Fine-tuning Annotation-based Autowiring with @Primary or @Fallback

由于按类型自动装配可能导致出现多个候选对象，因此通常需要对选择过程进行更多控制。一种实现此目的的方法是使用 Spring 的 @Primary 注解。@Primary 指示当多个 Bean 是要自动装配到单值依赖项的候选对象时，应优先考虑特定的 Bean。如果候选对象中恰好只有一个主要 Bean，则它将成为自动装配的值。

考虑将 firstMovieCatalog 定义为主要 MovieCatalog 的以下配置：

  • Java

  • Kotlin

@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 开始，有一个 @Fallback 注解，用于划分要注入的除常规 Bean 之外的任何 Bean。如果只保留了一个常规 Bean，则它实际上也是主要 Bean：

  • Java

  • Kotlin

@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 { ... }

	// ...
}

使用前述配置的两个变体，以下 MovieRecommender 将使用 firstMovieCatalog 自动装配：

  • Java

  • Kotlin

public class MovieRecommender {

	@Autowired
	private MovieCatalog movieCatalog;

	// ...
}
class MovieRecommender {

	@Autowired
	private lateinit var movieCatalog: MovieCatalog

	// ...
}

相应的 Bean 定义如下：

<?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>