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 */
017package org.apache.commons.lang3.concurrent;
018
019/**
020 * This class provides a generic implementation of the lazy initialization
021 * pattern.
022 *
023 * <p>
024 * Sometimes an application has to deal with an object only under certain
025 * circumstances, e.g. when the user selects a specific menu item or if a
026 * special event is received. If the creation of the object is costly or the
027 * consumption of memory or other system resources is significant, it may make
028 * sense to defer the creation of this object until it is really needed. This is
029 * a use case for the lazy initialization pattern.
030 * </p>
031 * <p>
032 * This abstract base class provides an implementation of the double-check idiom
033 * for an instance field as discussed in Joshua Bloch's "Effective Java", 2nd
034 * edition, item 71. The class already implements all necessary synchronization.
035 * A concrete subclass has to implement the {@code initialize()} method, which
036 * actually creates the wrapped data object.
037 * </p>
038 * <p>
039 * As an usage example consider that we have a class {@code ComplexObject} whose
040 * instantiation is a complex operation. In order to apply lazy initialization
041 * to this class, a subclass of {@link LazyInitializer} has to be created:
042 * </p>
043 *
044 * <pre>
045 * public class ComplexObjectInitializer extends LazyInitializer&lt;ComplexObject&gt; {
046 *     &#064;Override
047 *     protected ComplexObject initialize() {
048 *         return new ComplexObject();
049 *     }
050 * }
051 * </pre>
052 *
053 * <p>
054 * Access to the data object is provided through the {@code get()} method. So,
055 * code that wants to obtain the {@code ComplexObject} instance would simply
056 * look like this:
057 * </p>
058 *
059 * <pre>
060 * // Create an instance of the lazy initializer
061 * ComplexObjectInitializer initializer = new ComplexObjectInitializer();
062 * ...
063 * // When the object is actually needed:
064 * ComplexObject cobj = initializer.get();
065 * </pre>
066 *
067 * <p>
068 * If multiple threads call the {@code get()} method when the object has not yet
069 * been created, they are blocked until initialization completes. The algorithm
070 * guarantees that only a single instance of the wrapped object class is
071 * created, which is passed to all callers. Once initialized, calls to the
072 * {@code get()} method are pretty fast because no synchronization is needed
073 * (only an access to a <b>volatile</b> member field).
074 * </p>
075 *
076 * @since 3.0
077 * @param <T> the type of the object managed by this initializer class
078 */
079public abstract class LazyInitializer<T> implements ConcurrentInitializer<T> {
080
081    private static final Object NO_INIT = new Object();
082
083    @SuppressWarnings("unchecked")
084    // Stores the managed object.
085    private volatile T object = (T) NO_INIT;
086
087    /**
088     * Returns the object wrapped by this instance. On first access the object
089     * is created. After that it is cached and can be accessed pretty fast.
090     *
091     * @return the object initialized by this {@link LazyInitializer}
092     * @throws ConcurrentException if an error occurred during initialization of
093     * the object
094     */
095    @Override
096    public T get() throws ConcurrentException {
097        // use a temporary variable to reduce the number of reads of the
098        // volatile field
099        T result = object;
100
101        if (result == NO_INIT) {
102            synchronized (this) {
103                result = object;
104                if (result == NO_INIT) {
105                    object = result = initialize();
106                }
107            }
108        }
109
110        return result;
111    }
112
113    /**
114     * Creates and initializes the object managed by this {@code
115     * LazyInitializer}. This method is called by {@link #get()} when the object
116     * is accessed for the first time. An implementation can focus on the
117     * creation of the object. No synchronization is needed, as this is already
118     * handled by {@code get()}.
119     *
120     * @return the managed data object
121     * @throws ConcurrentException if an error occurs during object creation
122     */
123    protected abstract T initialize() throws ConcurrentException;
124}