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 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
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package org.apache.commons.lang3.concurrent;

import java.util.concurrent.atomic.AtomicReference;

 A specialized ConcurrentInitializer implementation which is similar to AtomicInitializer, but ensures that the initialize() method is called only once. 
 As AtomicInitializer this class is based on atomic variables, so it can create an object under concurrent access without synchronization. However, it implements an additional check to guarantee that the initialize() method which actually creates the object cannot be called multiple times. 
 Because of this additional check this implementation is slightly less efficient than AtomicInitializer, but if the object creation in the initialize() method is expensive or if multiple invocations of initialize() are problematic, it is the better alternative. 
 From its semantics this class has the same properties as LazyInitializer. It is a "save" implementation of the lazy initializer pattern. Comparing both classes in terms of efficiency is difficult because which one is faster depends on multiple factors. Because AtomicSafeInitializer does not use synchronization at all it probably outruns LazyInitializer, at least under low or moderate concurrent access. Developers should run their own benchmarks on the expected target platform to decide which implementation is suitable for their specific use case. 
Type parameters:
<T> – the type of the object managed by this initializer class
Since:
3.0/**
 * <p>
 * A specialized {@code ConcurrentInitializer} implementation which is similar
 * to {@link AtomicInitializer}, but ensures that the {@link #initialize()}
 * method is called only once.
 * </p>
 * <p>
 * As {@link AtomicInitializer} this class is based on atomic variables, so it
 * can create an object under concurrent access without synchronization.
 * However, it implements an additional check to guarantee that the
 * {@link #initialize()} method which actually creates the object cannot be
 * called multiple times.
 * </p>
 * <p>
 * Because of this additional check this implementation is slightly less
 * efficient than {@link AtomicInitializer}, but if the object creation in the
 * {@code initialize()} method is expensive or if multiple invocations of
 * {@code initialize()} are problematic, it is the better alternative.
 * </p>
 * <p>
 * From its semantics this class has the same properties as
 * {@link LazyInitializer}. It is a &quot;save&quot; implementation of the lazy
 * initializer pattern. Comparing both classes in terms of efficiency is
 * difficult because which one is faster depends on multiple factors. Because
 * {@code AtomicSafeInitializer} does not use synchronization at all it probably
 * outruns {@link LazyInitializer}, at least under low or moderate concurrent
 * access. Developers should run their own benchmarks on the expected target
 * platform to decide which implementation is suitable for their specific use
 * case.
 * </p>
 *
 * @since 3.0
 * @param <T> the type of the object managed by this initializer class
 */
public abstract class AtomicSafeInitializer<T> implements
        ConcurrentInitializer<T> {
    
A guard which ensures that initialize() is called only once. /** A guard which ensures that initialize() is called only once. */
    private final AtomicReference<AtomicSafeInitializer<T>> factory =
            new AtomicReference<>();

    
Holds the reference to the managed object. /** Holds the reference to the managed object. */
    private final AtomicReference<T> reference = new AtomicReference<>();

    
Get (and initialize, if not initialized yet) the required object
Throws:
ConcurrentException – if the initialization of the object causes an
exception
Returns:
lazily initialized object/**
     * Get (and initialize, if not initialized yet) the required object
     *
     * @return lazily initialized object
     * @throws ConcurrentException if the initialization of the object causes an
     * exception
     */
    @Override
    public final T get() throws ConcurrentException {
        T result;

        while ((result = reference.get()) == null) {
            if (factory.compareAndSet(null, this)) {
                reference.set(initialize());
            }
        }

        return result;
    }

    
Creates and initializes the object managed by this AtomicInitializer. This method is called by get() when the managed object is not available yet. An implementation can focus on the creation of the object. No synchronization is needed, as this is already handled by get(). This method is guaranteed to be called only once. 
Throws:
ConcurrentException – if an error occurs during object creation
Returns:
the managed data object/**
     * Creates and initializes the object managed by this
     * {@code AtomicInitializer}. This method is called by {@link #get()} when
     * the managed object is not available yet. An implementation can focus on
     * the creation of the object. No synchronization is needed, as this is
     * already handled by {@code get()}. This method is guaranteed to be called
     * only once.
     *
     * @return the managed data object
     * @throws ConcurrentException if an error occurs during object creation
     */
    protected abstract T initialize() throws ConcurrentException;
}