Spring扩展点在微服务应用(待完善)

ApplicationListener扩展

nacos注册服务，监听容器发布事件

# 容器发布事件

AbstractAutoServiceRegistration#onApplicationEvent

# 接收事件吗，注册服务到nacos

NacosServiceRegistry#register

Lifecycle扩展

#订阅服务实例更改的事件

NamingService#subscribe

SmartInitializingSingleton扩展

适用场景：对容器bean对象定制处理

场景：LoadBalance服务名替换成url。

说明： LoadBalancerAutoConfiguration自动配置类定义了SmartInitializingSingleton，在所有非懒加载实例化完后，回调该函数方法。再看RestTemplateCustomizer这个bean，会给restTemplate加工，添加一个loadBalancerInterceptor拦截器。

FactoryBean整合Feign

1. 开启注解@EnableFeignClients, 注解导入@Import({FeignClientsRegistrar.class})

@Retention(RetentionPolicy.RUNTIME)
@Target({ElementType.TYPE})
@Documented
@Import({FeignClientsRegistrar.class})
public @interface EnableFeignClients {String[] value() default {};String[] basePackages() default {};Class<?>[] basePackageClasses() default {};Class<?>[] defaultConfiguration() default {};Class<?>[] clients() default {};
}

2. 在处理 @Import 注解时, 会调用ImportBeanDefinitionRegistrar 的 registerBeanDefinitions

方法, ImportBeanDefinitionRegistrar 是一个用于通过 Java 类动态注册 bean 定义的接口，允许在容器启动阶段向容器中添加额外的 bean。

注册bean定义关键代码

public void registerFeignClients(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {ClassPathScanningCandidateComponentProvider scanner = getScanner();scanner.setResourceLoader(this.resourceLoader);Set<String> basePackages;Map<String, Object> attrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(FeignClient.class);final Class<?>[] clients = attrs == null ? null: (Class<?>[]) attrs.get("clients");if (clients == null || clients.length == 0) {scanner.addIncludeFilter(annotationTypeFilter);basePackages = getBasePackages(metadata);}else {final Set<String> clientClasses = new HashSet<>();basePackages = new HashSet<>();for (Class<?> clazz : clients) {basePackages.add(ClassUtils.getPackageName(clazz));clientClasses.add(clazz.getCanonicalName());}AbstractClassTestingTypeFilter filter = new AbstractClassTestingTypeFilter() {@Overrideprotected boolean match(ClassMetadata metadata) {String cleaned = metadata.getClassName().replaceAll("\\$", ".");return clientClasses.contains(cleaned);}};scanner.addIncludeFilter(new AllTypeFilter(Arrays.asList(filter, annotationTypeFilter)));}for (String basePackage : basePackages) {Set<BeanDefinition> candidateComponents = scanner.findCandidateComponents(basePackage);for (BeanDefinition candidateComponent : candidateComponents) {if (candidateComponent instanceof AnnotatedBeanDefinition) {// verify annotated class is an interfaceAnnotatedBeanDefinition 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 = getClientName(attributes);registerClientConfiguration(registry, name,attributes.get("configuration"));registerFeignClient(registry, annotationMetadata, attributes);}}}}private void registerFeignClient(BeanDefinitionRegistry registry,AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {String className = annotationMetadata.getClassName();BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(FeignClientFactoryBean.class);validate(attributes);definition.addPropertyValue("url", getUrl(attributes));definition.addPropertyValue("path", getPath(attributes));String name = getName(attributes);definition.addPropertyValue("name", name);String contextId = getContextId(attributes);definition.addPropertyValue("contextId", contextId);definition.addPropertyValue("type", className);definition.addPropertyValue("decode404", attributes.get("decode404"));definition.addPropertyValue("fallback", attributes.get("fallback"));definition.addPropertyValue("fallbackFactory", attributes.get("fallbackFactory"));definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);String alias = contextId + "FeignClient";AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();boolean primary = (Boolean) attributes.get("primary"); // has a default, won't be// nullbeanDefinition.setPrimary(primary);String qualifier = getQualifier(attributes);if (StringUtils.hasText(qualifier)) {alias = qualifier;}BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className,new String[] { alias });BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);}

3. FeignClientFactoryBean实现了FactoryBean和InitializingBean和ApplicationContextAware