| /* |
| * 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 $Id$ |
| * @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 |
| */ |
| 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; |
| } |