| /* | |
| * Copyright (c) 2012, 2013, Oracle and/or its affiliates. All rights reserved. | |
| * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. | |
| * | |
| * This code is free software; you can redistribute it and/or modify it | |
| * under the terms of the GNU General Public License version 2 only, as | |
| * published by the Free Software Foundation. Oracle designates this | |
| * particular file as subject to the "Classpath" exception as provided | |
| * by Oracle in the LICENSE file that accompanied this code. | |
| * | |
| * This code is distributed in the hope that it will be useful, but WITHOUT | |
| * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or | |
| * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License | |
| * version 2 for more details (a copy is included in the LICENSE file that | |
| * accompanied this code). | |
| * | |
| * You should have received a copy of the GNU General Public License version | |
| * 2 along with this work; if not, write to the Free Software Foundation, | |
| * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. | |
| * | |
| * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA | |
| * or visit www.oracle.com if you need additional information or have any | |
| * questions. | |
| */ | |
| package jdk.internal.vm.annotation; | |
| import java.lang.annotation.*; | |
| /** | |
| * A field may be annotated as stable if all of its component variables | |
| * changes value at most once. | |
| * A field's value counts as its component value. | |
| * If the field is typed as an array, then all the non-null components | |
| * of the array, of depth up to the rank of the field's array type, | |
| * also count as component values. | |
| * By extension, any variable (either array or field) which has annotated | |
| * as stable is called a stable variable, and its non-null or non-zero | |
| * value is called a stable value. | |
| * <p> | |
| * Since all fields begin with a default value of null for references | |
| * (resp., zero for primitives), it follows that this annotation indicates | |
| * that the first non-null (resp., non-zero) value stored in the field | |
| * will never be changed. | |
| * <p> | |
| * If the field is not of an array type, there are no array elements, | |
| * then the value indicated as stable is simply the value of the field. | |
| * If the dynamic type of the field value is an array but the static type | |
| * is not, the components of the array are <em>not</em> regarded as stable. | |
| * <p> | |
| * If the field is an array type, then both the field value and | |
| * all the components of the field value (if the field value is non-null) | |
| * are indicated to be stable. | |
| * If the field type is an array type with rank {@code N > 1}, | |
| * then each component of the field value (if the field value is non-null), | |
| * is regarded as a stable array of rank {@code N-1}. | |
| * <p> | |
| * Fields which are declared {@code final} may also be annotated as stable. | |
| * Since final fields already behave as stable values, such an annotation | |
| * conveys no additional information regarding change of the field's value, but | |
| * still conveys information regarding change of additional components values if | |
| * the type of the field is an array type (as described above). | |
| * <p> | |
| * The HotSpot VM relies on this annotation to promote a non-null (resp., | |
| * non-zero) component value to a constant, thereby enabling superior | |
| * optimizations of code depending on such a value (such as constant folding). | |
| * More specifically, the HotSpot VM will process non-null stable fields (final | |
| * or otherwise) in a similar manner to static final fields with respect to | |
| * promoting the field's value to a constant. Thus, placing aside the | |
| * differences for null/non-null values and arrays, a final stable field is | |
| * treated as if it is really final from both the Java language and the HotSpot | |
| * VM. | |
| * <p> | |
| * It is (currently) undefined what happens if a field annotated as stable | |
| * is given a third value (by explicitly updating a stable field, a component of | |
| * a stable array, or a final stable field via reflection or other means). | |
| * Since the HotSpot VM promotes a non-null component value to constant, it may | |
| * be that the Java memory model would appear to be broken, if such a constant | |
| * (the second value of the field) is used as the value of the field even after | |
| * the field value has changed (to a third value). | |
| * | |
| * @implNote | |
| * This annotation only takes effect for fields of classes loaded by the boot | |
| * loader. Annotations on fields of classes loaded outside of the boot loader | |
| * are ignored. | |
| */ | |
| @Target(ElementType.FIELD) | |
| @Retention(RetentionPolicy.RUNTIME) | |
| public @interface Stable { | |
| } |