jdk/src/share/classes/java/util/concurrent/atomic/package-info.java
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/*
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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 *
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 * This code is free software; you can redistribute it and/or modify it
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 * under the terms of the GNU General Public License version 2 only, as
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 * published by the Free Software Foundation.  Oracle designates this
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 * particular file as subject to the "Classpath" exception as provided
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 * by Oracle in the LICENSE file that accompanied this code.
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 *
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 * This code is distributed in the hope that it will be useful, but WITHOUT
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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
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 * version 2 for more details (a copy is included in the LICENSE file that
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 * accompanied this code).
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 *
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 * You should have received a copy of the GNU General Public License version
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 * 2 along with this work; if not, write to the Free Software Foundation,
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 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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 *
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 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 * or visit www.oracle.com if you need additional information or have any
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 * questions.
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 */
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/*
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 * This file is available under and governed by the GNU General Public
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 * License version 2 only, as published by the Free Software Foundation.
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 * However, the following notice accompanied the original version of this
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 * file:
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 *
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 * Written by Doug Lea with assistance from members of JCP JSR-166
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 * Expert Group and released to the public domain, as explained at
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 * http://creativecommons.org/licenses/publicdomain
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 */
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/**
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 * A small toolkit of classes that support lock-free thread-safe
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 * programming on single variables.  In essence, the classes in this
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 * package extend the notion of {@code volatile} values, fields, and
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 * array elements to those that also provide an atomic conditional update
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 * operation of the form:
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 *
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 * <pre>
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 *   boolean compareAndSet(expectedValue, updateValue);
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 * </pre>
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 *
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 * <p>This method (which varies in argument types across different
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 * classes) atomically sets a variable to the {@code updateValue} if it
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 * currently holds the {@code expectedValue}, reporting {@code true} on
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 * success.  The classes in this package also contain methods to get and
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 * unconditionally set values, as well as a weaker conditional atomic
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 * update operation {@code weakCompareAndSet} described below.
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 *
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 * <p>The specifications of these methods enable implementations to
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 * employ efficient machine-level atomic instructions that are available
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 * on contemporary processors.  However on some platforms, support may
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 * entail some form of internal locking.  Thus the methods are not
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 * strictly guaranteed to be non-blocking --
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 * a thread may block transiently before performing the operation.
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 *
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 * <p>Instances of classes
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 * {@link java.util.concurrent.atomic.AtomicBoolean},
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 * {@link java.util.concurrent.atomic.AtomicInteger},
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 * {@link java.util.concurrent.atomic.AtomicLong}, and
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 * {@link java.util.concurrent.atomic.AtomicReference}
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 * each provide access and updates to a single variable of the
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 * corresponding type.  Each class also provides appropriate utility
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 * methods for that type.  For example, classes {@code AtomicLong} and
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 * {@code AtomicInteger} provide atomic increment methods.  One
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 * application is to generate sequence numbers, as in:
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 *
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 * <pre>
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 * class Sequencer {
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 *   private final AtomicLong sequenceNumber
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 *     = new AtomicLong(0);
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 *   public long next() {
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 *     return sequenceNumber.getAndIncrement();
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 *   }
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 * }
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 * </pre>
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 *
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 * <p>The memory effects for accesses and updates of atomics generally
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 * follow the rules for volatiles, as stated in
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 * <a href="http://java.sun.com/docs/books/jls/"> The Java Language
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 * Specification, Third Edition (17.4 Memory Model)</a>:
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 *
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 * <ul>
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 *
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 *   <li> {@code get} has the memory effects of reading a
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 * {@code volatile} variable.
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 *
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 *   <li> {@code set} has the memory effects of writing (assigning) a
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 * {@code volatile} variable.
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 *
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 *   <li> {@code lazySet} has the memory effects of writing (assigning)
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 *   a {@code volatile} variable except that it permits reorderings with
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 *   subsequent (but not previous) memory actions that do not themselves
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 *   impose reordering constraints with ordinary non-{@code volatile}
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 *   writes.  Among other usage contexts, {@code lazySet} may apply when
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 *   nulling out, for the sake of garbage collection, a reference that is
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 *   never accessed again.
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 *
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 *   <li>{@code weakCompareAndSet} atomically reads and conditionally
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 *   writes a variable but does <em>not</em>
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 *   create any happens-before orderings, so provides no guarantees
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 *   with respect to previous or subsequent reads and writes of any
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 *   variables other than the target of the {@code weakCompareAndSet}.
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 *
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 *   <li> {@code compareAndSet}
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 *   and all other read-and-update operations such as {@code getAndIncrement}
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 *   have the memory effects of both reading and
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 *   writing {@code volatile} variables.
