1 /*
2 * Licensed to the Apache Software Foundation (ASF) under one or more
3 * contributor license agreements. See the NOTICE file distributed with
4 * this work for additional information regarding copyright ownership.
5 * The ASF licenses this file to You under the Apache License, Version 2.0
6 * (the "License"); you may not use this file except in compliance with
7 * the License. You may obtain a copy of the License at
8 *
9 * http://www.apache.org/licenses/LICENSE-2.0
10 *
11 * Unless required by applicable law or agreed to in writing, software
12 * distributed under the License is distributed on an "AS IS" BASIS,
13 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
14 * See the License for the specific language governing permissions and
15 * limitations under the License.
16 */
17 package org.apache.commons.lang3.concurrent;
18
19 /**
20 * <p>
21 * This class provides a generic implementation of the lazy initialization
22 * pattern.
23 * </p>
24 * <p>
25 * Sometimes an application has to deal with an object only under certain
26 * circumstances, e.g. when the user selects a specific menu item or if a
27 * special event is received. If the creation of the object is costly or the
28 * consumption of memory or other system resources is significant, it may make
29 * sense to defer the creation of this object until it is really needed. This is
30 * a use case for the lazy initialization pattern.
31 * </p>
32 * <p>
33 * This abstract base class provides an implementation of the double-check idiom
34 * for an instance field as discussed in Joshua Bloch's "Effective Java", 2nd
35 * edition, item 71. The class already implements all necessary synchronization.
36 * A concrete subclass has to implement the {@code initialize()} method, which
37 * actually creates the wrapped data object.
38 * </p>
39 * <p>
40 * As an usage example consider that we have a class {@code ComplexObject} whose
41 * instantiation is a complex operation. In order to apply lazy initialization
42 * to this class, a subclass of {@code LazyInitializer} has to be created:
43 *
44 * <pre>
45 * public class ComplexObjectInitializer extends LazyInitializer<ComplexObject> {
46 * @Override
47 * protected ComplexObject initialize() {
48 * return new ComplexObject();
49 * }
50 * }
51 * </pre>
52 *
53 * Access to the data object is provided through the {@code get()} method. So,
54 * code that wants to obtain the {@code ComplexObject} instance would simply
55 * look like this:
56 *
57 * <pre>
58 * // Create an instance of the lazy initializer
59 * ComplexObjectInitializer initializer = new ComplexObjectInitializer();
60 * ...
61 * // When the object is actually needed:
62 * ComplexObject cobj = initializer.get();
63 * </pre>
64 *
65 * </p>
66 * <p>
67 * If multiple threads call the {@code get()} method when the object has not yet
68 * been created, they are blocked until initialization completes. The algorithm
69 * guarantees that only a single instance of the wrapped object class is
70 * created, which is passed to all callers. Once initialized, calls to the
71 * {@code get()} method are pretty fast because no synchronization is needed
72 * (only an access to a <b>volatile</b> member field).
73 * </p>
74 *
75 * @since 3.0
76 * @version $Id: LazyInitializer.java 1309977 2012-04-05 17:53:39Z ggregory $
77 * @param <T> the type of the object managed by this initializer class
78 */
79 public abstract class LazyInitializer<T> implements ConcurrentInitializer<T> {
80 /** Stores the managed object. */
81 private volatile T object;
82
83 /**
84 * Returns the object wrapped by this instance. On first access the object
85 * is created. After that it is cached and can be accessed pretty fast.
86 *
87 * @return the object initialized by this {@code LazyInitializer}
88 * @throws ConcurrentException if an error occurred during initialization of
89 * the object
90 */
91 @Override
92 public T get() throws ConcurrentException {
93 // use a temporary variable to reduce the number of reads of the
94 // volatile field
95 T result = object;
96
97 if (result == null) {
98 synchronized (this) {
99 result = object;
100 if (result == null) {
101 object = result = initialize();
102 }
103 }
104 }
105
106 return result;
107 }
108
109 /**
110 * Creates and initializes the object managed by this {@code
111 * LazyInitializer}. This method is called by {@link #get()} when the object
112 * is accessed for the first time. An implementation can focus on the
113 * creation of the object. No synchronization is needed, as this is already
114 * handled by {@code get()}.
115 *
116 * @return the managed data object
117 * @throws ConcurrentException if an error occurs during object creation
118 */
119 protected abstract T initialize() throws ConcurrentException;
120 }