/*

  * 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

  * limitations under the License.

  */

 package org.apache.commons.lang3.concurrent;

 import java.util.concurrent.atomic.AtomicReference;

 /**

  * <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 "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

  * {@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

  * @version $Id: AtomicSafeInitializer.java 1309977 2012-04-05 17:53:39Z ggregory $

  * @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. */

     private final AtomicReference<AtomicSafeInitializer<T>> factory =

             new AtomicReference<AtomicSafeInitializer<T>>();

     /** Holds the reference to the managed object. */

     private final AtomicReference<T> reference = new AtomicReference<T>();

     /**

      * 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

      * {@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;

 }