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/licenses/publicdomain 34 */ 35 36 package java.util.concurrent.atomic; 37 import sun.misc.Unsafe; 38 import java.util.*; 39 40 /** 41 * An array of object references in which elements may be updated 42 * atomically. See the {@link java.util.concurrent.atomic} package 43 * specification for description of the properties of atomic 44 * variables. 45 * @since 1.5 46 * @author Doug Lea 47 * @param <E> The base class of elements held in this array 48 */ 49 public class AtomicReferenceArray<E> implements java.io.Serializable { 50 private static final long serialVersionUID = -6209656149925076980L; 51 52 private static final Unsafe unsafe = Unsafe.getUnsafe(); 53 private static final int base = unsafe.arrayBaseOffset(Object[].class); 54 private static final int scale = unsafe.arrayIndexScale(Object[].class); 55 private final Object[] array; 56 57 private long rawIndex(int i) { 58 if (i < 0 || i >= array.length) 59 throw new IndexOutOfBoundsException("index " + i); 60 return base + (long) i * scale; 61 } 62 63 /** 64 * Creates a new AtomicReferenceArray of given length. 65 * @param length the length of the array 66 */ 67 public AtomicReferenceArray(int length) { 68 array = new Object[length]; 69 // must perform at least one volatile write to conform to JMM 70 if (length > 0) 71 unsafe.putObjectVolatile(array, rawIndex(0), null); 72 } 73 74 /** 75 * Creates a new AtomicReferenceArray with the same length as, and 76 * all elements copied from, the given array. 77 * 78 * @param array the array to copy elements from 79 * @throws NullPointerException if array is null 80 */ 81 public AtomicReferenceArray(E[] array) { 82 if (array == null) 83 throw new NullPointerException(); 84 int length = array.length; 85 this.array = new Object[length]; 86 if (length > 0) { 87 int last = length-1; 88 for (int i = 0; i < last; ++i) 89 this.array[i] = array[i]; 90 // Do the last write as volatile 91 E e = array[last]; 92 unsafe.putObjectVolatile(this.array, rawIndex(last), e); 93 } 94 } 95 96 /** 97 * Returns the length of the array. 98 * 99 * @return the length of the array 100 */ 101 public final int length() { 102 return array.length; 103 } 104 105 /** 106 * Gets the current value at position {@code i}. 107 * 108 * @param i the index 109 * @return the current value 110 */ 111 public final E get(int i) { 112 return (E) unsafe.getObjectVolatile(array, rawIndex(i)); 113 } 114 115 /** 116 * Sets the element at position {@code i} to the given value. 117 * 118 * @param i the index 119 * @param newValue the new value 120 */ 121 public final void set(int i, E newValue) { 122 unsafe.putObjectVolatile(array, rawIndex(i), newValue); 123 } 124 125 /** 126 * Eventually sets the element at position {@code i} to the given value. 127 * 128 * @param i the index 129 * @param newValue the new value 130 * @since 1.6 131 */ 132 public final void lazySet(int i, E newValue) { 133 unsafe.putOrderedObject(array, rawIndex(i), newValue); 134 } 135 136 137 /** 138 * Atomically sets the element at position {@code i} to the given 139 * value and returns the old value. 140 * 141 * @param i the index 142 * @param newValue the new value 143 * @return the previous value 144 */ 145 public final E getAndSet(int i, E newValue) { 146 while (true) { 147 E current = get(i); 148 if (compareAndSet(i, current, newValue)) 149 return current; 150 } 151 } 152 153 /** 154 * Atomically sets the element at position {@code i} to the given 155 * updated value if the current value {@code ==} the expected value. 156 * @param i the index 157 * @param expect the expected value 158 * @param update the new value 159 * @return true if successful. False return indicates that 160 * the actual value was not equal to the expected value. 161 */ 162 public final boolean compareAndSet(int i, E expect, E update) { 163 return unsafe.compareAndSwapObject(array, rawIndex(i), 164 expect, update); 165 } 166 167 /** 168 * Atomically sets the element at position {@code i} to the given 169 * updated value if the current value {@code ==} the expected value. 170 * 171 * <p>May <a href="package-summary.html#Spurious">fail spuriously</a> 172 * and does not provide ordering guarantees, so is only rarely an 173 * appropriate alternative to {@code compareAndSet}. 174 * 175 * @param i the index 176 * @param expect the expected value 177 * @param update the new value 178 * @return true if successful. 179 */ 180 public final boolean weakCompareAndSet(int i, E expect, E update) { 181 return compareAndSet(i, expect, update); 182 } 183 184 /** 185 * Returns the String representation of the current values of array. 186 * @return the String representation of the current values of array. 187 */ 188 public String toString() { 189 if (array.length > 0) // force volatile read 190 get(0); 191 return Arrays.toString(array); 192 } 193 194 }