本文约2千字，主要知识

• OpenFeign的父子容器
• FeignClient的注册

背景

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。

环境

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

示例工程

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");
            }
        };
    }
}

参考资料

OpenFeign 的使用文档

FeignClient BeanDefinition的注册

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Spring applicationContext 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);
    }
}

registerFeignClients注册所有@FeignClient

public void registerFeignClients(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
    	// 保存clients BeanDefinition 的数组
		LinkedHashSet<BeanDefinition> candidateComponents = new LinkedHashSet<>();
		Map<String, Object> attrs = 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));
			Set<String> basePackages = getBasePackages(metadata);
			for (String basePackage : basePackages) {
				candidateComponents.addAll(scanner.findCandidateComponents(basePackage));
			}
		}
		else {
            // 已手动指定，直接添加
			for (Class<?> clazz : 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
				Map<String, Object> attributes = 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, Map<String, Object> attributes) {
		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

FeignContext 容器的实例化

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<Object>, InitializingBean, ApplicationContextAware, BeanFactoryAware {
    @Override    
    public Object getObject() {
        return getTarget();
    }


    <T> T getTarget() {
        // ③.3 以注入的FeignContext，构建Feign
        FeignContext context = beanFactory != null ? beanFactory.getBean(FeignContext.class)
            : applicationContext.getBean(FeignContext.class);
        // ③.5 构建feign，并创建子容器
        Feign.Builder builder = feign(context);

        String url = this.url + cleanPath();
        Client client = getOptional(context, Client.class);
        if (client != null) {
            // ... 
            builder.client(client);
        }
    	// 应用自定义的配置项
        applyBuildCustomizers(context, builder);

        Targeter targeter = get(context, Targeter.class);
        return (T) targeter.target(this, builder, context, new HardCodedTarget<>(type, name, url));
    }

    protected Feign.Builder feign(FeignContext context) {
        // ③.5.1 首次从子容器获取bean，将触发子容器创建
		FeignLoggerFactory loggerFactory = get(context, FeignLoggerFactory.class);
		Logger logger = loggerFactory.create(type);

		Feign.Builder builder = get(context, Feign.Builder.class)
				// required values
				.logger(logger)
				.encoder(get(context, Encoder.class))
				.decoder(get(context, Decoder.class))
				.contract(get(context, Contract.class));
	
        // ③.5.2  根据FeignClientProperties、FeignClientConfiguration、父容器配置Feign
		configureFeign(context, builder);

		return builder;
	}

    protected <T> T get(FeignContext context, Class<T> type) {
		T instance = context.getInstance(contextId, type);
		if (instance == null) {
			throw new IllegalStateException("No bean found of type " + type + " for " + contextId);
		}
		return instance;
	}

}

根据Spring Boot的SPI机制，可以发现在IOC 容器中引入了FeignAutoConfiguration。

FeignAutoConfiguration主要工作是注入一个FeignContext，与上方③.3 相呼应

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(Feign.class)
@EnableConfigurationProperties({ FeignClientProperties.class, FeignHttpClientProperties.class, FeignEncoderProperties.class })
public class FeignAutoConfiguration {
    // 注入第一节中的所有配置
    @Autowired(required = false)
    private List<FeignClientSpecification> configurations = new ArrayList<>();
    
    // ③.4 构建FeignContext
    @Bean
    public FeignContext feignContext() {
        FeignContext context = new FeignContext();
        context.setConfigurations(this.configurations);
        return context;
    }
}

在③.4 步骤时，断点查看configurations，可看到 步骤② 注册到Spring IOC容器中的FeignClientSpecification

public class FeignContext extends NamedContextFactory<FeignClientSpecification> {

    public FeignContext() {
        // 注意FeignClientsConfiguration 类，其中存放了默认Bean配置
        super(FeignClientsConfiguration.class, "feign", "feign.client.name");
    }
}

