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*
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* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
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*
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/*
* This file is available under and governed by the GNU General Public
* License version 2 only, as published by the Free Software Foundation.
* However, the following notice accompanied the original version of this
* file:
*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/licenses/publicdomain
*/
/**
* A small toolkit of classes that support lock-free thread-safe
* programming on single variables. In essence, the classes in this
* package extend the notion of {@code volatile} values, fields, and
* array elements to those that also provide an atomic conditional update
* operation of the form:
*
* <pre>
* boolean compareAndSet(expectedValue, updateValue);
* </pre>
*
* <p>This method (which varies in argument types across different
* classes) atomically sets a variable to the {@code updateValue} if it
* currently holds the {@code expectedValue}, reporting {@code true} on
* success. The classes in this package also contain methods to get and
* unconditionally set values, as well as a weaker conditional atomic
* update operation {@code weakCompareAndSet} described below.
*
* <p>The specifications of these methods enable implementations to
* employ efficient machine-level atomic instructions that are available
* on contemporary processors. However on some platforms, support may
* entail some form of internal locking. Thus the methods are not
* strictly guaranteed to be non-blocking --
* a thread may block transiently before performing the operation.
*
* <p>Instances of classes
* {@link java.util.concurrent.atomic.AtomicBoolean},
* {@link java.util.concurrent.atomic.AtomicInteger},
* {@link java.util.concurrent.atomic.AtomicLong}, and
* {@link java.util.concurrent.atomic.AtomicReference}
* each provide access and updates to a single variable of the
* corresponding type. Each class also provides appropriate utility
* methods for that type. For example, classes {@code AtomicLong} and
* {@code AtomicInteger} provide atomic increment methods. One
* application is to generate sequence numbers, as in:
*
* <pre>
* class Sequencer {
* private final AtomicLong sequenceNumber
* = new AtomicLong(0);
* public long next() {
* return sequenceNumber.getAndIncrement();
* }
* }
* </pre>
*
* <p>The memory effects for accesses and updates of atomics generally
* follow the rules for volatiles, as stated in
* <a href="http://java.sun.com/docs/books/jls/"> The Java Language
* Specification, Third Edition (17.4 Memory Model)</a>:
*
* <ul>
*
* <li> {@code get} has the memory effects of reading a
* {@code volatile} variable.
*
* <li> {@code set} has the memory effects of writing (assigning) a
* {@code volatile} variable.
*
* <li> {@code lazySet} has the memory effects of writing (assigning)
* a {@code volatile} variable except that it permits reorderings with
* subsequent (but not previous) memory actions that do not themselves
* impose reordering constraints with ordinary non-{@code volatile}
* writes. Among other usage contexts, {@code lazySet} may apply when
* nulling out, for the sake of garbage collection, a reference that is
* never accessed again.
*
* <li>{@code weakCompareAndSet} atomically reads and conditionally
* writes a variable but does <em>not</em>
* create any happens-before orderings, so provides no guarantees
* with respect to previous or subsequent reads and writes of any
* variables other than the target of the {@code weakCompareAndSet}.
*
* <li> {@code compareAndSet}
* and all other read-and-update operations such as {@code getAndIncrement}
* have the memory effects of both reading and
* writing {@code volatile} variables.
* </ul>
*
* <p>In addition to classes representing single values, this package
* contains <em>Updater</em> classes that can be used to obtain
* {@code compareAndSet} operations on any selected {@code volatile}
* field of any selected class.
*
* {@link java.util.concurrent.atomic.AtomicReferenceFieldUpdater},
* {@link java.util.concurrent.atomic.AtomicIntegerFieldUpdater}, and
* {@link java.util.concurrent.atomic.AtomicLongFieldUpdater} are
* reflection-based utilities that provide access to the associated
* field types. These are mainly of use in atomic data structures in
* which several {@code volatile} fields of the same node (for
* example, the links of a tree node) are independently subject to
* atomic updates. These classes enable greater flexibility in how
* and when to use atomic updates, at the expense of more awkward
* reflection-based setup, less convenient usage, and weaker
* guarantees.
*
* <p>The
* {@link java.util.concurrent.atomic.AtomicIntegerArray},
* {@link java.util.concurrent.atomic.AtomicLongArray}, and
* {@link java.util.concurrent.atomic.AtomicReferenceArray} classes
* further extend atomic operation support to arrays of these types.
* These classes are also notable in providing {@code volatile} access
* semantics for their array elements, which is not supported for
* ordinary arrays.
*
* <a name="Spurious">
* <p>The atomic classes also support method {@code weakCompareAndSet},
* which has limited applicability. On some platforms, the weak version
* may be more efficient than {@code compareAndSet} in the normal case,
* but differs in that any given invocation of the
* {@code weakCompareAndSet} method may return {@code false}
* <em>spuriously</em> (that is, for no apparent reason)</a>. A
* {@code false} return means only that the operation may be retried if
* desired, relying on the guarantee that repeated invocation when the
* variable holds {@code expectedValue} and no other thread is also
* attempting to set the variable will eventually succeed. (Such
* spurious failures may for example be due to memory contention effects
* that are unrelated to whether the expected and current values are
* equal.) Additionally {@code weakCompareAndSet} does not provide
* ordering guarantees that are usually needed for synchronization
* control. However, the method may be useful for updating counters and
* statistics when such updates are unrelated to the other
* happens-before orderings of a program. When a thread sees an update
* to an atomic variable caused by a {@code weakCompareAndSet}, it does
* not necessarily see updates to any <em>other</em> variables that
* occurred before the {@code weakCompareAndSet}. This may be
* acceptable when, for example, updating performance statistics, but
* rarely otherwise.
*
* <p>The {@link java.util.concurrent.atomic.AtomicMarkableReference}
* class associates a single boolean with a reference. For example, this
* bit might be used inside a data structure to mean that the object
* being referenced has logically been deleted.
*
* The {@link java.util.concurrent.atomic.AtomicStampedReference}
* class associates an integer value with a reference. This may be
* used for example, to represent version numbers corresponding to
* series of updates.
*
* <p>Atomic classes are designed primarily as building blocks for
* implementing non-blocking data structures and related infrastructure
* classes. The {@code compareAndSet} method is not a general
* replacement for locking. It applies only when critical updates for an
* object are confined to a <em>single</em> variable.
*
* <p>Atomic classes are not general purpose replacements for
* {@code java.lang.Integer} and related classes. They do <em>not</em>
* define methods such as {@code hashCode} and
* {@code compareTo}. (Because atomic variables are expected to be
* mutated, they are poor choices for hash table keys.) Additionally,
* classes are provided only for those types that are commonly useful in
* intended applications. For example, there is no atomic class for
* representing {@code byte}. In those infrequent cases where you would
* like to do so, you can use an {@code AtomicInteger} to hold
* {@code byte} values, and cast appropriately.
*
* You can also hold floats using
* {@link java.lang.Float#floatToIntBits} and
* {@link java.lang.Float#intBitsToFloat} conversions, and doubles using
* {@link java.lang.Double#doubleToLongBits} and
* {@link java.lang.Double#longBitsToDouble} conversions.
*
* @since 1.5
*/package java.util.concurrent.atomic;
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