Spring知識點-@Configuration使用

1、定義

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Component
public @interface Configuration {

    @AliasFor(annotation = Component.class)
    String value() default "";

}

@Configuration的做用，指定一個或多個@Bean方法，由spring加載和生成bean,至關於xml的root標籤。

2、例子

@Configuration
 public class AppConfig {

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

3、如何讓spring加載它？

一、經過ApplicationContext加載

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

二、經過xml加載

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

三、經過@ComponentScan

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

待加載的類放到掃描包裏面

4、其它用法

一、@Component

• 被@Configuration註解的類也是一個@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;
    }
}

二、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());
    }
}

三、搭配Profile區分多環境

@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
    }
}

或者用這種

@Configuration
public class ProfileDatabaseConfig {

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

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

四、加載xml裏面的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);
    }
}

五、多個嵌套使用

@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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