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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  import org.apache.commons.lang3.function.FailableConsumer;
22  import org.apache.commons.lang3.function.FailableSupplier;
23  
24  /**
25   * A specialized implementation of the {@link ConcurrentInitializer} interface
26   * based on an {@link AtomicReference} variable.
27   *
28   * <p>
29   * This class maintains a member field of type {@link AtomicReference}. It
30   * implements the following algorithm to create and initialize an object in its
31   * {@link #get()} method:
32   * </p>
33   * <ul>
34   * <li>First it is checked whether the {@link AtomicReference} variable contains
35   * already a value. If this is the case, the value is directly returned.</li>
36   * <li>Otherwise the {@link #initialize()} method is called. This method must be
37   * defined in concrete subclasses to actually create the managed object.</li>
38   * <li>After the object was created by {@link #initialize()} it is checked
39   * whether the {@link AtomicReference} variable is still undefined. This has to
40   * be done because in the meantime another thread may have initialized the
41   * object. If the reference is still empty, the newly created object is stored
42   * in it and returned by this method.</li>
43   * <li>Otherwise the value stored in the {@link AtomicReference} is returned.</li>
44   * </ul>
45   * <p>
46   * Because atomic variables are used this class does not need any
47   * synchronization. So there is no danger of deadlock, and access to the managed
48   * object is efficient. However, if multiple threads access the {@code
49   * AtomicInitializer} object before it has been initialized almost at the same
50   * time, it can happen that {@link #initialize()} is called multiple times. The
51   * algorithm outlined above guarantees that {@link #get()} always returns the
52   * same object though.
53   * </p>
54   * <p>
55   * Compared with the {@link LazyInitializer} class, this class can be more
56   * efficient because it does not need synchronization. The drawback is that the
57   * {@link #initialize()} method can be called multiple times which may be
58   * problematic if the creation of the managed object is expensive. As a rule of
59   * thumb this initializer implementation is preferable if there are not too many
60   * threads involved and the probability that multiple threads access an
61   * uninitialized object is small. If there is high parallelism,
62   * {@link LazyInitializer} is more appropriate.
63   * </p>
64   *
65   * @since 3.0
66   * @param <T> the type of the object managed by this initializer class
67   */
68  public class AtomicInitializer<T> extends AbstractConcurrentInitializer<T, ConcurrentException> {
69  
70      /**
71       * Builds a new instance.
72       *
73       * @param <T> the type of the object managed by the initializer.
74       * @param <I> the type of the initializer managed by this builder.
75       * @since 3.14.0
76       */
77      public static class Builder<I extends AtomicInitializer<T>, T> extends AbstractBuilder<I, T, Builder<I, T>, ConcurrentException> {
78  
79          @SuppressWarnings("unchecked")
80          @Override
81          public I get() {
82              return (I) new AtomicInitializer(getInitializer(), getCloser());
83          }
84  
85      }
86  
87      private static final Object NO_INIT = new Object();
88  
89      /**
90       * Creates a new builder.
91       *
92       * @param <T> the type of object to build.
93       * @return a new builder.
94       * @since 3.14.0
95       */
96      public static <T> Builder<AtomicInitializer<T>, T> builder() {
97          return new Builder<>();
98      }
99  
100     /** Holds the reference to the managed object. */
101     private final AtomicReference<T> reference = new AtomicReference<>(getNoInit());
102 
103     /**
104      * Constructs a new instance.
105      */
106     public AtomicInitializer() {
107         // empty
108     }
109 
110     /**
111      * Constructs a new instance.
112      *
113      * @param initializer the initializer supplier called by {@link #initialize()}.
114      * @param closer the closer consumer called by {@link #close()}.
115      */
116     private AtomicInitializer(final FailableSupplier<T, ConcurrentException> initializer, final FailableConsumer<T, ConcurrentException> closer) {
117         super(initializer, closer);
118     }
119 
120     /**
121      * Returns the object managed by this initializer. The object is created if
122      * it is not available yet and stored internally. This method always returns
123      * the same object.
124      *
125      * @return the object created by this {@link AtomicInitializer}
126      * @throws ConcurrentException if an error occurred during initialization of
127      * the object
128      */
129     @Override
130     public T get() throws ConcurrentException {
131         T result = reference.get();
132 
133         if (result == getNoInit()) {
134             result = initialize();
135             if (!reference.compareAndSet(getNoInit(), result)) {
136                 // another thread has initialized the reference
137                 result = reference.get();
138             }
139         }
140 
141         return result;
142     }
143 
144     /** Gets the internal no-init object cast for this instance. */
145     @SuppressWarnings("unchecked")
146     private T getNoInit() {
147         return (T) NO_INIT;
148     }
149 
150     /**
151      * {@inheritDoc}
152      */
153     @Override
154     protected ConcurrentException getTypedException(Exception e) {
155         return new ConcurrentException(e);
156     }
157 
158     /**
159      * Tests whether this instance is initialized. Once initialized, always returns true.
160      *
161      * @return whether this instance is initialized. Once initialized, always returns true.
162      * @since 3.14.0
163      */
164     @Override
165     public boolean isInitialized() {
166         return reference.get() != NO_INIT;
167     }
168 }