OpenFeignAutoConfiguration源码解析-创新互联

本文约2千字，主要知识

• OpenFeign的父子容器
• FeignClient的注册

在使用Spring Cloud时，经常使用OpenFeign 作为远程服务调用的类库；
Feign 是一种声明式服务调用组件，它在 RestTemplate 的基础上做了进一步的封装。通过 Feign，我们只需要声明一个接口并通过注解进行简单的配置（类似于 Dao 接口上面的 Mapper 注解一样）即可实现对 HTTP 接口的绑定。通过 Feign，我们可以像调用本地方法一样来调用远程服务，而完全感觉不到这是在进行远程调用。
Spring Boot3中，也引入了一套全新的声明式HTTP调用，这也可说明声明式的调用方式收到足够的青睐。

Spring Boot : 2.7.6
spring-cloud：2021.0.5
OpenFeign-core : 11.10
jdk：17

https://github.com/Fudeveloper/openfeign-demo.git

以下为示例工程的主要类

@FeignClient(name = "api-proxy"
             ,url = "http://127.0.0.1:8080"
             ,configuration = MyApiConfiguration.class)
public interface MyApi {@GetMapping("/test/my")
    String testForGet();
}

public class MyApiConfiguration {@Bean
    public Encoder myEncoder() {return new Encoder() {@Override
            public void encode(Object object, Type bodyType, RequestTemplate template) throws EncodeException {System.out.println("trigger myEncoder#encode");
            }
        };
    }
}

OpenFeign 的使用文档

按照国际惯例，要启用SpringBoot的某个组件，需要先标注@EnableXXX注解；OpenFeign 也不例外，其注解类@EnableFeignClients主要内容如下：

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
// 核心：导入FeignClientsRegistrar
@Import(FeignClientsRegistrar.class)
public @interface EnableFeignClients {	String[] basePackages() default {};
	Class[] clients() default {};
	// ... 
}

其中，最核心的动作是@Import(FeignClientsRegistrar.class)
FeignClientsRegistrar实现了ImportBeanDefinitionRegistrar接口，实现了一套自定义的BeanDefinition注册流程。在SpringapplicationContext容器refresh的过程中（具体为invokeBeanFactoryPostProcessors->postProcessBeanDefinitionRegistry->processConfigBeanDefinitions）时，将调用FeignClientsRegistrar#registerBeanDefinitions

class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware {@Override
    public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {// 注册 默认配置
        registerDefaultConfiguration(metadata, registry);
        // 注册 FeignClient
        registerFeignClients(metadata, registry);
    }
}

public void registerFeignClients(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {// 保存clients BeanDefinition 的数组
		LinkedHashSetcandidateComponents = new LinkedHashSet<>();
		Mapattrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());
		final Class[] clients = attrs == null ? null : (Class[]) attrs.get("clients");
		if (clients == null || clients.length == 0) {// 如果未手动指定 clients，则扫描后添加
			ClassPathScanningCandidateComponentProvider scanner = getScanner();
			scanner.setResourceLoader(this.resourceLoader);
			scanner.addIncludeFilter(new AnnotationTypeFilter(FeignClient.class));
			SetbasePackages = getBasePackages(metadata);
			for (String basePackage : basePackages) {		candidateComponents.addAll(scanner.findCandidateComponents(basePackage));
			}
		}
		else {// 已手动指定，直接添加
			for (Classclazz : clients) {		candidateComponents.add(new AnnotatedGenericBeanDefinition(clazz));
			}
		}
        // ①遍历所有client，并注册
		for (BeanDefinition candidateComponent : candidateComponents) {	if (candidateComponent instanceof AnnotatedBeanDefinition) {// ... 

