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 
  38 /**
  39  * An {@code AtomicMarkableReference} maintains an object reference
  40  * along with a mark bit, that can be updated atomically.
  41  *
  42  * <p>Implementation note: This implementation maintains markable
  43  * references by creating internal objects representing "boxed"
  44  * [reference, boolean] pairs.
  45  *
  46  * @since 1.5
  47  * @author Doug Lea
  48  * @param <V> The type of object referred to by this reference
  49  */
  50 public class AtomicMarkableReference<V> {
  51 
  52     private static class Pair<T> {
  53         final T reference;
  54         final boolean mark;
  55         private Pair(T reference, boolean mark) {
  56             this.reference = reference;
  57             this.mark = mark;
  58         }
  59         static <T> Pair<T> of(T reference, boolean mark) {
  60             return new Pair<T>(reference, mark);
  61         }
  62     }
  63 
  64     private volatile Pair<V> pair;
  65 
  66     /**
  67      * Creates a new {@code AtomicMarkableReference} with the given
  68      * initial values.
  69      *
  70      * @param initialRef the initial reference
  71      * @param initialMark the initial mark
  72      */
  73     public AtomicMarkableReference(V initialRef, boolean initialMark) {
  74         pair = Pair.of(initialRef, initialMark);
  75     }
  76 
  77     /**
  78      * Returns the current value of the reference.
  79      *
  80      * @return the current value of the reference
  81      */
  82     public V getReference() {
  83         return pair.reference;
  84     }
  85 
  86     /**
  87      * Returns the current value of the mark.
  88      *
  89      * @return the current value of the mark
  90      */
  91     public boolean isMarked() {
  92         return pair.mark;
  93     }
  94 
  95     /**
  96      * Returns the current values of both the reference and the mark.
  97      * Typical usage is {@code boolean[1] holder; ref = v.get(holder); }.
  98      *
  99      * @param markHolder an array of size of at least one. On return,
 100      * {@code markHolder[0]} will hold the value of the mark.
 101      * @return the current value of the reference
 102      */
 103     public V get(boolean[] markHolder) {
 104         Pair<V> pair = this.pair;
 105         markHolder[0] = pair.mark;
 106         return pair.reference;
 107     }
 108 
 109     /**
 110      * Atomically sets the value of both the reference and mark
 111      * to the given update values if the
 112      * current reference is {@code ==} to the expected reference
 113      * and the current mark is equal to the expected mark.
 114      *
 115      * <p><a href="package-summary.html#weakCompareAndSet">May fail
 116      * spuriously and does not provide ordering guarantees</a>, so is
 117      * only rarely an appropriate alternative to {@code compareAndSet}.
 118      *
 119      * @param expectedReference the expected value of the reference
 120      * @param newReference the new value for the reference
 121      * @param expectedMark the expected value of the mark
 122      * @param newMark the new value for the mark
 123      * @return {@code true} if successful
 124      */
 125     public boolean weakCompareAndSet(V       expectedReference,
 126                                      V       newReference,
 127                                      boolean expectedMark,
 128                                      boolean newMark) {
 129         return compareAndSet(expectedReference, newReference,
 130                              expectedMark, newMark);
 131     }
 132 
 133     /**
 134      * Atomically sets the value of both the reference and mark
 135      * to the given update values if the
 136      * current reference is {@code ==} to the expected reference
 137      * and the current mark is equal to the expected mark.
 138      *
 139      * @param expectedReference the expected value of the reference
 140      * @param newReference the new value for the reference
 141      * @param expectedMark the expected value of the mark
 142      * @param newMark the new value for the mark
 143      * @return {@code true} if successful
 144      */
 145     public boolean compareAndSet(V       expectedReference,
 146                                  V       newReference,
 147                                  boolean expectedMark,
 148                                  boolean newMark) {
 149         Pair<V> current = pair;
 150         return
 151             expectedReference == current.reference &&
 152             expectedMark == current.mark &&
 153             ((newReference == current.reference &&
 154               newMark == current.mark) ||
 155              casPair(current, Pair.of(newReference, newMark)));
 156     }
 157 
 158     /**
 159      * Unconditionally sets the value of both the reference and mark.
 160      *
 161      * @param newReference the new value for the reference
 162      * @param newMark the new value for the mark
 163      */
 164     public void set(V newReference, boolean newMark) {
 165         Pair<V> current = pair;
 166         if (newReference != current.reference || newMark != current.mark)
 167             this.pair = Pair.of(newReference, newMark);
 168     }
 169 
 170     /**
 171      * Atomically sets the value of the mark to the given update value
 172      * if the current reference is {@code ==} to the expected
 173      * reference.  Any given invocation of this operation may fail
 174      * (return {@code false}) spuriously, but repeated invocation
 175      * when the current value holds the expected value and no other
 176      * thread is also attempting to set the value will eventually
 177      * succeed.
 178      *
 179      * @param expectedReference the expected value of the reference
 180      * @param newMark the new value for the mark
 181      * @return {@code true} if successful
 182      */
 183     public boolean attemptMark(V expectedReference, boolean newMark) {
 184         Pair<V> current = pair;
 185         return
 186             expectedReference == current.reference &&
 187             (newMark == current.mark ||
 188              casPair(current, Pair.of(expectedReference, newMark)));
 189     }
 190 
 191     // Unsafe mechanics
 192 
 193     private static final jdk.internal.misc.Unsafe U = jdk.internal.misc.Unsafe.getUnsafe();
 194     private static final long PAIR;
 195     static {
 196         try {
 197             PAIR = U.objectFieldOffset
 198                 (AtomicMarkableReference.class.getDeclaredField("pair"));
 199         } catch (ReflectiveOperationException e) {
 200             throw new Error(e);
 201         }
 202     }
 203 
 204     private boolean casPair(Pair<V> cmp, Pair<V> val) {
 205         return U.compareAndSwapObject(this, PAIR, cmp, val);
 206     }
 207 }