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.  Sun designates this

     7  * particular file as subject to the "Classpath" exception as provided

     8  * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

    21  * CA 95054 USA or visit www.sun.com if you need additional information or

    22  * have any 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 

    39 /**

    40  * An object reference that may be updated atomically. See the {@link

    41  * java.util.concurrent.atomic} package specification for description

    42  * of the properties of atomic variables.

    43  * @since 1.5

    44  * @author Doug Lea

    45  * @param <V> The type of object referred to by this reference

    46  */

    47 public class AtomicReference<V>  implements java.io.Serializable {

    48     private static final long serialVersionUID = -1848883965231344442L;

    49 

    50     private static final Unsafe unsafe = Unsafe.getUnsafe();

    51     private static final long valueOffset;

    52 

    53     static {

    54       try {

    55         valueOffset = unsafe.objectFieldOffset

    56             (AtomicReference.class.getDeclaredField("value"));

    57       } catch (Exception ex) { throw new Error(ex); }

    58     }

    59 

    60     private volatile V value;

    61 

    62     /**

    63      * Creates a new AtomicReference with the given initial value.

    64      *

    65      * @param initialValue the initial value

    66      */

    67     public AtomicReference(V initialValue) {

    68         value = initialValue;

    69     }

    70 

    71     /**

    72      * Creates a new AtomicReference with null initial value.

    73      */

    74     public AtomicReference() {

    75     }

    76 

    77     /**

    78      * Gets the current value.

    79      *

    80      * @return the current value

    81      */

    82     public final V get() {

    83         return value;

    84     }

    85 

    86     /**

    87      * Sets to the given value.

    88      *

    89      * @param newValue the new value

    90      */

    91     public final void set(V newValue) {

    92         value = newValue;

    93     }

    94 

    95     /**

    96      * Eventually sets to the given value.

    97      *

    98      * @param newValue the new value

    99      * @since 1.6

   100      */

   101     public final void lazySet(V newValue) {

   102         unsafe.putOrderedObject(this, valueOffset, newValue);

   103     }

   104 

   105     /**

   106      * Atomically sets the value to the given updated value

   107      * if the current value {@code ==} the expected value.

   108      * @param expect the expected value

   109      * @param update the new value

   110      * @return true if successful. False return indicates that

   111      * the actual value was not equal to the expected value.

   112      */

   113     public final boolean compareAndSet(V expect, V update) {

   114         return unsafe.compareAndSwapObject(this, valueOffset, expect, update);

   115     }

   116 

   117     /**

   118      * Atomically sets the value to the given updated value

   119      * if the current value {@code ==} the expected value.

   120      *

   121      * <p>May <a href="package-summary.html#Spurious">fail spuriously</a>

   122      * and does not provide ordering guarantees, so is only rarely an

   123      * appropriate alternative to {@code compareAndSet}.

   124      *

   125      * @param expect the expected value

   126      * @param update the new value

   127      * @return true if successful.

   128      */

   129     public final boolean weakCompareAndSet(V expect, V update) {

   130         return unsafe.compareAndSwapObject(this, valueOffset, expect, update);

   131     }

   132 

   133     /**

   134      * Atomically sets to the given value and returns the old value.

   135      *

   136      * @param newValue the new value

   137      * @return the previous value

   138      */

   139     public final V getAndSet(V newValue) {

   140         while (true) {

   141             V x = get();

   142             if (compareAndSet(x, newValue))

   143                 return x;

   144         }

   145     }

   146 

   147     /**

   148      * Returns the String representation of the current value.

   149      * @return the String representation of the current value.

   150      */

   151     public String toString() {

   152         return String.valueOf(get());

   153     }

   154 

   155 }