一、是什么？

查看接口注釋：根據給定的注釋元數據，根據需要注冊bean定義......spring會遍歷所有的beanDefinition，逐個創建對應的bean。

public interface ImportBeanDefinitionRegistrar {
	/**
	 * Register bean definitions as necessary based on the given annotation metadata of
	 * the importing {@code @Configuration} class.......(截取部分注釋)
	 */
	default void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry,
			BeanNameGenerator importBeanNameGenerator) {
		registerBeanDefinitions(importingClassMetadata, registry);
	}
	default void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
	}
}

PS：創建bean還有哪些方式？

1、@Component、@Bean等等
2、@Import(導入類)
3、實現ImportSelector接口，重寫selectImports方法，返回需要導入的全類名
4、實現ImportBeanDefinitionRegistrar接口，注冊bean定義

二、demo案例

StudentBean：學生類

public class StudentBean {
    private String stuName;
    public String getStuName() {
        return stuName;
    }
    public void setStuName(String stuName) {
        this.stuName = stuName;
    }
}

StudentImportBeanDefinitionRegistrar：實現ImportBeanDefinitionRegistrar接口

@Component
public class StudentImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {
    @Override
    public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
        // 創建beanDefinitionBuilder
        BeanDefinitionBuilder beanDefinitionBuilder = BeanDefinitionBuilder.genericBeanDefinition(StudentBean.class);
        beanDefinitionBuilder.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
        // 獲取beanDefinition
        AbstractBeanDefinition beanDefinition = beanDefinitionBuilder.getBeanDefinition();
        BeanDefinitionHolder beanDefinitionHolder = new BeanDefinitionHolder(beanDefinition, StudentBean.class.getSimpleName());
        // 注冊beanDefinition
        BeanDefinitionReaderUtils.registerBeanDefinition(beanDefinitionHolder, registry);
    }
}

啟動類：使用@Import導入
StudentImportBeanDefinitionRegistrar類

@Import(StudentImportBeanDefinitionRegistrar.class)
@SpringBootApplication
public class DemoApplication {
    public static void mAIn(String[] args) {
        ConfigurableApplicationContext applicationContext = SpringApplication.run(DemoApplication.class, args);
        StudentBean studentBean = applicationContext.getBean(StudentBean.class);
        System.out.println("studentBean: " + studentBean);
    }
}

我們大都習慣使用@Enable***方式引入某個組件，此處可稍作改造。

新增注解：EnableStudentBean

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(StudentImportBeanDefinitionRegistrar.class)
public @interface EnableStudentBean {
}

啟動類修改：

@EnableStudentBean
@SpringBootApplication
public class DemoApplication {
    public static void main(String[] args) {
        ConfigurableApplicationContext applicationContext = SpringApplication.run(DemoApplication.class, args);
        StudentBean studentBean = applicationContext.getBean(StudentBean.class);
        System.out.println("studentBean: " + studentBean);
    }
}

三、ImportBeanDefinitionRegistrar在其他開源項目中的使用

1. MyBatis

在使用mybatis時，我們需要指定mapper的掃描路徑：

@MapperScan(basePackages = "com.test.demo.dao.mapper")

查看@MapperScan源碼，發現導入了MapperScannerRegistrar類，該類實現了
ImportBeanDefinitionRegistrar接口

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(MapperScannerRegistrar.class)
@Repeatable(MapperScans.class)
public @interface MapperScan

MapperScannerRegistrar類(截取部分代碼)：

public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar {
  @Override
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    // 獲取MapperScan注解
    AnnotationAttributes mapperScanAttrs = AnnotationAttributes
        .fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
    if (mapperScanAttrs != null) {
      // 掃描對應的mapper接口，并注冊BeanDefinition(bean定義)
      registerBeanDefinitions(importingClassMetadata, mapperScanAttrs, registry,
          generateBaseBeanName(importingClassMetadata, 0));
    }
  }
}

2. Openfeign

在使用OpenFeign時，我們需要指定feign接口掃描路徑：

@EnableFeignClients(basePackages = ""com.test.demo")

查看@EnableFeignClients源碼，發現導入了FeignClientsRegistrar類，該類實現了
ImportBeanDefinitionRegistrar接口

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(FeignClientsRegistrar.class)
public @interface EnableFeignClients

FeignClientsRegistrar類(截取部分代碼)：

class FeignClientsRegistrar	implements ImportBeanDefinitionRegistrar {
	@Override
	public void registerBeanDefinitions(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {
		registerDefaultConfiguration(metadata, registry);
    registerFeignClients(metadata, registry);
	}
}

掃描所有加了@FeignClient注解的接口，接著注冊FeignClientFactoryBean類型的BeanDefinition到容器中，需要使用時生成具體的接口代理實現服務調用。