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 import java.lang.invoke.MethodHandles; 39 import java.lang.invoke.VarHandle; 40 41 /** 42 * An {@code AtomicMarkableReference} maintains an object reference 43 * along with a mark bit, that can be updated atomically. 44 * 45 * <p>Implementation note: This implementation maintains markable 46 * references by creating internal objects representing "boxed" 47 * [reference, boolean] pairs. 48 * 49 * @since 1.5 50 * @author Doug Lea 51 * @param <V> The type of object referred to by this reference 52 */ 53 public class AtomicMarkableReference<V> { 54 55 private static class Pair<T> { 56 final T reference; 57 final boolean mark; Pair(T reference, boolean mark)58 private Pair(T reference, boolean mark) { 59 this.reference = reference; 60 this.mark = mark; 61 } of(T reference, boolean mark)62 static <T> Pair<T> of(T reference, boolean mark) { 63 return new Pair<T>(reference, mark); 64 } 65 } 66 67 private volatile Pair<V> pair; 68 69 /** 70 * Creates a new {@code AtomicMarkableReference} with the given 71 * initial values. 72 * 73 * @param initialRef the initial reference 74 * @param initialMark the initial mark 75 */ AtomicMarkableReference(V initialRef, boolean initialMark)76 public AtomicMarkableReference(V initialRef, boolean initialMark) { 77 pair = Pair.of(initialRef, initialMark); 78 } 79 80 /** 81 * Returns the current value of the reference. 82 * 83 * @return the current value of the reference 84 */ getReference()85 public V getReference() { 86 return pair.reference; 87 } 88 89 /** 90 * Returns the current value of the mark. 91 * 92 * @return the current value of the mark 93 */ isMarked()94 public boolean isMarked() { 95 return pair.mark; 96 } 97 98 /** 99 * Returns the current values of both the reference and the mark. 100 * Typical usage is {@code boolean[1] holder; ref = v.get(holder); }. 101 * 102 * @param markHolder an array of size of at least one. On return, 103 * {@code markHolder[0]} will hold the value of the mark. 104 * @return the current value of the reference 105 */ get(boolean[] markHolder)106 public V get(boolean[] markHolder) { 107 Pair<V> pair = this.pair; 108 markHolder[0] = pair.mark; 109 return pair.reference; 110 } 111 112 /** 113 * Atomically sets the value of both the reference and mark 114 * to the given update values if the 115 * current reference is {@code ==} to the expected reference 116 * and the current mark is equal to the expected mark. 117 * 118 * <p><a href="package-summary.html#weakCompareAndSet">May fail 119 * spuriously and does not provide ordering guarantees</a>, so is 120 * only rarely an appropriate alternative to {@code compareAndSet}. 121 * 122 * @param expectedReference the expected value of the reference 123 * @param newReference the new value for the reference 124 * @param expectedMark the expected value of the mark 125 * @param newMark the new value for the mark 126 * @return {@code true} if successful 127 */ weakCompareAndSet(V expectedReference, V newReference, boolean expectedMark, boolean newMark)128 public boolean weakCompareAndSet(V expectedReference, 129 V newReference, 130 boolean expectedMark, 131 boolean newMark) { 132 return compareAndSet(expectedReference, newReference, 133 expectedMark, newMark); 134 } 135 136 /** 137 * Atomically sets the value of both the reference and mark 138 * to the given update values if the 139 * current reference is {@code ==} to the expected reference 140 * and the current mark is equal to the expected mark. 141 * 142 * @param expectedReference the expected value of the reference 143 * @param newReference the new value for the reference 144 * @param expectedMark the expected value of the mark 145 * @param newMark the new value for the mark 146 * @return {@code true} if successful 147 */ compareAndSet(V expectedReference, V newReference, boolean expectedMark, boolean newMark)148 public boolean compareAndSet(V expectedReference, 149 V newReference, 150 boolean expectedMark, 151 boolean newMark) { 152 Pair<V> current = pair; 153 return 154 expectedReference == current.reference && 155 expectedMark == current.mark && 156 ((newReference == current.reference && 157 newMark == current.mark) || 158 casPair(current, Pair.of(newReference, newMark))); 159 } 160 161 /** 162 * Unconditionally sets the value of both the reference and mark. 163 * 164 * @param newReference the new value for the reference 165 * @param newMark the new value for the mark 166 */ set(V newReference, boolean newMark)167 public void set(V newReference, boolean newMark) { 168 Pair<V> current = pair; 169 if (newReference != current.reference || newMark != current.mark) 170 this.pair = Pair.of(newReference, newMark); 171 } 172 173 /** 174 * Atomically sets the value of the mark to the given update value 175 * if the current reference is {@code ==} to the expected 176 * reference. Any given invocation of this operation may fail 177 * (return {@code false}) spuriously, but repeated invocation 178 * when the current value holds the expected value and no other 179 * thread is also attempting to set the value will eventually 180 * succeed. 181 * 182 * @param expectedReference the expected value of the reference 183 * @param newMark the new value for the mark 184 * @return {@code true} if successful 185 */ attemptMark(V expectedReference, boolean newMark)186 public boolean attemptMark(V expectedReference, boolean newMark) { 187 Pair<V> current = pair; 188 return 189 expectedReference == current.reference && 190 (newMark == current.mark || 191 casPair(current, Pair.of(expectedReference, newMark))); 192 } 193 194 // VarHandle mechanics 195 private static final VarHandle PAIR; 196 static { 197 try { 198 MethodHandles.Lookup l = MethodHandles.lookup(); 199 PAIR = l.findVarHandle(AtomicMarkableReference.class, "pair", 200 Pair.class); 201 } catch (ReflectiveOperationException e) { 202 throw new ExceptionInInitializerError(e); 203 } 204 } 205 casPair(Pair<V> cmp, Pair<V> val)206 private boolean casPair(Pair<V> cmp, Pair<V> val) { 207 return PAIR.compareAndSet(this, cmp, val); 208 } 209 } 210