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 /**
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 *
044 * <pre>
045 * public class ComplexObjectInitializer extends LazyInitializer<ComplexObject> {
046 * @Override
047 * protected ComplexObject initialize() {
048 * return new ComplexObject();
049 * }
050 * }
051 * </pre>
052 *
053 * Access to the data object is provided through the {@code get()} method. So,
054 * code that wants to obtain the {@code ComplexObject} instance would simply
055 * look like this:
056 *
057 * <pre>
058 * // Create an instance of the lazy initializer
059 * ComplexObjectInitializer initializer = new ComplexObjectInitializer();
060 * ...
061 * // When the object is actually needed:
062 * ComplexObject cobj = initializer.get();
063 * </pre>
064 *
065 * </p>
066 * <p>
067 * If multiple threads call the {@code get()} method when the object has not yet
068 * been created, they are blocked until initialization completes. The algorithm
069 * guarantees that only a single instance of the wrapped object class is
070 * created, which is passed to all callers. Once initialized, calls to the
071 * {@code get()} method are pretty fast because no synchronization is needed
072 * (only an access to a <b>volatile</b> member field).
073 * </p>
074 *
075 * @since 3.0
076 * @version $Id: LazyInitializer.java 1088899 2011-04-05 05:31:27Z bayard $
077 * @param <T> the type of the object managed by this initializer class
078 */
079 public abstract class LazyInitializer<T> implements ConcurrentInitializer<T> {
080 /** Stores the managed object. */
081 private volatile T object;
082
083 /**
084 * Returns the object wrapped by this instance. On first access the object
085 * is created. After that it is cached and can be accessed pretty fast.
086 *
087 * @return the object initialized by this {@code LazyInitializer}
088 * @throws ConcurrentException if an error occurred during initialization of
089 * the object
090 */
091 public T get() throws ConcurrentException {
092 // use a temporary variable to reduce the number of reads of the
093 // volatile field
094 T result = object;
095
096 if (result == null) {
097 synchronized (this) {
098 result = object;
099 if (result == null) {
100 object = result = initialize();
101 }
102 }
103 }
104
105 return result;
106 }
107
108 /**
109 * Creates and initializes the object managed by this {@code
110 * LazyInitializer}. This method is called by {@link #get()} when the object
111 * is accessed for the first time. An implementation can focus on the
112 * creation of the object. No synchronization is needed, as this is already
113 * handled by {@code get()}.
114 *
115 * @return the managed data object
116 * @throws ConcurrentException if an error occurs during object creation
117 */
118 protected abstract T initialize() throws ConcurrentException;
119 }