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