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package javax.management;

import java.lang.annotation.*;


Meta-annotation that describes how an annotation element relates to a field in a 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 CacheControlMBean, the usual rules mean that it will have an attribute called CacheSize of type long. The @Units annotation, given the above definition, will ensure that the MBeanAttributeInfo for this attribute will have a Descriptor that has a field called units with corresponding value 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 Descriptor will contain the following fields:



Descriptor Fields

NameValue


units"bytes"


descriptionResourceKey"bytes.key"


descriptionResourceBundleBaseName
    "com.example.foo.MBeanResources"




An annotation such as @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 @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 
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:



Descriptor Field Types

Annotation elementDescriptor field


Primitive value (5, false, etc)
    Wrapped value (Integer.valueOf(5), Boolean.FALSE, etc)


Class constant (e.g. Thread.class)
    Class name from Class.getName() (e.g. "java.lang.Thread")


Enum constant (e.g. ElementType.FIELD)
    Constant name from Enum.name() (e.g. "FIELD")


Array of class constants or enum constants
    String array derived by applying these rules to each
        element


Value of any other type
 (String, String[], int[], etc)		
    The same value




Since:1.6
/**
 * <p>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.</p>
 *
 * <p>Consider this annotation for example:</p>
 *
 * <pre>
 * &#64;Documented
 * &#64;Target(ElementType.METHOD)
 * &#64;Retention(RetentionPolicy.RUNTIME)
 * public &#64;interface Units {
 *     <b>&#64;DescriptorKey("units")</b>
 *     String value();
 * }
 * </pre>
 *
 * <p>and this use of the annotation:</p>
 *
 * <pre>
 * public interface CacheControlMBean {
 *     <b>&#64;Units("bytes")</b>
 *     public long getCacheSize();
 * }
 * </pre>
 *
 * <p>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}
 * annotation, 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}.</p>
 *
 * <p>Similarly, if the annotation looks like this:</p>
 *
 * <pre>
 * &#64;Documented
 * &#64;Target(ElementType.METHOD)
 * &#64;Retention(RetentionPolicy.RUNTIME)
 * public &#64;interface Units {
 *     <b>&#64;DescriptorKey("units")</b>
 *     String value();
 *
 *     <b>&#64;DescriptorKey("descriptionResourceKey")</b>
 *     String resourceKey() default "";
 *
 *     <b>&#64;DescriptorKey("descriptionResourceBundleBaseName")</b>
 *     String resourceBundleBaseName() default "";
 * }
 * </pre>
 *
 * <p>and it is used like this:</p>
 *
 * <pre>
 * public interface CacheControlMBean {
 *     <b>&#64;Units("bytes",
 *            resourceKey="bytes.key",
 *            resourceBundleBaseName="com.example.foo.MBeanResources")</b>
 *     public long getCacheSize();
 * }
 * </pre>
 *
 * <p>then the resulting {@code Descriptor} will contain the following
 * fields:</p>
 *
 * <table border="1">
 * <caption style="display:none">Descriptor Fields</caption>
 * <tr><th>Name</th><th>Value</th></tr>
 * <tr><td>units</td><td>"bytes"</td></tr>
 * <tr><td>descriptionResourceKey</td><td>"bytes.key"</td></tr>
 * <tr><td>descriptionResourceBundleBaseName</td>
 *     <td>"com.example.foo.MBeanResources"</td></tr>
 * </table>
 *
 * <p>An annotation such as {@code @Units} can be applied to:</p>
 *
 * <ul>
 * <li>a Standard MBean or MXBean interface;
 * <li>a method in such an interface;
 * <li>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);
 * <li>a public constructor in the class that implements a Standard MBean
 * or MXBean;
 * <li>a parameter in such a constructor.
 * </ul>
 *
 * <p>Other uses of the annotation are ignored.</p>
 *
 * <p>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.
 *
 * <p>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.</p>
 *
 * <p>The Descriptor resulting from these annotations will be merged
 * with any Descriptor fields provided by the implementation, such as
 * the <a href="Descriptor.html#immutableInfo">{@code
 * immutableInfo}</a> field for an MBean.  The fields from the annotations
 * must be consistent with these fields provided by the implementation.</p>
 *
 * <p>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:</p>
 *
 * <table border="1">
 * <caption style="display:none">Descriptor Field Types</caption>
 * <tr><th>Annotation element</th><th>Descriptor field</th></tr>
 * <tr><td>Primitive value ({@code 5}, {@code false}, etc)</td>
 *     <td>Wrapped value ({@code Integer.valueOf(5)},
 *         {@code Boolean.FALSE}, etc)</td></tr>
 * <tr><td>Class constant (e.g. {@code Thread.class})</td>
 *     <td>Class name from {@link Class#getName()}
 *         (e.g. {@code "java.lang.Thread"})</td></tr>
 * <tr><td>Enum constant (e.g. {@link ElementType#FIELD})</td>
 *     <td>Constant name from {@link Enum#name()}
 *         (e.g. {@code "FIELD"})</td></tr>
 * <tr><td>Array of class constants or enum constants</td>
 *     <td>String array derived by applying these rules to each
 *         element</td></tr>
 * <tr><td>Value of any other type<br>
 *         ({@code String}, {@code String[]}, {@code int[]}, etc)</td>
 *     <td>The same value</td></tr>
 * </table>
 *
 * @since 1.6
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface DescriptorKey {
    
Returns the descriptor key.

Returns:the descriptor key
/**
     * Returns the descriptor key.
     * @return the descriptor key
     */
    String value();
}