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 * https://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.exception.ExceptionUtils;
22 import org.apache.commons.lang3.function.FailableConsumer;
23 import org.apache.commons.lang3.function.FailableSupplier;
24
25 /**
26 * A specialized {@link ConcurrentInitializer} implementation which is similar
27 * to {@link AtomicInitializer}, but ensures that the {@link #initialize()}
28 * method is called only once.
29 *
30 * <p>
31 * As {@link AtomicInitializer} this class is based on atomic variables, so it
32 * can create an object under concurrent access without synchronization.
33 * However, it implements an additional check to guarantee that the
34 * {@link #initialize()} method which actually creates the object cannot be
35 * called multiple times.
36 * </p>
37 * <p>
38 * Because of this additional check this implementation is slightly less
39 * efficient than {@link AtomicInitializer}, but if the object creation in the
40 * {@code initialize()} method is expensive or if multiple invocations of
41 * {@code initialize()} are problematic, it is the better alternative.
42 * </p>
43 * <p>
44 * From its semantics this class has the same properties as
45 * {@link LazyInitializer}. It is a "save" implementation of the lazy
46 * initializer pattern. Comparing both classes in terms of efficiency is
47 * difficult because which one is faster depends on multiple factors. Because
48 * {@link AtomicSafeInitializer} does not use synchronization at all it probably
49 * outruns {@link LazyInitializer}, at least under low or moderate concurrent
50 * access. Developers should run their own benchmarks on the expected target
51 * platform to decide which implementation is suitable for their specific use
52 * case.
53 * </p>
54 *
55 * @param <T> the type of the object managed by this initializer class
56 * @since 3.0
57 */
58 public class AtomicSafeInitializer<T> extends AbstractConcurrentInitializer<T, ConcurrentException> {
59
60 /**
61 * Builds a new instance.
62 *
63 * @param <T> The type of results supplied by this builder.
64 * @param <I> The type of the initializer managed by this builder.
65 * @since 3.14.0
66 */
67 public static class Builder<I extends AtomicSafeInitializer<T>, T> extends AbstractBuilder<I, T, Builder<I, T>, ConcurrentException> {
68
69 /**
70 * Constructs a new instance.
71 */
72 public Builder() {
73 // empty
74 }
75
76 @SuppressWarnings("unchecked")
77 @Override
78 public I get() {
79 return (I) new AtomicSafeInitializer(getInitializer(), getCloser());
80 }
81
82 }
83
84 private static final Object NO_INIT = new Object();
85
86 /**
87 * Creates a new builder.
88 *
89 * @param <T> the type of object to build.
90 * @return a new builder.
91 * @since 3.14.0
92 */
93 public static <T> Builder<AtomicSafeInitializer<T>, T> builder() {
94 return new Builder<>();
95 }
96
97 /** A guard which ensures that initialize() is called only once. */
98 private final AtomicReference<AtomicSafeInitializer<T>> factory = new AtomicReference<>();
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 AtomicSafeInitializer() {
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 AtomicSafeInitializer(final FailableSupplier<T, ConcurrentException> initializer, final FailableConsumer<T, ConcurrentException> closer) {
117 super(initializer, closer);
118 }
119
120 /**
121 * Gets (and initialize, if not initialized yet) the required object.
122 *
123 * @return lazily initialized object.
124 * @throws ConcurrentException if the initialization of the object causes an exception.
125 */
126 @Override
127 public final T get() throws ConcurrentException {
128 T result;
129 while ((result = reference.get()) == getNoInit()) {
130 if (factory.compareAndSet(null, this)) {
131 try {
132 reference.set(initialize());
133 } catch (final Throwable t) {
134 // Allow retry on failure; otherwise callers spin forever.
135 factory.set(null);
136 // Rethrow preserving original semantics: unchecked as-is, checked wrapped.
137 final Throwable checked = ExceptionUtils.throwUnchecked(t);
138 throw checked instanceof ConcurrentException ? (ConcurrentException) checked : new ConcurrentException(checked);
139 }
140 }
141 }
142 return result;
143 }
144
145 /** Gets the internal no-init object cast for this instance. */
146 @SuppressWarnings("unchecked")
147 private T getNoInit() {
148 return (T) NO_INIT;
149 }
150
151 /**
152 * {@inheritDoc}
153 */
154 @Override
155 protected ConcurrentException getTypedException(final Exception e) {
156 return new ConcurrentException(e);
157 }
158
159 /**
160 * Tests whether this instance is initialized. Once initialized, always returns true.
161 *
162 * @return whether this instance is initialized. Once initialized, always returns true.
163 * @since 3.14.0
164 */
165 @Override
166 public boolean isInitialized() {
167 return reference.get() != NO_INIT;
168 }
169 }