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213 lines
8.8 KiB
213 lines
8.8 KiB
/*
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* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*
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*/
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/*
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*
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*
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*
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*
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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/publicdomain/zero/1.0/
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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> {@code boolean compareAndSet(expectedValue, updateValue);}</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> {@code
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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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* }}</pre>
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*
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* <p>It is straightforward to define new utility functions that, like
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* {@code getAndIncrement}, apply a function to a value atomically.
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* For example, given some transformation
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* <pre> {@code long transform(long input)}</pre>
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*
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* write your utility method as follows:
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* <pre> {@code
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* long getAndTransform(AtomicLong var) {
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* long prev, next;
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* do {
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* prev = var.get();
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* next = transform(prev);
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* } while (!var.compareAndSet(prev, next));
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* return prev; // return next; for transformAndGet
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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="https://docs.oracle.com/javase/specs/jls/se7/html/jls-17.html#jls-17.4">
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* The Java Language Specification (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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* <p id="weakCompareAndSet">The atomic classes also support method
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* {@code weakCompareAndSet}, which has limited applicability. On some
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* platforms, the weak version may be more efficient than {@code
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* compareAndSet} in the normal case, but differs in that any given
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* invocation of the {@code weakCompareAndSet} method may return {@code
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* false} <em>spuriously</em> (that is, for no apparent reason). 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 equals}, {@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#floatToRawIntBits} and
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* {@link java.lang.Float#intBitsToFloat} conversions, and doubles using
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* {@link java.lang.Double#doubleToRawLongBits} 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;
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