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  * 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  * object is often accessed by a different thread.
  46  *
  47  * <p>A {@code @Contended} field annotation may optionally include a
  48  * contention group tag. All fields with the same tag are considered
  49  * as a group with respect to isolation from other groups. A default
  50  * annotation without a tag indicates contention with all other
  51  * fields, including other {@code @Contended} ones.
  52  *
  53  * <p>When the annotation is used at the class level, all unannotated
  54  * fields of the object are considered to be in the same default
  55  * group, separate from any fields that carry their own (possibly
  56  * tagged) {@code @Contended} annotations.
  57  *
  58  * @since 1.8
  59  */
  60 @Retention(RetentionPolicy.RUNTIME)
  61 @Target({ElementType.FIELD, ElementType.TYPE})
  62 public @interface Contended {
  63 
  64     /**
  65      * The (optional) contention group tag.
  66      */
  67     String value() default "";
  68 }