Spring 通过@Import实现Bean的注册

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);

        }

      }

    }

  }