Meta-annotation that describes how an annotation element relates
to a field in a {@link Descriptor}. This can be the Descriptor for
an MBean, or for an attribute, operation, or constructor in an
MBean, or for a parameter of an operation or constructor.
Consider this annotation for example:
@Documented
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Units {
@DescriptorKey("units")
String value();
}
and this use of the annotation:
public interface CacheControlMBean {
@Units("bytes")
public long getCacheSize();
}
When a Standard MBean is made from the {@code CacheControlMBean},
the usual rules mean that it will have an attribute called
{@code CacheSize} of type {@code long}. The {@code @Units}
attribute, given the above definition, will ensure that the
{@link MBeanAttributeInfo} for this attribute will have a
{@code Descriptor} that has a field called {@code units} with
corresponding value {@code bytes}.
Similarly, if the annotation looks like this:
@Documented
@Target(ElementType.METHOD)
@Retention(RetentionPolicy.RUNTIME)
public @interface Units {
@DescriptorKey("units")
String value();
@DescriptorKey("descriptionResourceKey")
String resourceKey() default "";
@DescriptorKey("descriptionResourceBundleBaseName")
String resourceBundleBaseName() default "";
}
and it is used like this:
public interface CacheControlMBean {
@Units("bytes",
resourceKey="bytes.key",
resourceBundleBaseName="com.example.foo.MBeanResources")
public long getCacheSize();
}
then the resulting {@code Descriptor} will contain the following
fields:
Name | Value |
units | "bytes" |
descriptionResourceKey | "bytes.key" |
descriptionResourceBundleBaseName |
"com.example.foo.MBeanResources" |
An annotation such as {@code @Units} can be applied to:
- a Standard MBean or MXBean interface;
- a method in such an interface;
- a parameter of a method in a Standard MBean or MXBean interface
when that method is an operation (not a getter or setter for an attribute);
- a public constructor in the class that implements a Standard MBean
or MXBean;
- a parameter in such a constructor.
Other uses of the annotation are ignored.
Interface annotations are checked only on the exact interface
that defines the management interface of a Standard MBean or an
MXBean, not on its parent interfaces. Method annotations are
checked only in the most specific interface in which the method
appears; in other words, if a child interface overrides a method
from a parent interface, only {@code @DescriptorKey} annotations in
the method in the child interface are considered.
The Descriptor fields contributed in this way by different
annotations on the same program element must be consistent. That
is, two different annotations, or two members of the same
annotation, must not define a different value for the same
Descriptor field. Fields from annotations on a getter method must
also be consistent with fields from annotations on the
corresponding setter method.
The Descriptor resulting from these annotations will be merged
with any Descriptor fields provided by the implementation, such as
the {@code
immutableInfo} field for an MBean. The fields from the annotations
must be consistent with these fields provided by the implementation.
An annotation element to be converted into a descriptor field
can be of any type allowed by the Java language, except an annotation
or an array of annotations. The value of the field is derived from
the value of the annotation element as follows:
Annotation element | Descriptor field |
Primitive value ({@code 5}, {@code false}, etc) |
Wrapped value ({@code Integer.valueOf(5)},
{@code Boolean.FALSE}, etc) |
Class constant (e.g. {@code Thread.class}) |
Class name from {@link Class#getName()}
(e.g. {@code "java.lang.Thread"}) |
Enum constant (e.g. {@link ElementType#FIELD}) |
Constant name from {@link Enum#name()}
(e.g. {@code "FIELD"}) |
Array of class constants or enum constants |
String array derived by applying these rules to each
element |
Value of any other type
({@code String}, {@code String[]}, {@code int[]}, etc) |
The same value |
|