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 }
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