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 }