34  * This annotation marks classes and fields as considered to be contended.

    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  *

    42       Defines the contention group tag.

    77      * The (optional) contention group tag.

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

    79      *

    80      * @return contention group tag.