spring扩展点之ImportBeanDefinitionRegistrar _spring

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
@Componentpublic class StudentImportBeanDefinitionRegistrar implements ImportBeanDefinitionRegistrar {@Overridepublic void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry) {// 创建beanDefinitionBuilderBeanDefinitionBuilder beanDefinitionBuilder = BeanDefinitionBuilder.genericBeanDefinition(StudentBean.class);beanDefinitionBuilder.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);// 获取beanDefinitionAbstractBeanDefinition beanDefinition = beanDefinitionBuilder.getBeanDefinition();BeanDefinitionHolder beanDefinitionHolder = new BeanDefinitionHolder(beanDefinition, StudentBean.class.getSimpleName());// 注册beanDefinitionBeanDefinitionReaderUtils.registerBeanDefinition(beanDefinitionHolder, registry);}}启动类：使用@Import导入
StudentImportBeanDefinitionRegistrar类
@Import(StudentImportBeanDefinitionRegistrar.class)@SpringBootApplicationpublic 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);}}

文章插图

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@SpringBootApplicationpublic 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

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 {@Overridepublic 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));}}}