使用 @Primary 或 @Fallback 微调基于注解的自动装配

ChatGPT-4o 中英对照 Fine-tuning Annotation-based Autowiring with @Primary or @Fallback Fine-tuning Annotation-based Autowiring with @Primary or @Fallback

由于按类型自动装配可能会导致多个候选项，因此通常需要对选择过程进行更多控制。实现这一点的一种方法是使用 Spring 的 @Primary 注解。@Primary 表示当多个 bean 是自动装配到单值依赖项的候选项时，应该优先选择特定的 bean。如果候选项中恰好存在一个主 bean，它将成为自动装配的值。

考虑以下配置，将 firstMovieCatalog 定义为主 MovieCatalog：

Java

@Configuration
public class MovieConfiguration {

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

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

    // ...
}

java

Kotlin

@Configuration
class MovieConfiguration {

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

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

    // ...
}

kotlin

或者，从 6.2 版本开始，有一个 @Fallback 注解用于标记除常规 bean 之外的任何其他 bean 以进行注入。如果只剩下一个常规 bean，它实际上也是主要的：

Java

@Configuration
public class MovieConfiguration {

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

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

    // ...
}

java

Kotlin

@Configuration
class MovieConfiguration {

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

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

    // ...
}

kotlin

在上述配置的两种变体中，以下 MovieRecommender 被自动装配为 firstMovieCatalog：

Java

public class MovieRecommender {

    @Autowired
    private MovieCatalog movieCatalog;

    // ...
}

java

Kotlin

class MovieRecommender {

    @Autowired
    private lateinit var movieCatalog: MovieCatalog

    // ...
}

kotlin

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>

xml