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Diffstat (limited to 'src/main/java/org/apache/commons/lang3/concurrent/AtomicSafeInitializer.java')
-rw-r--r-- | src/main/java/org/apache/commons/lang3/concurrent/AtomicSafeInitializer.java | 95 |
1 files changed, 95 insertions, 0 deletions
diff --git a/src/main/java/org/apache/commons/lang3/concurrent/AtomicSafeInitializer.java b/src/main/java/org/apache/commons/lang3/concurrent/AtomicSafeInitializer.java new file mode 100644 index 000000000..3616a86da --- /dev/null +++ b/src/main/java/org/apache/commons/lang3/concurrent/AtomicSafeInitializer.java @@ -0,0 +1,95 @@ +/* + * 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 {@link ConcurrentInitializer} implementation which is similar + * to {@link AtomicInitializer}, but ensures that the {@link #initialize()} + * method is called only once. + * + * <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 + * {@link 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. */ + private final AtomicReference<AtomicSafeInitializer<T>> factory = + new AtomicReference<>(); + + /** Holds the reference to the managed object. */ + private final AtomicReference<T> reference = new AtomicReference<>(); + + /** + * Gets (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 + * {@link 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; +} |