                // 获取@FeignClient 标注类的属性
				Mapattributes = annotationMetadata.getAnnotationAttributes(FeignClient.class.getCanonicalName());

				String name = getClientName(attributes);
                // ② 注册 ${name}.FeignClientSpecification 
				registerClientConfiguration(registry, name, attributes.get("configuration"));
            	// ③ 注册client的bean
				registerFeignClient(registry, annotationMetadata, attributes);
			}
		}
	}

在①处断点后可以发现，candidateComponents已装载我们自定义的FeignClient

private void registerFeignClient(BeanDefinitionRegistry registry, AnnotationMetadata annotationMetadata, Mapattributes) {String className = annotationMetadata.getClassName();
		Class clazz = ClassUtils.resolveClassName(className, null);
        // 强转
		ConfigurableBeanFactory beanFactory = registry instanceof ConfigurableBeanFactory
				? (ConfigurableBeanFactory) registry : null;
		String contextId = getContextId(beanFactory, attributes);
		String name = getName(attributes);
        // ③.1 构建factoryBean
		FeignClientFactoryBean factoryBean = new FeignClientFactoryBean();
		factoryBean.setBeanFactory(beanFactory);
		factoryBean.setName(name);
		factoryBean.setContextId(contextId);
		factoryBean.setType(clazz);
        // 是否可刷新，从feign.client.refresh-enabled配置解析
		factoryBean.setRefreshableClient(isClientRefreshEnabled());

        // 将@FeignClinet标注类的配置信息构建为BeanDefinition
        // clazz即为@FeignClinet标注类的class
		BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(clazz, () ->{	factoryBean.setUrl(getUrl(beanFactory, attributes));
			factoryBean.setPath(getPath(beanFactory, attributes));
			factoryBean.setDecode404(Boolean.parseBoolean(String.valueOf(attributes.get("decode404"))));
			Object fallback = attributes.get("fallback");
			if (fallback != null) {		factoryBean.setFallback(fallback instanceof Class ? (Class) fallback
						: ClassUtils.resolveClassName(fallback.toString(), null));
			}
			Object fallbackFactory = attributes.get("fallbackFactory");
			if (fallbackFactory != null) {		factoryBean.setFallbackFactory(fallbackFactory instanceof Class ? (Class) fallbackFactory
						: ClassUtils.resolveClassName(fallbackFactory.toString(), null));
			}
            // ③.2 注意此处是从factoryBean 获取对象，具体为FeignClientFactoryBean#getObject
			return factoryBean.getObject();
		});
		definition.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE);
		definition.setLazyInit(true);
		validate(attributes);

		AbstractBeanDefinition beanDefinition = definition.getBeanDefinition();
		beanDefinition.setAttribute(FactoryBean.OBJECT_TYPE_ATTRIBUTE, className);
		beanDefinition.setAttribute("feignClientsRegistrarFactoryBean", factoryBean);

		// has a default, won't be null
		boolean primary = (Boolean) attributes.get("primary");

		beanDefinition.setPrimary(primary);

		String[] qualifiers = getQualifiers(attributes);
		if (ObjectUtils.isEmpty(qualifiers)) {	qualifiers = new String[] {contextId + "FeignClient" };
		}

		BeanDefinitionHolder holder = new BeanDefinitionHolder(beanDefinition, className, qualifiers);
        // 委托给DefaultListableBeanFactory#registerBeanDefinition
        // ④ 将@FeignClinet标注类注册入Spring IOC父容器
		BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
    	// 支持Request.Options的refresh 操作，非本章重点
		registerOptionsBeanDefinition(registry, contextId);
	}

registerDefaultConfiguration、registerFeignClients，最终都将调用registerClientConfiguration方法。
② 和③ 方法则将实际操作委托给DefaultListableBeanFactory#registerBeanDefinition；这涉及到Spring IOC的核心流程，此处不再赘述。在本文中只需记住如下现象。若要了解更多相关知识，可参考： 《Spring 揭秘》第二部分：Spring IOC容器

BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
if (existingDefinition != null) {// 不存在则加入集合
    this.beanDefinitionMap.put(beanName, beanDefinition);
    this.beanDefinitionNames.add(beanName);
}

在③结束后，查看DefaultListableBeanFactory#beanDefinitionMap,可发现我们自定义的FeignClient已注册到IOC 容器中

回看③.1 、③.2的操作，即创建FeignClientFactoryBean并从中获取对象；在Spring中，FactoryBean是个举足轻重的类，可参考：What’s a FactoryBean?

注意③.2的后续操作：FeignClientFactoryBean#getObject

public class FeignClientFactoryBean implements FactoryBean