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+++ b/jdk/src/share/classes/java/util/concurrent/atomic/package-info.java Sat Dec 01 00:00:00 2007 +0000
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+/*
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation. Sun designates this
+ * particular file as subject to the "Classpath" exception as provided
+ * by Sun in the LICENSE file that accompanied this code.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ */
+
+/*
+ * 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;