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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 implementation of the {@link ConcurrentInitializer} interface
* based on an {@link AtomicReference} variable.
*
* <p>
* This class maintains a member field of type {@link AtomicReference}. It
* implements the following algorithm to create and initialize an object in its
* {@link #get()} method:
* </p>
* <ul>
* <li>First it is checked whether the {@link AtomicReference} variable contains
* already a value. If this is the case, the value is directly returned.</li>
* <li>Otherwise the {@link #initialize()} method is called. This method must be
* defined in concrete subclasses to actually create the managed object.</li>
* <li>After the object was created by {@link #initialize()} it is checked
* whether the {@link AtomicReference} variable is still undefined. This has to
* be done because in the meantime another thread may have initialized the
* object. If the reference is still empty, the newly created object is stored
* in it and returned by this method.</li>
* <li>Otherwise the value stored in the {@link AtomicReference} is returned.</li>
* </ul>
* <p>
* Because atomic variables are used this class does not need any
* synchronization. So there is no danger of deadlock, and access to the managed
* object is efficient. However, if multiple threads access the {@code
* AtomicInitializer} object before it has been initialized almost at the same
* time, it can happen that {@link #initialize()} is called multiple times. The
* algorithm outlined above guarantees that {@link #get()} always returns the
* same object though.
* </p>
* <p>
* Compared with the {@link LazyInitializer} class, this class can be more
* efficient because it does not need synchronization. The drawback is that the
* {@link #initialize()} method can be called multiple times which may be
* problematic if the creation of the managed object is expensive. As a rule of
* thumb this initializer implementation is preferable if there are not too many
* threads involved and the probability that multiple threads access an
* uninitialized object is small. If there is high parallelism,
* {@link LazyInitializer} is more appropriate.
* </p>
*
* @since 3.0
* @param <T> the type of the object managed by this initializer class
*/
public abstract class AtomicInitializer<T> implements ConcurrentInitializer<T> {
/** Holds the reference to the managed object. */
private final AtomicReference<T> reference = new AtomicReference<>();
/**
* Returns the object managed by this initializer. The object is created if
* it is not available yet and stored internally. This method always returns
* the same object.
*
* @return the object created by this {@link AtomicInitializer}
* @throws ConcurrentException if an error occurred during initialization of
* the object
*/
@Override
public T get() throws ConcurrentException {
T result = reference.get();
if (result == null) {
result = initialize();
if (!reference.compareAndSet(null, result)) {
// another thread has initialized the reference
result = reference.get();
}
}
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()}. As stated by the class comment, it is possible
* that this method is called multiple times.
*
* @return the managed data object
* @throws ConcurrentException if an error occurs during object creation
*/
protected abstract T initialize() throws ConcurrentException;
}
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