001/*
002 * Copyright 2002-2017 the original author or authors.
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 *      https://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package org.springframework.stereotype;
018
019import java.lang.annotation.Documented;
020import java.lang.annotation.ElementType;
021import java.lang.annotation.Retention;
022import java.lang.annotation.RetentionPolicy;
023import java.lang.annotation.Target;
024
025/**
026 * Indicate that the annotated element represents a stereotype for the index.
027 *
028 * <p>The {@code CandidateComponentsIndex} is an alternative to classpath
029 * scanning that uses a metadata file generated at compilation time. The
030 * index allows retrieving the candidate components (i.e. fully qualified
031 * name) based on a stereotype. This annotation instructs the generator to
032 * index the element on which the annotated element is present or if it
033 * implements or extends from the annotated element. The stereotype is the
034 * fully qualified name of the annotated element.
035 *
036 * <p>Consider the default {@link Component} annotation that is meta-annotated
037 * with this annotation. If a component is annotated with {@link Component},
038 * an entry for that component will be added to the index using the
039 * {@code org.springframework.stereotype.Component} stereotype.
040 *
041 * <p>This annotation is also honored on meta-annotations. Consider this
042 * custom annotation:
043 * <pre class="code">
044 * package com.example;
045 *
046 * &#064;Target(ElementType.TYPE)
047 * &#064;Retention(RetentionPolicy.RUNTIME)
048 * &#064;Documented
049 * &#064;Indexed
050 * &#064;Service
051 * public @interface PrivilegedService { ... }
052 * </pre>
053 *
054 * If the above annotation is present on a type, it will be indexed with two
055 * stereotypes: {@code org.springframework.stereotype.Component} and
056 * {@code com.example.PrivilegedService}. While {@link Service} isn't directly
057 * annotated with {@code Indexed}, it is meta-annotated with {@link Component}.
058 *
059 * <p>It is also possible to index all implementations of a certain interface or
060 * all the subclasses of a given class by adding {@code @Indexed} on it.
061 *
062 * Consider this base interface:
063 * <pre class="code">
064 * package com.example;
065 *
066 * &#064;Indexed
067 * public interface AdminService { ... }
068 * </pre>
069 *
070 * Now, consider an implementation of this {@code AdminService} somewhere:
071 * <pre class="code">
072 * package com.example.foo;
073 *
074 * import com.example.AdminService;
075 *
076 * public class ConfigurationAdminService implements AdminService { ... }
077 * </pre>
078 *
079 * Because this class implements an interface that is indexed, it will be
080 * automatically included with the {@code com.example.AdminService} stereotype.
081 * If there are more {@code @Indexed} interfaces and/or superclasses in the
082 * hierarchy, the class will map to all their stereotypes.
083 *
084 * @author Stephane Nicoll
085 * @since 5.0
086 */
087@Target(ElementType.TYPE)
088@Retention(RetentionPolicy.RUNTIME)
089@Documented
090public @interface Indexed {
091}