001 /*
002 * Licensed to the Apache Software Foundation (ASF) under one or more
003 * contributor license agreements. See the NOTICE file distributed with
004 * this work for additional information regarding copyright ownership.
005 * The ASF licenses this file to You under the Apache License, Version 2.0
006 * (the "License"); you may not use this file except in compliance with
007 * the License. You may obtain a copy of the License at
008 *
009 * http://www.apache.org/licenses/LICENSE-2.0
010 *
011 * Unless required by applicable law or agreed to in writing, software
012 * distributed under the License is distributed on an "AS IS" BASIS,
013 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
014 * See the License for the specific language governing permissions and
015 * limitations under the License.
016 */
017 package org.apache.commons.lang3.concurrent;
018
019 import java.util.concurrent.atomic.AtomicReference;
020
021 /**
022 * <p>
023 * A specialized implementation of the {@code ConcurrentInitializer} interface
024 * based on an {@link AtomicReference} variable.
025 * </p>
026 * <p>
027 * This class maintains a member field of type {@code AtomicReference}. It
028 * implements the following algorithm to create and initialize an object in its
029 * {@link #get()} method:
030 * <ul>
031 * <li>First it is checked whether the {@code AtomicReference} variable contains
032 * already a value. If this is the case, the value is directly returned.</li>
033 * <li>Otherwise the {@link #initialize()} method is called. This method must be
034 * defined in concrete subclasses to actually create the managed object.</li>
035 * <li>After the object was created by {@link #initialize()} it is checked
036 * whether the {@code AtomicReference} variable is still undefined. This has to
037 * be done because in the meantime another thread may have initialized the
038 * object. If the reference is still empty, the newly created object is stored
039 * in it and returned by this method.</li>
040 * <li>Otherwise the value stored in the {@code AtomicReference} is returned.</li>
041 * </ul>
042 * </p>
043 * <p>
044 * Because atomic variables are used this class does not need any
045 * synchronization. So there is no danger of deadlock, and access to the managed
046 * object is efficient. However, if multiple threads access the {@code
047 * AtomicInitializer} object before it has been initialized almost at the same
048 * time, it can happen that {@link #initialize()} is called multiple times. The
049 * algorithm outlined above guarantees that {@link #get()} always returns the
050 * same object though.
051 * </p>
052 * <p>
053 * Compared with the {@link LazyInitializer} class, this class can be more
054 * efficient because it does not need synchronization. The drawback is that the
055 * {@link #initialize()} method can be called multiple times which may be
056 * problematic if the creation of the managed object is expensive. As a rule of
057 * thumb this initializer implementation is preferable if there are not too many
058 * threads involved and the probability that multiple threads access an
059 * uninitialized object is small. If there is high parallelism,
060 * {@link LazyInitializer} is more appropriate.
061 * </p>
062 *
063 * @since 3.0
064 * @version $Id: AtomicInitializer.java 1088899 2011-04-05 05:31:27Z bayard $
065 * @param <T> the type of the object managed by this initializer class
066 */
067 public abstract class AtomicInitializer<T> implements ConcurrentInitializer<T> {
068 /** Holds the reference to the managed object. */
069 private final AtomicReference<T> reference = new AtomicReference<T>();
070
071 /**
072 * Returns the object managed by this initializer. The object is created if
073 * it is not available yet and stored internally. This method always returns
074 * the same object.
075 *
076 * @return the object created by this {@code AtomicInitializer}
077 * @throws ConcurrentException if an error occurred during initialization of
078 * the object
079 */
080 public T get() throws ConcurrentException {
081 T result = reference.get();
082
083 if (result == null) {
084 result = initialize();
085 if (!reference.compareAndSet(null, result)) {
086 // another thread has initialized the reference
087 result = reference.get();
088 }
089 }
090
091 return result;
092 }
093
094 /**
095 * Creates and initializes the object managed by this {@code
096 * AtomicInitializer}. This method is called by {@link #get()} when the
097 * managed object is not available yet. An implementation can focus on the
098 * creation of the object. No synchronization is needed, as this is already
099 * handled by {@code get()}. As stated by the class comment, it is possible
100 * that this method is called multiple times.
101 *
102 * @return the managed data object
103 * @throws ConcurrentException if an error occurs during object creation
104 */
105 protected abstract T initialize() throws ConcurrentException;
106 }