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    }