FeignContext继承于NamedContextFactory：NamedContextFactory 可以创建一个子容器（或者说子上下文）。OpenFeign 则是依赖它来实现子容器机制的。

FeignContext 子容器的创建

在③.5中，多处调用了FeignClientFactoryBean#get方法，此方法将调用NamedContextFactory#createContext方法创建子容器。

// 子容器 map集合
private Map<String, AnnotationConfigApplicationContext> contexts = new ConcurrentHashMap<>();


public <T> T getInstance(String name, Class<T> type) {
    AnnotationConfigApplicationContext context = getContext(name);
    try {
        // 从容器中获取Bean
        return context.getBean(type);
    }
    catch (NoSuchBeanDefinitionException e) {
        // ignore
    }
    return null;
}

protected AnnotationConfigApplicationContext getContext(String name) {
    if (!this.contexts.containsKey(name)) {
        synchronized (this.contexts) {
            if (!this.contexts.containsKey(name)) {
                // 不存在时，将创建子容器
                this.contexts.put(name, createContext(name));
            }
        }
    }
    // 从子容器中获取
    return this.contexts.get(name);
}

// 创建子容器的方法
protected AnnotationConfigApplicationContext createContext(String name) {
    AnnotationConfigApplicationContext context;
    if (this.parent != null) {
        // 处理父类信息
        DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
        if (parent instanceof ConfigurableApplicationContext) {
            beanFactory.setBeanClassLoader(
                ((ConfigurableApplicationContext) parent).getBeanFactory().getBeanClassLoader());
        }
        else {
            beanFactory.setBeanClassLoader(parent.getClassLoader());
        }
        context = new AnnotationConfigApplicationContext(beanFactory);
        context.setClassLoader(this.parent.getClassLoader());
    }
    else {
        // 当前已为顶级
        context = new AnnotationConfigApplicationContext();
    }
    
    if (this.configurations.containsKey(name)) {
        for (Class<?> configuration : this.configurations.get(name).getConfiguration()) {
            // 处理@FeignClient 中定义的configuration，对应背景中的`MyApiConfiguration`
            context.register(configuration);
        }
    }
    for (Map.Entry<String, C> entry : this.configurations.entrySet()) {
        if (entry.getKey().startsWith("default.")) {
            for (Class<?> configuration : entry.getValue().getConfiguration()) {
                context.register(configuration);
            }
        }
    }
    context.register(PropertyPlaceholderAutoConfiguration.class, this.defaultConfigType);
    context.getEnvironment().getPropertySources().addFirst(new MapPropertySource(this.propertySourceName,
                                                                                 Collections.<String, Object>singletonMap(this.propertyName, name)));
    if (this.parent != null) {
        // 设置父容器，目的是让所有bean也使用父容器的上下文信息
        context.setParent(this.parent);
    }
    context.setDisplayName(generateDisplayName(name));
    // 和Spring IOC容器相同，走refresh流程
    context.refresh();
    return context;
}

同样的，在子容器的refresh 过程中（具体为finishBeanFactoryInitialization->beanFactory.preInstantiateSingletons），将初始化子容器所需要的Bean，如背景中定义的MyEncoder，以及FeignClientsConfiguration类中定义的名为feignXX的各个bean。
之后情况则与IOC 容器初始化过程类似，此处不再赘述。

子容器初始化完成后，执行到③.5.2 处的代码，其根据FeignClientProperties、FeignClientConfiguration、父容器配置Feign；其中，FeignClientConfiguration主要是注入一些默认Bean，如下所示

@Configuration(proxyBeanMethods = false)
public class FeignClientsConfiguration {
    @Bean
    @ConditionalOnMissingBean
    public Decoder feignDecoder() {
        return new OptionalDecoder(new ResponseEntityDecoder(new SpringDecoder(this.messageConverters)));
    }

    //... 省略其他默认Encoder、Capability 等
}

之后，执行至④，将@FeignClinet标注类，以bean的形式注册入Spring IOC父容器。至此，FeignClient的注册已完成。

总结