使用 @Primary 或 @Fallback 微調基於註解的自動裝配

由於依類型自動裝配可能會導致多個候選者，因此通常需要更精確地控制選擇過程。一種方法是使用 Spring 的 @Primary 註解。@Primary 表示當多個 Bean 成為自動裝配到單值相依性的候選者時，應優先考慮特定的 Bean。如果候選者中正好存在一個主要 Bean，則它將成為自動裝配的值。

考慮以下組態，其將 firstMovieCatalog 定義為主要的 MovieCatalog

  • 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，從 6.2 開始，有一個 @Fallback 註解，用於標記要注入的任何非常規 Bean。如果只剩下一個常規 Bean，它實際上也成為主要的 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 { ... }

	// ...
}

使用上述兩種組態變體，以下 MovieRecommender 都會自動裝配 firstMovieCatalog

  • Java

  • Kotlin

public class MovieRecommender {

	@Autowired
	private MovieCatalog movieCatalog;

	// ...
}
class MovieRecommender {

	@Autowired
	private lateinit var movieCatalog: MovieCatalog

	// ...
}

對應的 Bean 定義如下

<?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>