    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 {@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 * @param <T> the type of the object managed by this initializer class
    054 */
    055public abstract class AtomicSafeInitializer<T> implements
    056        ConcurrentInitializer<T> {
    057    /** A guard which ensures that initialize() is called only once. */
    058    private final AtomicReference<AtomicSafeInitializer<T>> factory =
    059            new AtomicReference<>();
    060
    061    /** Holds the reference to the managed object. */
    062    private final AtomicReference<T> reference = new AtomicReference<>();
    063
    064    /**
    065     * Get (and initialize, if not initialized yet) the required object
    066     *
    067     * @return lazily initialized object
    068     * @throws ConcurrentException if the initialization of the object causes an
    069     * exception
    070     */
    071    @Override
    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}