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. + * + *
A {@code @Contended} field annotation may optionally include a + * contention group 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 + * distinct and anonymous contention group. + * + *
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. + * + *
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 ""; +}