一、是什么？

查看接口注释：根据给定的注释元数据，根据需要注册bean定义......spring会遍历所有的beanDefinition，逐个创建对应的bean。

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

PS：创建bean还有哪些方式？

1、@Component、@Bean等等
2、@Import(导入类)
3、实现ImportSelector接口，重写selectImports方法，返回需要导入的全类名
4、实现ImportBeanDefinitionRegistrar接口，注册bean定义

二、demo案例

StudentBean：学生类

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

StudentImportBeanDefinitionRegistrar：实现ImportBeanDefinitionRegistrar接口

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

启动类：使用@Import导入
StudentImportBeanDefinitionRegistrar类

@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);
    }
}

我们大都习惯使用@Enable***方式引入某个组件，此处可稍作改造。

新增注解：EnableStudentBean

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

启动类修改：

@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在其他开源项目中的使用

1. MyBatis

在使用mybatis时，我们需要指定mapper的扫描路径：

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

查看@MapperScan源码，发现导入了MapperScannerRegistrar类，该类实现了
ImportBeanDefinitionRegistrar接口

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

MapperScannerRegistrar类(截取部分代码)：

public class MapperScannerRegistrar implements ImportBeanDefinitionRegistrar {
  @Override
  public void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {
    // 获取MapperScan注解
    AnnotationAttributes mapperScanAttrs = AnnotationAttributes
        .fromMap(importingClassMetadata.getAnnotationAttributes(MapperScan.class.getName()));
    if (mapperScanAttrs != null) {
      // 扫描对应的mapper接口，并注册BeanDefinition(bean定义)
      registerBeanDefinitions(importingClassMetadata, mapperScanAttrs, registry,
          generateBaseBeanName(importingClassMetadata, 0));
    }
  }
}

2. Openfeign

在使用OpenFeign时，我们需要指定feign接口扫描路径：

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

查看@EnableFeignClients源码，发现导入了FeignClientsRegistrar类，该类实现了
ImportBeanDefinitionRegistrar接口

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

FeignClientsRegistrar类(截取部分代码)：

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

扫描所有加了@FeignClient注解的接口，接着注册FeignClientFactoryBean类型的BeanDefinition到容器中，需要使用时生成具体的接口代理实现服务调用。