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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.
* 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.
* 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}.
* 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).
* 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.
* 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 {
}