/*

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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.  Oracle designates this

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

 * by Oracle 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,

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 *

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package jdk.internal.vm.annotation;

import java.lang.annotation.ElementType;

import java.lang.annotation.Retention;

import java.lang.annotation.RetentionPolicy;

import java.lang.annotation.Target;

/**

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

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

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

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

 * objects or fields. Susceptibility to memory contention is a

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

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

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

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

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

 * in concurrent contexts in which each instance of the annotated

 * class is often accessed by a different thread.

 *

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

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

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

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

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

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

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

 *

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

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

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

 * With the class level annotation, implementations may choose different

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

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

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

 *

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

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

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

 * subclass instances, providing isolation of all the defined contention

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

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

 *

 * @since 1.8

 */

@Retention(RetentionPolicy.RUNTIME)

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

public @interface Contended {

    /**

     * The (optional) contention group tag.

     * This tag is only meaningful for field level annotations.

     *

     * @return contention group tag.

     */

    String value() default "";

}