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 import java.util.concurrent.atomic.AtomicReference;
20
21 /**
22 * <p>
23 * A specialized implementation of the {@code ConcurrentInitializer} interface
24 * based on an {@link AtomicReference} variable.
25 * </p>
26 * <p>
27 * This class maintains a member field of type {@code AtomicReference}. It
28 * implements the following algorithm to create and initialize an object in its
29 * {@link #get()} method:
30 * <ul>
31 * <li>First it is checked whether the {@code AtomicReference} variable contains
32 * already a value. If this is the case, the value is directly returned.</li>
33 * <li>Otherwise the {@link #initialize()} method is called. This method must be
34 * defined in concrete subclasses to actually create the managed object.</li>
35 * <li>After the object was created by {@link #initialize()} it is checked
36 * whether the {@code AtomicReference} variable is still undefined. This has to
37 * be done because in the meantime another thread may have initialized the
38 * object. If the reference is still empty, the newly created object is stored
39 * in it and returned by this method.</li>
40 * <li>Otherwise the value stored in the {@code AtomicReference} is returned.</li>
41 * </ul>
42 * </p>
43 * <p>
44 * Because atomic variables are used this class does not need any
45 * synchronization. So there is no danger of deadlock, and access to the managed
46 * object is efficient. However, if multiple threads access the {@code
47 * AtomicInitializer} object before it has been initialized almost at the same
48 * time, it can happen that {@link #initialize()} is called multiple times. The
49 * algorithm outlined above guarantees that {@link #get()} always returns the
50 * same object though.
51 * </p>
52 * <p>
53 * Compared with the {@link LazyInitializer} class, this class can be more
54 * efficient because it does not need synchronization. The drawback is that the
55 * {@link #initialize()} method can be called multiple times which may be
56 * problematic if the creation of the managed object is expensive. As a rule of
57 * thumb this initializer implementation is preferable if there are not too many
58 * threads involved and the probability that multiple threads access an
59 * uninitialized object is small. If there is high parallelism,
60 * {@link LazyInitializer} is more appropriate.
61 * </p>
62 *
63 * @since 3.0
64 * @version $Id: AtomicInitializer.java 1309977 2012-04-05 17:53:39Z ggregory $
65 * @param <T> the type of the object managed by this initializer class
66 */
67 public abstract class AtomicInitializer<T> implements ConcurrentInitializer<T> {
68 /** Holds the reference to the managed object. */
69 private final AtomicReference<T> reference = new AtomicReference<T>();
70
71 /**
72 * Returns the object managed by this initializer. The object is created if
73 * it is not available yet and stored internally. This method always returns
74 * the same object.
75 *
76 * @return the object created by this {@code AtomicInitializer}
77 * @throws ConcurrentException if an error occurred during initialization of
78 * the object
79 */
80 @Override
81 public T get() throws ConcurrentException {
82 T result = reference.get();
83
84 if (result == null) {
85 result = initialize();
86 if (!reference.compareAndSet(null, result)) {
87 // another thread has initialized the reference
88 result = reference.get();
89 }
90 }
91
92 return result;
93 }
94
95 /**
96 * Creates and initializes the object managed by this {@code
97 * AtomicInitializer}. This method is called by {@link #get()} when the
98 * managed object is not available yet. An implementation can focus on the
99 * creation of the object. No synchronization is needed, as this is already
100 * handled by {@code get()}. As stated by the class comment, it is possible
101 * that this method is called multiple times.
102 *
103 * @return the managed data object
104 * @throws ConcurrentException if an error occurs during object creation
105 */
106 protected abstract T initialize() throws ConcurrentException;
107 }