/*
* Copyright (c) 2013, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
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* 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).
*
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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 "";
}