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 }