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 * @version $Id: AtomicSafeInitializer.java 1309977 2012-04-05 17:53:39Z ggregory $
054 * @param <T> the type of the object managed by this initializer class
055 */
056public 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    @Override
073    public final T get() throws ConcurrentException {
074        T result;
075
076        while ((result = reference.get()) == null) {
077            if (factory.compareAndSet(null, this)) {
078                reference.set(initialize());
079            }
080        }
081
082        return result;
083    }
084
085    /**
086     * Creates and initializes the object managed by this
087     * {@code AtomicInitializer}. This method is called by {@link #get()} when
088     * the managed object is not available yet. An implementation can focus on
089     * the creation of the object. No synchronization is needed, as this is
090     * already handled by {@code get()}. This method is guaranteed to be called
091     * only once.
092     *
093     * @return the managed data object
094     * @throws ConcurrentException if an error occurs during object creation
095     */
096    protected abstract T initialize() throws ConcurrentException;
097}