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方法，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里面

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区分多环境

@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里面的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、多个嵌套使用

@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();
        }
    }
}

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