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 jdk.internal.vm.annotation; 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 // Android-note: Contended-specific implementation notes not relevant for Android. 34 /** 35 * <p>An annotation expressing that objects and/or their fields are 36 * expected to encounter memory contention, generally in the form of 37 * "false sharing". This annotation serves as a hint that such objects 38 * and fields should reside in locations isolated from those of other 39 * objects or fields. Susceptibility to memory contention is a 40 * property of the intended usages of objects and fields, not their 41 * types or qualifiers. The effects of this annotation will nearly 42 * always add significant space overhead to objects. The use of 43 * {@code @Contended} is warranted only when the performance impact of 44 * this time/space tradeoff is intrinsically worthwhile; for example, 45 * in concurrent contexts in which each instance of the annotated 46 * class is often accessed by a different thread. 47 * 48 * <p>A {@code @Contended} field annotation may optionally include a 49 * <i>contention group</i> tag. A contention group defines a set of one 50 * or more fields that collectively must be isolated from all other 51 * contention groups. The fields in the same contention group may not be 52 * pairwise isolated. With no contention group tag (or with the default 53 * empty tag: "") each {@code @Contended} field resides in its own 54 * <i>distinct</i> and <i>anonymous</i> contention group. 55 * 56 * <p>When the annotation is used at the class level, the effect is 57 * equivalent to grouping all the declared fields not already having the 58 * {@code @Contended} annotation into the same anonymous group. 59 * With the class level annotation, implementations may choose different 60 * isolation techniques, such as isolating the entire object, rather than 61 * isolating distinct fields. A contention group tag has no meaning 62 * in a class level {@code @Contended} annotation, and is ignored. 63 * 64 * <p>The class level {@code @Contended} annotation is not inherited and has 65 * no effect on the fields declared in any sub-classes. The effects of all 66 * {@code @Contended} annotations, however, remain in force for all 67 * subclass instances, providing isolation of all the defined contention 68 * groups. Contention group tags are not inherited, and the same tag used 69 * in a superclass and subclass, represent distinct contention groups. 70 * 71 * @since 1.8 72 */ 73 // Android-changed: RetentionPolicy.SOURCE is sufficient as this is no-op on Android. 74 // @Retention(RetentionPolicy.RUNTIME) 75 @Retention(RetentionPolicy.SOURCE) 76 @Target({ElementType.FIELD, ElementType.TYPE}) 77 public @interface Contended { 78 79 /** 80 * The (optional) contention group tag. 81 * This tag is only meaningful for field level annotations. 82 * 83 * @return contention group tag. 84 */ value()85 String value() default ""; 86 } 87