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 * </ul>
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 *
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 * <p>In addition to classes representing single values, this package
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 * contains <em>Updater</em> classes that can be used to obtain
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 * {@code compareAndSet} operations on any selected {@code volatile}
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 * field of any selected class.
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 *
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 * {@link java.util.concurrent.atomic.AtomicReferenceFieldUpdater},
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 * {@link java.util.concurrent.atomic.AtomicIntegerFieldUpdater}, and
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 * {@link java.util.concurrent.atomic.AtomicLongFieldUpdater} are
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 * reflection-based utilities that provide access to the associated
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 * field types.  These are mainly of use in atomic data structures in
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 * which several {@code volatile} fields of the same node (for
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 * example, the links of a tree node) are independently subject to
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 * atomic updates.  These classes enable greater flexibility in how
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 * and when to use atomic updates, at the expense of more awkward
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 * reflection-based setup, less convenient usage, and weaker
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 * guarantees.
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 *
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 * <p>The
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 * {@link java.util.concurrent.atomic.AtomicIntegerArray},
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 * {@link java.util.concurrent.atomic.AtomicLongArray}, and
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 * {@link java.util.concurrent.atomic.AtomicReferenceArray} classes
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 * further extend atomic operation support to arrays of these types.
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 * These classes are also notable in providing {@code volatile} access
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 * semantics for their array elements, which is not supported for
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 * ordinary arrays.
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 *
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 * <a name="Spurious">
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 * <p>The atomic classes also support method {@code weakCompareAndSet},
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 * which has limited applicability.  On some platforms, the weak version
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 * may be more efficient than {@code compareAndSet} in the normal case,
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 * but differs in that any given invocation of the
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 * {@code weakCompareAndSet} method may return {@code false}
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 * <em>spuriously</em> (that is, for no apparent reason)</a>.  A
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 * {@code false} return means only that the operation may be retried if
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 * desired, relying on the guarantee that repeated invocation when the
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 * variable holds {@code expectedValue} and no other thread is also
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 * attempting to set the variable will eventually succeed.  (Such
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 * spurious failures may for example be due to memory contention effects
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 * that are unrelated to whether the expected and current values are
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 * equal.)  Additionally {@code weakCompareAndSet} does not provide
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 * ordering guarantees that are usually needed for synchronization
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 * control.  However, the method may be useful for updating counters and
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 * statistics when such updates are unrelated to the other
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 * happens-before orderings of a program.  When a thread sees an update
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 * to an atomic variable caused by a {@code weakCompareAndSet}, it does
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 * not necessarily see updates to any <em>other</em> variables that
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 * occurred before the {@code weakCompareAndSet}.  This may be
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 * acceptable when, for example, updating performance statistics, but
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 * rarely otherwise.
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 *
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 * <p>The {@link java.util.concurrent.atomic.AtomicMarkableReference}
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 * class associates a single boolean with a reference.  For example, this
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 * bit might be used inside a data structure to mean that the object
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 * being referenced has logically been deleted.
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 *
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 * The {@link java.util.concurrent.atomic.AtomicStampedReference}
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 * class associates an integer value with a reference.  This may be
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 * used for example, to represent version numbers corresponding to
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 * series of updates.
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 *
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 * <p>Atomic classes are designed primarily as building blocks for
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 * implementing non-blocking data structures and related infrastructure
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 * classes.  The {@code compareAndSet} method is not a general
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 * replacement for locking.  It applies only when critical updates for an
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 * object are confined to a <em>single</em> variable.
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 *
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 * <p>Atomic classes are not general purpose replacements for
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 * {@code java.lang.Integer} and related classes.  They do <em>not</em>
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 * define methods such as {@code hashCode} and
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 * {@code compareTo}.  (Because atomic variables are expected to be
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 * mutated, they are poor choices for hash table keys.)  Additionally,
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 * classes are provided only for those types that are commonly useful in
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 * intended applications.  For example, there is no atomic class for
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 * representing {@code byte}.  In those infrequent cases where you would
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 * like to do so, you can use an {@code AtomicInteger} to hold
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 * {@code byte} values, and cast appropriately.
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 *
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 * You can also hold floats using
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 * {@link java.lang.Float#floatToIntBits} and
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 * {@link java.lang.Float#intBitsToFloat} conversions, and doubles using
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 * {@link java.lang.Double#doubleToLongBits} and
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 * {@link java.lang.Double#longBitsToDouble} conversions.
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 *
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 * @since 1.5
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 */
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package java.util.concurrent.atomic;