1 /*

     2  * Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.

     3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

     4  *

     5  * This code is free software; you can redistribute it and/or modify it

     6  * under the terms of the GNU General Public License version 2 only, as

     7  * published by the Free Software Foundation.  Oracle designates this

     8  * particular file as subject to the "Classpath" exception as provided

     9  * by Oracle in the LICENSE file that accompanied this code.

    10  *

    11  * This code is distributed in the hope that it will be useful, but WITHOUT

    12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

    13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

    14  * version 2 for more details (a copy is included in the LICENSE file that

    15  * accompanied this code).

    16  *

    17  * You should have received a copy of the GNU General Public License version

    18  * 2 along with this work; if not, write to the Free Software Foundation,

    19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

    20  *

    21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

    22  * or visit www.oracle.com if you need additional information or have any

    23  * questions.

    24  */

    25 

    26 package sun.misc;

    27 

    28 import java.lang.annotation.ElementType;

    29 import java.lang.annotation.Retention;

    30 import java.lang.annotation.RetentionPolicy;

    31 import java.lang.annotation.Target;

    32 

    33 /**

    34  * <p>An annotation expressing that objects and/or their fields are

    35  * expected to encounter memory contention, generally in the form of

    36  * "false sharing". This annotation serves as a hint that such objects

    37  * and fields should reside in locations isolated from those of other

    38  * objects or fields. Susceptibility to memory contention is a

    39  * property of the intended usages of objects and fields, not their

    40  * types or qualifiers. The effects of this annotation will nearly

    41  * always add significant space overhead to objects. The use of

    42  * {@code @Contended} is warranted only when the performance impact of

    43  * this time/space tradeoff is intrinsically worthwhile; for example,

    44  * in concurrent contexts in which each instance of the annotated

    45  * class is often accessed by a different thread.

    46  *

    47  * <p>A {@code @Contended} field annotation may optionally include a

    48  * <i>contention group</i> tag. A contention group defines a set of one

    49  * or more fields that collectively must be isolated from all other

    50  * contention groups. The fields in the same contention group may not be

    51  * pairwise isolated. With no contention group tag (or with the default

    52  * empty tag: "") each {@code @Contended} field resides in its own

    53  * <i>distinct</i> and <i>anonymous</i> contention group.

    54  *

    55  * <p>When the annotation is used at the class level, the effect is

    56  * equivalent to grouping all the declared fields not already having the

    57  * {@code @Contended} annotation into the same anonymous group.

    58  * With the class level annotation, implementations may choose different

    59  * isolation techniques, such as isolating the entire object, rather than

    60  * isolating distinct fields. A contention group tag has no meaning

    61  * in a class level {@code @Contended} annotation, and is ignored.

    62  *

    63  * <p>The class level {@code @Contended} annotation is not inherited and has

    64  * no effect on the fields declared in any sub-classes. The effects of all

    65  * {@code @Contended} annotations, however, remain in force for all

    66  * subclass instances, providing isolation of all the defined contention

    67  * groups. Contention group tags are not inherited, and the same tag used

    68  * in a superclass and subclass, represent distinct contention groups.

    69  *

    70  * @since 1.8

    71  */

    72 @Retention(RetentionPolicy.RUNTIME)

    73 @Target({ElementType.FIELD, ElementType.TYPE})

    74 public @interface Contended {

    75 

    76     /**

    77      * The (optional) contention group tag.

    78      * This tag is only meaningful for field level annotations.

    79      *

    80      * @return contention group tag.

    81      */

    82     String value() default "";

    83 }