一、定义

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {@AliasFor(annotation = Component.class)
    String value() default "";}

@Configuration 的作用，指定一个或多个 @Bean 办法，由 spring 加载和生成 bean, 相当于 xml 的 root 标签。

二、例子

@Configuration
 public class AppConfig {

     @Bean
     public MyBean myBean() {// instantiate, configure and return bean ...}
 }

三、如何让 spring 加载它？

1、通过 ApplicationContext 加载

AnnotationConfigApplicationContext ctx = new AnnotationConfigApplicationContext();
 ctx.register(AppConfig.class);
 ctx.refresh();
 MyBean myBean = ctx.getBean(MyBean.class);
 // use myBean ...
 

2、通过 xml 加载

 <beans>
    <context:annotation-config/>
    <bean class="com.acme.AppConfig"/>
 </beans>

3、通过 @ComponentScan

@ComponentScan("com.acme.app.services")

待加载的类放到扫描包外面

四、其它用法

1、@Component

• 被 @Configuration 注解的类也是一个 @Component
• @Autowired, @Value 都能用

 @Configuration
 public class AppConfig {

    @Autowired Environment env;

    @Value("${bean.name}") String beanName;

    @Bean
    public MyBean myBean() {MyBean myBean = new MyBean();
        myBean.setName(env.getProperty("bean.name"));
        return myBean;
    }
}

2、搭配 Import

@Configuration
public class DatabaseConfig {

    @Bean
    public DataSource dataSource() {// instantiate, configure and return DataSource}
}

@Configuration
@Import(DatabaseConfig.class)
public class AppConfig {

    private final DatabaseConfig dataConfig;

    public AppConfig(DatabaseConfig dataConfig) {this.dataConfig = dataConfig;}

    @Bean
    public MyBean myBean() {// reference the dataSource() bean method
        return new MyBean(dataConfig.dataSource());
    }
}

3、搭配 Profile 辨别多环境

@Profile("development")
@Configuration
public class EmbeddedDatabaseConfig {

    @Bean
    public DataSource dataSource() {// instantiate, configure and return embedded DataSource}
}

@Profile("production")
@Configuration
public class ProductionDatabaseConfig {

    @Bean
    public DataSource dataSource() {// instantiate, configure and return production DataSource}
}

或者用这种

@Configuration
public class ProfileDatabaseConfig {@Bean("dataSource")
    @Profile("development")
    public DataSource embeddedDatabase() { ...}

    @Bean("dataSource")
    @Profile("production")
    public DataSource productionDatabase() { ...}
}

4、加载 xml 外面的 bean

@Configuration
@ImportResource("classpath:/com/acme/database-config.xml")
public class AppConfig {

    @Inject DataSource dataSource; // from XML

    @Bean
    public MyBean myBean() {
        // inject the XML-defined dataSource bean
        return new MyBean(this.dataSource);
    }
}

5、多个嵌套应用

@Configuration
public class AppConfig {

    @Inject DataSource dataSource;

    @Bean
    public MyBean myBean() {return new MyBean(dataSource);
    }

    @Configuration
    static class DatabaseConfig {
        @Bean
        DataSource dataSource() {return new EmbeddedDatabaseBuilder().build();}
    }
}

欢送关注微信公众号：丰极，更多技术学习分享。