luni/src/main/java/java/util/concurrent/atomic/package-info.java

/*
 * 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;