    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 */
    017package org.apache.commons.lang3.concurrent;
    018
    019import 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 * </p>
    031 * <ul>
    032 * <li>First it is checked whether the {@code AtomicReference} variable contains
    033 * already a value. If this is the case, the value is directly returned.</li>
    034 * <li>Otherwise the {@link #initialize()} method is called. This method must be
    035 * defined in concrete subclasses to actually create the managed object.</li>
    036 * <li>After the object was created by {@link #initialize()} it is checked
    037 * whether the {@code AtomicReference} variable is still undefined. This has to
    038 * be done because in the meantime another thread may have initialized the
    039 * object. If the reference is still empty, the newly created object is stored
    040 * in it and returned by this method.</li>
    041 * <li>Otherwise the value stored in the {@code AtomicReference} is returned.</li>
    042 * </ul>
    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 * @param <T> the type of the object managed by this initializer class
    065 */
    066public abstract class AtomicInitializer<T> implements ConcurrentInitializer<T> {
    067    /** Holds the reference to the managed object. */
    068    private final AtomicReference<T> reference = new AtomicReference<>();
    069
    070    /**
    071     * Returns the object managed by this initializer. The object is created if
    072     * it is not available yet and stored internally. This method always returns
    073     * the same object.
    074     *
    075     * @return the object created by this {@code AtomicInitializer}
    076     * @throws ConcurrentException if an error occurred during initialization of
    077     * the object
    078     */
    079    @Override
    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}