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 /** 37 * A small toolkit of classes that support lock-free thread-safe 38 * programming on single variables. Instances of Atomic classes 39 * maintain values that are accessed and updated using methods 40 * otherwise available for fields using associated atomic {@link 41 * java.lang.invoke.VarHandle} operations. 42 * 43 * <p>Instances of classes 44 * {@link java.util.concurrent.atomic.AtomicBoolean}, 45 * {@link java.util.concurrent.atomic.AtomicInteger}, 46 * {@link java.util.concurrent.atomic.AtomicLong}, and 47 * {@link java.util.concurrent.atomic.AtomicReference} 48 * each provide access and updates to a single variable of the 49 * corresponding type. Each class also provides appropriate utility 50 * methods for that type. For example, classes {@code AtomicLong} and 51 * {@code AtomicInteger} provide atomic increment methods. One 52 * application is to generate sequence numbers, as in: 53 * 54 * <pre> {@code 55 * class Sequencer { 56 * private final AtomicLong sequenceNumber 57 * = new AtomicLong(17); 58 * public long next() { 59 * return sequenceNumber.getAndIncrement(); 60 * } 61 * }}</pre> 62 * 63 * <p>Arbitrary transformations of the contained value are provided both 64 * by low-level read-modify-write operations such as {@code compareAndSet} 65 * and by higher-level methods such as {@code getAndUpdate}. 66 * 67 * <p>These classes are not general purpose replacements for {@code 68 * java.lang.Integer} and related classes. They do <em>not</em> 69 * define methods such as {@code equals}, {@code hashCode} and {@code 70 * compareTo}. Because atomic variables are expected to be mutated, 71 * they are poor choices for hash table keys. 72 * 73 * <p>The 74 * {@link java.util.concurrent.atomic.AtomicIntegerArray}, 75 * {@link java.util.concurrent.atomic.AtomicLongArray}, and 76 * {@link java.util.concurrent.atomic.AtomicReferenceArray} classes 77 * further extend atomic operation support to arrays of these types. 78 * These classes are also notable in providing {@code volatile} access 79 * semantics for their array elements. 80 * 81 * <p>In addition to classes representing single values and arrays, 82 * this package contains <em>Updater</em> classes that can be used to 83 * obtain {@code compareAndSet} and related operations on any selected 84 * {@code volatile} field of any selected class. These classes 85 * predate the introduction of {@link 86 * java.lang.invoke.VarHandle}, and are of more limited use. 87 * {@link java.util.concurrent.atomic.AtomicReferenceFieldUpdater}, 88 * {@link java.util.concurrent.atomic.AtomicIntegerFieldUpdater}, and 89 * {@link java.util.concurrent.atomic.AtomicLongFieldUpdater} are 90 * reflection-based utilities that provide access to the associated 91 * field types. These are mainly of use in atomic data structures in 92 * which several {@code volatile} fields of the same node (for 93 * example, the links of a tree node) are independently subject to 94 * atomic updates. These classes enable greater flexibility in how 95 * and when to use atomic updates, at the expense of more awkward 96 * reflection-based setup, less convenient usage, and weaker 97 * guarantees. 98 * 99 * <p>The {@link java.util.concurrent.atomic.AtomicMarkableReference} 100 * class associates a single boolean with a reference. For example, this 101 * bit might be used inside a data structure to mean that the object 102 * being referenced has logically been deleted. 103 * 104 * The {@link java.util.concurrent.atomic.AtomicStampedReference} 105 * class associates an integer value with a reference. This may be 106 * used for example, to represent version numbers corresponding to 107 * series of updates. 108 * 109 * @since 1.5 110 */ 111 package java.util.concurrent.atomic; 112