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package org.glassfish.config.support;

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.Target;

import static java.lang.annotation.RetentionPolicy.RUNTIME;

/**
 * Annotation that qualifies a configuration element that can only exist
 * as a singleton in the configuration tree.
 *
 * When configuration elements are added to their parents through a
 * subclassing pattern, it can be difficult for the system to determine if this
 * element can exist as a singleton or a collection.
 *
 * For instance, when the parent contains :
 * 
 * @Element("*")
 * List getExtensions();
 * 
 *
 * A subclass of Extension can have a single or multiple instances stored
 * in the extensions list. Adding this annotation will qualify that only
 * one instance of the annotated configuration can be found in that
 * collection. Note that the collection can contain other instances of other
 * subclasses of Extension.
 *
 * @author Jerome Dochez
 */
@Retention(RUNTIME)
@Target(ElementType.TYPE)
public @interface Singleton {
}