DescriptorKey.java

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 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
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 * accompanied this code).
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package javax.management;

import java.lang.annotation.*;

/**
 * <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 class="striped">
 * <caption style="display:none">Descriptor Fields</caption>
 * <thead>
 * <tr><th scope="col">Name</th><th scope="col">Value</th></tr>
 * </thead>
 * <tbody style="text-align:left">
 * <tr><th scope="row">units</th><td>"bytes"</td></tr>
 * <tr><th scope="row">descriptionResourceKey</th><td>"bytes.key"</td></tr>
 * <tr><th scope="row">descriptionResourceBundleBaseName</th>
 *     <td>"com.example.foo.MBeanResources"</td></tr>
 * </tbody>
 * </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 class="striped">
 * <caption style="display:none">Descriptor Field Types</caption>
 * <thead>
 * <tr><th scope="col">Annotation element</th><th scope="col">Descriptor field</th></tr>
 * </thead>
 * <tbody style="text-align:left">
 * <tr><th scope="row">Primitive value ({@code 5}, {@code false}, etc)</th>
 *     <td>Wrapped value ({@code Integer.valueOf(5)},
 *         {@code Boolean.FALSE}, etc)</td></tr>
 * <tr><th scope="row">Class constant (e.g. {@code Thread.class})</th>
 *     <td>Class name from {@link Class#getName()}
 *         (e.g. {@code "java.lang.Thread"})</td></tr>
 * <tr><th scope="row">Enum constant (e.g. {@link ElementType#FIELD})</th>
 *     <td>Constant name from {@link Enum#name()}
 *         (e.g. {@code "FIELD"})</td></tr>
 * <tr><th scope="row">Array of class constants or enum constants</th>
 *     <td>String array derived by applying these rules to each
 *         element</td></tr>
 * <tr><th scope="row">Value of any other type<br>
 *         ({@code String}, {@code String[]}, {@code int[]}, etc)</th>
 *     <td>The same value</td></tr>
 * </tbody>
 * </table>
 *
 * @since 1.6
 */
@Documented
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface DescriptorKey {
    /**
     * Returns the descriptor key.
     * @return the descriptor key
     */
    String value();
}