今天看到一个神奇的用法, Spring可以通过@Import导入实现了ImportBeanDefinitionRegistrar接口的类来注册那个类.
ImportBeanDefinitionRegistrar接口是用来被实现以使Spring在处理Configuration的时候注册额外的bean. @Import注解可以引入一个Configuration类, 或ImportSelector实现类以根据特定条件导入合适的类. 其次就是可以引入实现了ImportBeanDefinitionRegistrar接口的类来直接注册类.

当然, 除了ImportBeanDefinitionRegistrar实现类, 还可以使用ClassPathBeanDefinitionScanner这个扫描器, 获取需要注册的bean. ClassPathBeanDefinitionScanner继承了ClassPathScanningCandidateComponentProvider, 其中有两个TypeFilter, includeFilters和excludeFilters, 可以用来过滤类.

比如以下即实现了ImportBeanDefinitionRegistrar接口:

class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware {public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {this.registerDefaultConfiguration(metadata, registry);this.registerFeignClients(metadata, registry);}
}

在registerFeignClients使用ClassPathScanningCandidateComponentProvider来过滤类, 注入包含了@FeignClient注解的类.

  public void registerFeignClients(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {ClassPathScanningCandidateComponentProvider scanner = this.getScanner();scanner.setResourceLoader(this.resourceLoader);Map<String, Object> attrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(FeignClient.class);Class<?>[] clients = attrs == null ? null : (Class[])((Class[])attrs.get("clients"));Object basePackages;if (clients != null && clients.length != 0) {final Set<String> clientClasses = new HashSet();basePackages = new HashSet();Class[] var9 = clients;int var10 = clients.length;for(int var11 = 0; var11 < var10; ++var11) {Class<?> clazz = var9[var11];((Set)basePackages).add(ClassUtils.getPackageName(clazz));clientClasses.add(clazz.getCanonicalName());}AbstractClassTestingTypeFilter filter = new AbstractClassTestingTypeFilter() {protected boolean match(ClassMetadata metadata) {String cleaned = metadata.getClassName().replaceAll("\\$", ".");return clientClasses.contains(cleaned);}};scanner.addIncludeFilter(new FeignClientsRegistrar.AllTypeFilter(Arrays.asList(filter, annotationTypeFilter)));} else {scanner.addIncludeFilter(annotationTypeFilter);basePackages = this.getBasePackages(metadata);}Iterator var17 = ((Set)basePackages).iterator();while(var17.hasNext()) {String basePackage = (String)var17.next();Set<BeanDefinition> candidateComponents = scanner.findCandidateComponents(basePackage);Iterator var21 = candidateComponents.iterator();while(var21.hasNext()) {BeanDefinition candidateComponent = (BeanDefinition)var21.next();if (candidateComponent instanceof AnnotatedBeanDefinition) {AnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition)candidateComponent;AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();Assert.isTrue(annotationMetadata.isInterface(), "@FeignClient can only be specified on an interface");Map<String, Object> attributes = annotationMetadata.getAnnotationAttributes(FeignClient.class.getCanonicalName());String name = this.getClientName(attributes);this.registerClientConfiguration(registry, name, attributes.get("configuration"));this.registerFeignClient(registry, annotationMetadata, attributes);}}}}