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
38 import java.util.Arrays;
39 import java.lang.reflect.Array;
40 import sun.misc.Unsafe;
41
42 /**
43 * An array of object references in which elements may be updated
44 * atomically. See the {@link java.util.concurrent.atomic} package
45 * specification for description of the properties of atomic
46 * variables.
47 * @since 1.5
48 * @author Doug Lea
49 * @param <E> The base class of elements held in this array
50 */
51 public class AtomicReferenceArray<E> implements java.io.Serializable {
52 private static final long serialVersionUID = -6209656149925076980L;
53
54 private static final Unsafe unsafe;
55 private static final int base;
56 private static final int shift;
57 private static final long arrayFieldOffset;
58 private final Object[] array; // must have exact type Object[]
59
60 static {
61 int scale;
62 try {
63 unsafe = Unsafe.getUnsafe();
64 arrayFieldOffset = unsafe.objectFieldOffset
65 (AtomicReferenceArray.class.getDeclaredField("array"));
66 base = unsafe.arrayBaseOffset(Object[].class);
67 scale = unsafe.arrayIndexScale(Object[].class);
68 } catch (Exception e) {
69 throw new Error(e);
70 }
71 if ((scale & (scale - 1)) != 0)
72 throw new Error("data type scale not a power of two");
73 shift = 31 - Integer.numberOfLeadingZeros(scale);
74 }
75
76 private long checkedByteOffset(int i) {
77 if (i < 0 || i >= array.length)
78 throw new IndexOutOfBoundsException("index " + i);
79
80 return byteOffset(i);
81 }
82
83 private static long byteOffset(int i) {
84 return ((long) i << shift) + base;
85 }
86
87 /**
88 * Creates a new AtomicReferenceArray of the given length, with all
89 * elements initially null.
90 *
91 * @param length the length of the array
92 */
93 public AtomicReferenceArray(int length) {
94 array = new Object[length];
95 }
96
97 /**
98 * Creates a new AtomicReferenceArray with the same length as, and
99 * all elements copied from, the given array.
100 *
101 * @param array the array to copy elements from
102 * @throws NullPointerException if array is null
103 */
104 public AtomicReferenceArray(E[] array) {
105 // Visibility guaranteed by final field guarantees
106 this.array = Arrays.copyOf(array, array.length, Object[].class);
107 }
108
109 /**
110 * Returns the length of the array.
111 *
112 * @return the length of the array
113 */
114 public final int length() {
115 return array.length;
116 }
117
118 /**
119 * Gets the current value at position {@code i}.
120 *
121 * @param i the index
122 * @return the current value
123 */
124 public final E get(int i) {
125 return getRaw(checkedByteOffset(i));
126 }
127
128 @SuppressWarnings("unchecked")
129 private E getRaw(long offset) {
130 return (E) unsafe.getObjectVolatile(array, offset);
131 }
132
133 /**
134 * Sets the element at position {@code i} to the given value.
135 *
136 * @param i the index
137 * @param newValue the new value
138 */
139 public final void set(int i, E newValue) {
140 unsafe.putObjectVolatile(array, checkedByteOffset(i), newValue);
141 }
142
143 /**
144 * Eventually sets the element at position {@code i} to the given value.
145 *
146 * @param i the index
147 * @param newValue the new value
148 * @since 1.6
149 */
150 public final void lazySet(int i, E newValue) {
151 unsafe.putOrderedObject(array, checkedByteOffset(i), newValue);
152 }
153
154 /**
155 * Atomically sets the element at position {@code i} to the given
156 * value and returns the old value.
157 *
158 * @param i the index
159 * @param newValue the new value
160 * @return the previous value
161 */
162 public final E getAndSet(int i, E newValue) {
163 long offset = checkedByteOffset(i);
164 while (true) {
165 E current = getRaw(offset);
166 if (compareAndSetRaw(offset, current, newValue))
167 return current;
168 }
169 }
170
171 /**
172 * Atomically sets the element at position {@code i} to the given
173 * updated value if the current value {@code ==} the expected value.
174 *
175 * @param i the index
176 * @param expect the expected value
177 * @param update the new value
178 * @return true if successful. False return indicates that
179 * the actual value was not equal to the expected value.
180 */
181 public final boolean compareAndSet(int i, E expect, E update) {
182 return compareAndSetRaw(checkedByteOffset(i), expect, update);
183 }
184
185 private boolean compareAndSetRaw(long offset, E expect, E update) {
186 return unsafe.compareAndSwapObject(array, offset, expect, update);
187 }
188
189 /**
190 * Atomically sets the element at position {@code i} to the given
191 * updated value if the current value {@code ==} the expected value.
192 *
193 * <p>May <a href="package-summary.html#Spurious">fail spuriously</a>
194 * and does not provide ordering guarantees, so is only rarely an
195 * appropriate alternative to {@code compareAndSet}.
196 *
197 * @param i the index
198 * @param expect the expected value
199 * @param update the new value
200 * @return true if successful.
201 */
202 public final boolean weakCompareAndSet(int i, E expect, E update) {
203 return compareAndSet(i, expect, update);
204 }
205
206 /**
207 * Returns the String representation of the current values of array.
208 * @return the String representation of the current values of array
209 */
210 public String toString() {
211 int iMax = array.length - 1;
212 if (iMax == -1)
213 return "[]";
214
215 StringBuilder b = new StringBuilder();
216 b.append('[');
217 for (int i = 0; ; i++) {
218 b.append(getRaw(byteOffset(i)));
219 if (i == iMax)
220 return b.append(']').toString();
221 b.append(',').append(' ');
222 }
223 }
224
225 /**
226 * Reconstitutes the instance from a stream (that is, deserializes it).
227 */
228 private void readObject(java.io.ObjectInputStream s)
229 throws java.io.IOException, ClassNotFoundException,
230 java.io.InvalidObjectException {
231 // Note: This must be changed if any additional fields are defined
232 Object a = s.readFields().get("array", null);
233 if (a == null || !a.getClass().isArray())
234 throw new java.io.InvalidObjectException("Not array type");
235 if (a.getClass() != Object[].class)
236 a = Arrays.copyOf((Object[])a, Array.getLength(a), Object[].class);
237 unsafe.putObjectVolatile(this, arrayFieldOffset, a);
238 }
239
240 }