1 /*
2 * Copyright (c) 2012, 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.*;
29
30 /**
31 * A field may be annotated as stable if all of its component variables
32 * changes value at most once.
33 * A field's value counts as its component value.
34 * If the field is typed as an array, then all the non-null components
35 * of the array, of depth up to the rank of the field's array type,
36 * also count as component values.
37 * By extension, any variable (either array or field) which has annotated
38 * as stable is called a stable variable, and its non-null or non-zero
39 * value is called a stable value.
40 * <p>
41 * Since all fields begin with a default value of null for references
42 * (resp., zero for primitives), it follows that this annotation indicates
43 * that the first non-null (resp., non-zero) value stored in the field
44 * will never be changed.
45 * <p>
46 * If the field is not of an array type, there are no array elements,
47 * then the value indicated as stable is simply the value of the field.
48 * If the dynamic type of the field value is an array but the static type
49 * is not, the components of the array are <em>not</em> regarded as stable.
50 * <p>
51 * If the field is an array type, then both the field value and
52 * all the components of the field value (if the field value is non-null)
53 * are indicated to be stable.
54 * If the field type is an array type with rank {@code N > 1},
55 * then each component of the field value (if the field value is non-null),
56 * is regarded as a stable array of rank {@code N-1}.
57 * <p>
58 * Fields which are declared {@code final} may also be annotated as stable.
59 * Since final fields already behave as stable values, such an annotation
60 * conveys no additional information regarding change of the field's value, but
61 * still conveys information regarding change of additional components values if
62 * the type of the field is an array type (as described above).
63 * <p>
64 * The HotSpot VM relies on this annotation to promote a non-null (resp.,
65 * non-zero) component value to a constant, thereby enabling superior
66 * optimizations of code depending on such a value (such as constant folding).
67 * More specifically, the HotSpot VM will process non-null stable fields (final
68 * or otherwise) in a similar manner to static final fields with respect to
69 * promoting the field's value to a constant. Thus, placing aside the
70 * differences for null/non-null values and arrays, a final stable field is
71 * treated as if it is really final from both the Java language and the HotSpot
72 * VM.
73 * <p>
74 * It is (currently) undefined what happens if a field annotated as stable
75 * is given a third value (by explicitly updating a stable field, a component of
76 * a stable array, or a final stable field via reflection or other means).
77 * Since the HotSpot VM promotes a non-null component value to constant, it may
78 * be that the Java memory model would appear to be broken, if such a constant
79 * (the second value of the field) is used as the value of the field even after
80 * the field value has changed (to a third value).
81 *
82 * @implNote
83 * This annotation only takes effect for fields of classes loaded by the boot
84 * loader. Annotations on fields of classes loaded outside of the boot loader
85 * are ignored.
86 */
87 @Target(ElementType.FIELD)
88 @Retention(RetentionPolicy.RUNTIME)
89 public @interface Stable {
90 }