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 shift; 55 private final Object[] array; 56 57 static { 58 int scale = unsafe.arrayIndexScale(Object[].class); 59 if ((scale & (scale - 1)) != 0) 60 throw new Error("data type scale not a power of two"); 61 shift = 31 - Integer.numberOfLeadingZeros(scale); 62 } 63 64 private long checkedByteOffset(int i) { 65 if (i < 0 || i >= array.length) 66 throw new IndexOutOfBoundsException("index " + i); 67 68 return byteOffset(i); 69 } 70 71 private static long byteOffset(int i) { 72 return ((long) i << shift) + base; 73 } 74 75 /** 76 * Creates a new AtomicReferenceArray of the given length, with all 77 * elements initially zero. 78 * 79 * @param length the length of the array 80 */ 81 public AtomicReferenceArray(int length) { 82 array = new Object[length]; 83 } 84 85 /** 86 * Creates a new AtomicReferenceArray with the same length as, and 87 * all elements copied from, the given array. 88 * 89 * @param array the array to copy elements from 90 * @throws NullPointerException if array is null 91 */ 92 public AtomicReferenceArray(E[] array) { 93 // Visibility guaranteed by final field guarantees 94 this.array = array.clone(); 95 } 96 97 /** 98 * Returns the length of the array. 99 * 100 * @return the length of the array 101 */ 102 public final int length() { 103 return array.length; 104 } 105 106 /** 107 * Gets the current value at position {@code i}. 108 * 109 * @param i the index 110 * @return the current value 111 */ 112 public final E get(int i) { 113 return getRaw(checkedByteOffset(i)); 114 } 115 116 private E getRaw(long offset) { 117 return (E) unsafe.getObjectVolatile(array, offset); 118 } 119 120 /** 121 * Sets the element at position {@code i} to the given value. 122 * 123 * @param i the index 124 * @param newValue the new value 125 */ 126 public final void set(int i, E newValue) { 127 unsafe.putObjectVolatile(array, checkedByteOffset(i), newValue); 128 } 129 130 /** 131 * Eventually sets the element at position {@code i} to the given value. 132 * 133 * @param i the index 134 * @param newValue the new value 135 * @since 1.6 136 */ 137 public final void lazySet(int i, E newValue) { 138 unsafe.putOrderedObject(array, checkedByteOffset(i), newValue); 139 } 140 141 142 /** 143 * Atomically sets the element at position {@code i} to the given 144 * value and returns the old value. 145 * 146 * @param i the index 147 * @param newValue the new value 148 * @return the previous value 149 */ 150 public final E getAndSet(int i, E newValue) { 151 long offset = checkedByteOffset(i); 152 while (true) { 153 E current = (E) getRaw(offset); 154 if (compareAndSetRaw(offset, current, newValue)) 155 return current; 156 } 157 } 158 159 /** 160 * Atomically sets the element at position {@code i} to the given 161 * updated value if the current value {@code ==} the expected value. 162 * 163 * @param i the index 164 * @param expect the expected value 165 * @param update the new value 166 * @return true if successful. False return indicates that 167 * the actual value was not equal to the expected value. 168 */ 169 public final boolean compareAndSet(int i, E expect, E update) { 170 return compareAndSetRaw(checkedByteOffset(i), expect, update); 171 } 172 173 private boolean compareAndSetRaw(long offset, E expect, E update) { 174 return unsafe.compareAndSwapObject(array, offset, expect, update); 175 } 176 177 /** 178 * Atomically sets the element at position {@code i} to the given 179 * updated value if the current value {@code ==} the expected value. 180 * 181 * <p>May <a href="package-summary.html#Spurious">fail spuriously</a> 182 * and does not provide ordering guarantees, so is only rarely an 183 * appropriate alternative to {@code compareAndSet}. 184 * 185 * @param i the index 186 * @param expect the expected value 187 * @param update the new value 188 * @return true if successful. 189 */ 190 public final boolean weakCompareAndSet(int i, E expect, E update) { 191 return compareAndSet(i, expect, update); 192 } 193 194 /** 195 * Returns the String representation of the current values of array. 196 * @return the String representation of the current values of array 197 */ 198 public String toString() { 199 int iMax = array.length - 1; 200 if (iMax == -1) 201 return "[]"; 202 203 StringBuilder b = new StringBuilder(); 204 b.append('['); 205 for (int i = 0; ; i++) { 206 b.append(getRaw(byteOffset(i))); 207 if (i == iMax) 208 return b.append(']').toString(); 209 b.append(',').append(' '); 210 } 211 } 212 213 } --- EOF ---