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Fine-tuning Annotation-based Autowiring with @Primary
or @Fallback
Because autowiring by type may lead to multiple candidates, it is often necessary to have
more control over the selection process. One way to accomplish this is with Spring’s
@Primary
annotation. @Primary
indicates that a particular bean should be given
preference when multiple beans are candidates to be autowired to a single-valued
dependency. If exactly one primary bean exists among the candidates, it becomes the
autowired value.
Consider the following configuration that defines firstMovieCatalog
as the
primary 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 { ... }
// ...
}
Alternatively, as of 6.2, there is a @Fallback
annotation for demarcating
any beans other than the regular ones to be injected. If only one regular
bean is left, it is effectively primary as well:
-
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 { ... }
// ...
}
With both variants of the preceding configuration, the following
MovieRecommender
is autowired with the firstMovieCatalog
:
-
Java
-
Kotlin
public class MovieRecommender {
@Autowired
private MovieCatalog movieCatalog;
// ...
}
class MovieRecommender {
@Autowired
private lateinit var movieCatalog: MovieCatalog
// ...
}
The corresponding bean definitions follow:
<?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>