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}