    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 implementation of the {@code ConcurrentInitializer} interface
    024     * based on an {@link AtomicReference} variable.
    025     * </p>
    026     * <p>
    027     * This class maintains a member field of type {@code AtomicReference}. It
    028     * implements the following algorithm to create and initialize an object in its
    029     * {@link #get()} method:
    030     * <ul>
    031     * <li>First it is checked whether the {@code AtomicReference} variable contains
    032     * already a value. If this is the case, the value is directly returned.</li>
    033     * <li>Otherwise the {@link #initialize()} method is called. This method must be
    034     * defined in concrete subclasses to actually create the managed object.</li>
    035     * <li>After the object was created by {@link #initialize()} it is checked
    036     * whether the {@code AtomicReference} variable is still undefined. This has to
    037     * be done because in the meantime another thread may have initialized the
    038     * object. If the reference is still empty, the newly created object is stored
    039     * in it and returned by this method.</li>
    040     * <li>Otherwise the value stored in the {@code AtomicReference} is returned.</li>
    041     * </ul>
    042     * </p>
    043     * <p>
    044     * Because atomic variables are used this class does not need any
    045     * synchronization. So there is no danger of deadlock, and access to the managed
    046     * object is efficient. However, if multiple threads access the {@code
    047     * AtomicInitializer} object before it has been initialized almost at the same
    048     * time, it can happen that {@link #initialize()} is called multiple times. The
    049     * algorithm outlined above guarantees that {@link #get()} always returns the
    050     * same object though.
    051     * </p>
    052     * <p>
    053     * Compared with the {@link LazyInitializer} class, this class can be more
    054     * efficient because it does not need synchronization. The drawback is that the
    055     * {@link #initialize()} method can be called multiple times which may be
    056     * problematic if the creation of the managed object is expensive. As a rule of
    057     * thumb this initializer implementation is preferable if there are not too many
    058     * threads involved and the probability that multiple threads access an
    059     * uninitialized object is small. If there is high parallelism,
    060     * {@link LazyInitializer} is more appropriate.
    061     * </p>
    062     *
    063     * @since 3.0
    064     * @version $Id: AtomicInitializer.java 1088899 2011-04-05 05:31:27Z bayard $
    065     * @param <T> the type of the object managed by this initializer class
    066     */
    067    public abstract class AtomicInitializer<T> implements ConcurrentInitializer<T> {
    068        /** Holds the reference to the managed object. */
    069        private final AtomicReference<T> reference = new AtomicReference<T>();
    070
    071        /**
    072         * Returns the object managed by this initializer. The object is created if
    073         * it is not available yet and stored internally. This method always returns
    074         * the same object.
    075         *
    076         * @return the object created by this {@code AtomicInitializer}
    077         * @throws ConcurrentException if an error occurred during initialization of
    078         * the object
    079         */
    080        public T get() throws ConcurrentException {
    081            T result = reference.get();
    082
    083            if (result == null) {
    084                result = initialize();
    085                if (!reference.compareAndSet(null, result)) {
    086                    // another thread has initialized the reference
    087                    result = reference.get();
    088                }
    089            }
    090
    091            return result;
    092        }
    093
    094        /**
    095         * Creates and initializes the object managed by this {@code
    096         * AtomicInitializer}. This method is called by {@link #get()} when the
    097         * managed object is not available yet. An implementation can focus on the
    098         * creation of the object. No synchronization is needed, as this is already
    099         * handled by {@code get()}. As stated by the class comment, it is possible
    100         * that this method is called multiple times.
    101         *
    102         * @return the managed data object
    103         * @throws ConcurrentException if an error occurs during object creation
    104         */
    105        protected abstract T initialize() throws ConcurrentException;
    106    }