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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
 * limitations under the License.
 */
package org.apache.commons.lang3.concurrent;

import java.util.concurrent.atomic.AtomicReference;

/**
 * 

* A specialized {@code ConcurrentInitializer} implementation which is similar * to {@link AtomicInitializer}, but ensures that the {@link #initialize()} * method is called only once. *

*

* 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. *

*

* 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. *

*

* 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. *

* * @since 3.0 * @param the type of the object managed by this initializer class */ public abstract class AtomicSafeInitializer implements ConcurrentInitializer { /** A guard which ensures that initialize() is called only once. */ private final AtomicReference> factory = new AtomicReference<>(); /** Holds the reference to the managed object. */ private final AtomicReference reference = new AtomicReference<>(); /** * 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; }




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