067         *
    068         * @return lazily initialized object
    069         * @throws ConcurrentException if the initialization of the object causes an
    070         * exception
    071         */
    072        public final T get() throws ConcurrentException {
    073            T result;
    074
    075            while ((result = reference.get()) == null) {
    076                if (factory.compareAndSet(null, this)) {
    077                    reference.set(initialize());
    078                }
    079            }
    080
    081            return result;
    082        }
    083
    084        /**
    085         * Creates and initializes the object managed by this
    086         * {@code AtomicInitializer}. This method is called by {@link #get()} when
    087         * the managed object is not available yet. An implementation can focus on
    088         * the creation of the object. No synchronization is needed, as this is
    089         * already handled by {@code get()}. This method is guaranteed to be called
    090         * only once.
    091         *
    092         * @return the managed data object
    093         * @throws ConcurrentException if an error occurs during object creation
    094         */
    095        protected abstract T initialize() throws ConcurrentException;
    096    }