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/publicdomain/zero/1.0/
  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     @SuppressWarnings("unchecked")
 140     public final V getAndSet(V newValue) {
 141         return (V)unsafe.getAndSetObject(this, valueOffset, newValue);
 142     }
 143 
 144     /**
 145      * Returns the String representation of the current value.
 146      * @return the String representation of the current value
 147      */
 148     public String toString() {
 149         return String.valueOf(get());
 150     }
 151 
 152 }