1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent.atomic;
37 import java.util.function.UnaryOperator;
38 import java.util.function.BinaryOperator;
39 import sun.misc.Unsafe;
40
41 /**
42 * An object reference that may be updated atomically. See the {@link
43 * java.util.concurrent.atomic} package specification for description
44 * of the properties of atomic variables.
45 * @since 1.5
46 * @author Doug Lea
47 * @param <V> The type of object referred to by this reference
48 */
49 public class AtomicReference<V> implements java.io.Serializable {
50 private static final long serialVersionUID = -1848883965231344442L;
51
52 private static final Unsafe unsafe = Unsafe.getUnsafe();
53 private static final long valueOffset;
54
55 static {
56 try {
57 valueOffset = unsafe.objectFieldOffset
58 (AtomicReference.class.getDeclaredField("value"));
59 } catch (Exception ex) { throw new Error(ex); }
60 }
61
62 private volatile V value;
63
64 /**
65 * Creates a new AtomicReference with the given initial value.
66 *
67 * @param initialValue the initial value
68 */
69 public AtomicReference(V initialValue) {
70 value = initialValue;
71 }
72
73 /**
74 * Creates a new AtomicReference with null initial value.
75 */
76 public AtomicReference() {
77 }
78
79 /**
80 * Gets the current value.
81 *
82 * @return the current value
83 */
84 public final V get() {
85 return value;
86 }
87
88 /**
89 * Sets to the given value.
90 *
91 * @param newValue the new value
92 */
93 public final void set(V newValue) {
94 value = newValue;
95 }
96
97 /**
98 * Eventually sets to the given value.
99 *
100 * @param newValue the new value
101 * @since 1.6
102 */
103 public final void lazySet(V newValue) {
104 unsafe.putOrderedObject(this, valueOffset, newValue);
105 }
106
107 /**
108 * Atomically sets the value to the given updated value
109 * if the current value {@code ==} the expected value.
110 * @param expect the expected value
111 * @param update the new value
112 * @return true if successful. False return indicates that
113 * the actual value was not equal to the expected value.
114 */
115 public final boolean compareAndSet(V expect, V update) {
116 return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
117 }
118
119 /**
120 * Atomically sets the value to the given updated value
121 * if the current value {@code ==} the expected value.
122 *
123 * <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
124 * and does not provide ordering guarantees, so is only rarely an
125 * appropriate alternative to {@code compareAndSet}.
126 *
127 * @param expect the expected value
128 * @param update the new value
129 * @return true if successful
130 */
131 public final boolean weakCompareAndSet(V expect, V update) {
132 return unsafe.compareAndSwapObject(this, valueOffset, expect, update);
133 }
134
135 /**
136 * Atomically sets to the given value and returns the old value.
137 *
138 * @param newValue the new value
139 * @return the previous value
140 */
141 @SuppressWarnings("unchecked")
142 public final V getAndSet(V newValue) {
143 return (V)unsafe.getAndSetObject(this, valueOffset, newValue);
144 }
145
146 /**
147 * Atomically updates the current value with the results of
148 * applying the given function. The function should be
149 * side-effect-free, since it may be re-applied when attempted
150 * updates fail due to contention among threads.
151 *
152 * @param updateFunction a side-effect-free function
153 * @return the previous value
154 * @since 1.8
155 */
156 public final V getAndUpdate(UnaryOperator<V> updateFunction) {
157 V prev, next;
158 do {
159 prev = get();
160 next = updateFunction.operate(prev);
161 } while (!compareAndSet(prev, next));
162 return prev;
163 }
164
165 /**
166 * Atomically updates the current value with the results of
167 * applying the given function. The function should be
168 * side-effect-free, since it may be re-applied when attempted
169 * updates fail due to contention among threads.
170 *
171 * @param updateFunction a side-effect-free function
172 * @return the updated value
173 * @since 1.8
174 */
175 public final V updateAndGet(UnaryOperator<V> updateFunction) {
176 V prev, next;
177 do {
178 prev = get();
179 next = updateFunction.operate(prev);
180 } while (!compareAndSet(prev, next));
181 return next;
182 }
183
184 /**
185 * Atomically updates the current value with the results of
186 * applying the given function to the current and given values.
187 * The function should be side-effect-free, since it may be
188 * re-applied when attempted updates fail due to contention among
189 * threads. The function is applied with the current value as its
190 * first argument, and the given update as the second argument.
191 *
192 * @param x the update value
193 * @param accumulatorFunction a side-effect-free function of two arguments
194 * @return the previous value
195 * @since 1.8
196 */
197 public final V getAndAccumulate(V x,
198 BinaryOperator<V> accumulatorFunction) {
199 V prev, next;
200 do {
201 prev = get();
202 next = accumulatorFunction.operate(prev, x);
203 } while (!compareAndSet(prev, next));
204 return prev;
205 }
206
207 /**
208 * Atomically updates the current value with the results of
209 * applying the given function to the current and given values.
210 * The function should be side-effect-free, since it may be
211 * re-applied when attempted updates fail due to contention among
212 * threads. The function is applied with the current value as its
213 * first argument, and the given update as the second argument.
214 *
215 * @param x the update value
216 * @param accumulatorFunction a side-effect-free function of two arguments
217 * @return the updated value
218 * @since 1.8
219 */
220 public final V accumulateAndGet(V x,
221 BinaryOperator<V> accumulatorFunction) {
222 V prev, next;
223 do {
224 prev = get();
225 next = accumulatorFunction.operate(prev, x);
226 } while (!compareAndSet(prev, next));
227 return next;
228 }
229
230 /**
231 * Returns the String representation of the current value.
232 * @return the String representation of the current value
233 */
234 public String toString() {
235 return String.valueOf(get());
236 }
237
238 }