001/*
002 * Copyright 2002-2016 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.context.annotation;
018
019import java.lang.annotation.Documented;
020import java.lang.annotation.ElementType;
021import java.lang.annotation.Inherited;
022import java.lang.annotation.Retention;
023import java.lang.annotation.RetentionPolicy;
024import java.lang.annotation.Target;
025
026/**
027 * Indicates that a bean should be given preference when multiple candidates
028 * are qualified to autowire a single-valued dependency. If exactly one
029 * 'primary' bean exists among the candidates, it will be the autowired value.
030 *
031 * <p>This annotation is semantically equivalent to the {@code <bean>} element's
032 * {@code primary} attribute in Spring XML.
033 *
034 * <p>May be used on any class directly or indirectly annotated with
035 * {@code @Component} or on methods annotated with @{@link Bean}.
036 *
037 * <h2>Example</h2>
038 * <pre class="code">
039 * &#064;Component
040 * public class FooService {
041 *
042 *     private FooRepository fooRepository;
043 *
044 *     &#064;Autowired
045 *     public FooService(FooRepository fooRepository) {
046 *         this.fooRepository = fooRepository;
047 *     }
048 * }
049 *
050 * &#064;Component
051 * public class JdbcFooRepository extends FooRepository {
052 *
053 *     public JdbcFooRepository(DataSource dataSource) {
054 *         // ...
055 *     }
056 * }
057 *
058 * &#064;Primary
059 * &#064;Component
060 * public class HibernateFooRepository extends FooRepository {
061 *
062 *     public HibernateFooRepository(SessionFactory sessionFactory) {
063 *         // ...
064 *     }
065 * }
066 * </pre>
067 *
068 * <p>Because {@code HibernateFooRepository} is marked with {@code @Primary},
069 * it will be injected preferentially over the jdbc-based variant assuming both
070 * are present as beans within the same Spring application context, which is
071 * often the case when component-scanning is applied liberally.
072 *
073 * <p>Note that using {@code @Primary} at the class level has no effect unless
074 * component-scanning is being used. If a {@code @Primary}-annotated class is
075 * declared via XML, {@code @Primary} annotation metadata is ignored, and
076 * {@code <bean primary="true|false"/>} is respected instead.
077 *
078 * @author Chris Beams
079 * @since 3.0
080 * @see Lazy
081 * @see Bean
082 * @see ComponentScan
083 * @see org.springframework.stereotype.Component
084 */
085@Target({ElementType.TYPE, ElementType.METHOD})
086@Retention(RetentionPolicy.RUNTIME)
087@Inherited
088@Documented
089public @interface Primary {
090
091}