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使用 @Primary 或 @Fallback 微调基于注解的自动装配

由于按类型自动连线可能导致多个候选项，因此通常需要对选择过程有更多控制。一种实现此目的的方法是使用Spring的 @Primary注解。@Primary表示当多个bean是单值依赖项的自动连线候选项时，应优先考虑特定的bean。如果候选项中恰好有一个主要bean，它将成为自动连线的值。spring-doc.cadn.net.cn

考虑以下配置，其中 firstMovieCatalog 被定义为 主 MovieCatalog：spring-doc.cadn.net.cn

@Configuration
public class MovieConfiguration {

	@Bean
	@Primary
	public MovieCatalog firstMovieCatalog() { ... }

	@Bean
	public MovieCatalog secondMovieCatalog() { ... }

	// ...
}
@Configuration
class MovieConfiguration {

	@Bean
	@Primary
	fun firstMovieCatalog(): MovieCatalog { ... }

	@Bean
	fun secondMovieCatalog(): MovieCatalog { ... }

	// ...
}

或者，从6.2版本开始，可以使用 @Fallback 注解来标记除普通 Bean 之外的其他 Bean 以进行注入。如果只剩下一种普通 Bean，则它实际上也具有首要性：spring-doc.cadn.net.cn

@Configuration
public class MovieConfiguration {

	@Bean
	public MovieCatalog firstMovieCatalog() { ... }

	@Bean
	@Fallback
	public MovieCatalog secondMovieCatalog() { ... }

	// ...
}
@Configuration
class MovieConfiguration {

	@Bean
	fun firstMovieCatalog(): MovieCatalog { ... }

	@Bean
	@Fallback
	fun secondMovieCatalog(): MovieCatalog { ... }

	// ...
}

通过上述配置的两种变体，以下 MovieRecommender 将被自动装配到 firstMovieCatalog 中：spring-doc.cadn.net.cn

public class MovieRecommender {

	@Autowired
	private MovieCatalog movieCatalog;

	// ...
}
class MovieRecommender {

	@Autowired
	private lateinit var movieCatalog: MovieCatalog

	// ...
}

对应的bean定义如下：spring-doc.cadn.net.cn

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	xmlns:context="http://www.springframework.org/schema/context"
	xsi:schemaLocation="http://www.springframework.org/schema/beans
		https://www.springframework.org/schema/beans/spring-beans.xsd
		http://www.springframework.org/schema/context
		https://www.springframework.org/schema/context/spring-context.xsd">

	<context:annotation-config/>

	<bean class="example.SimpleMovieCatalog" primary="true">
		<!-- inject any dependencies required by this bean -->
	</bean>

	<bean class="example.SimpleMovieCatalog">
		<!-- inject any dependencies required by this bean -->
	</bean>

	<bean id="movieRecommender" class="example.MovieRecommender"/>

</beans>