Openfegin 源码分析

在使用 fegin 的过程中，最重要的一点就是在 application 中增加@EnableFeignClients，来启用 feign。

我们来看看这个注解的声明。

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@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(FeignClientsRegistrar.class)
public @interface EnableFeignClients {
  ...
}

在这个注解中，主要是通过 import 动态装载了 FeignClientsRegistrar 类，这个就是 spring 的动态装载 registrar 的实现，我们来看看 FeignClientsRegistrar 的具体实现

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@Override
public void registerBeanDefinitions(AnnotationMetadata metadata,
		BeanDefinitionRegistry registry) {
	registerDefaultConfiguration(metadata, registry);
	registerFeignClients(metadata, registry);
}

重写 registerBeanDefinitions 方法，注入 bean。

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public void registerFeignClients(AnnotationMetadata metadata,
		BeanDefinitionRegistry registry) {
 	...
     	for (String basePackage : basePackages) {
		Set<BeanDefinition> candidateComponents = scanner
				.findCandidateComponents(basePackage);
		for (BeanDefinition candidateComponent : candidateComponents) {
			if (candidateComponent instanceof AnnotatedBeanDefinition) {
				// verify annotated class is an interface
				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 = getClientName(attributes);
				registerClientConfiguration(registry, name,
						attributes.get("configuration"));

				registerFeignClient(registry, annotationMetadata, attributes);
			}
		}
	}
 }

registerFeignClients 这个方法中就是扫描所有配置的包路径，然后找到所有被@FeignClient 注解的类，添加到 BeanDefinition

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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();
		beanDefinition.setAttribute(FactoryBean.OBJECT_TYPE_ATTRIBUTE, className);

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

		beanDefinition.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);
	}

在 registerFeignClient 这个方法中，定义了一个 FeignClientFactoryBean 的工厂 bean，通过这个工厂 bean 把被@FeignClient 注解的类进行了封装。

我们来看看 FeignClientFactoryBean 的实现

FeignClientFactoryBean 实现了 FactoryBean 接口，这个接口最重要的方法 getObject 被 FeignClientFactoryBean 进行了重写。

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@Override
public Object getObject() throws Exception {
	return getTarget();
}

<T> T getTarget() {
	FeignContext context = this.applicationContext.getBean(FeignContext.class);
	Feign.Builder builder = feign(context);

	if (!StringUtils.hasText(this.url)) {
		if (!this.name.startsWith("http")) {
			this.url = "http://" + this.name;
		}
		else {
			this.url = this.name;
		}
		this.url += cleanPath();
		return (T) loadBalance(builder, context,
				new HardCodedTarget<>(this.type, this.name, this.url));
	}
	if (StringUtils.hasText(this.url) && !this.url.startsWith("http")) {
		this.url = "http://" + this.url;
	}
	String url = this.url + cleanPath();
	Client client = getOptional(context, Client.class);
	if (client != null) {
		if (client instanceof LoadBalancerFeignClient) {
			// not load balancing because we have a url,
			// but ribbon is on the classpath, so unwrap
			client = ((LoadBalancerFeignClient) client).getDelegate();
		}
		if (client instanceof FeignBlockingLoadBalancerClient) {
			// not load balancing because we have a url,
			// but Spring Cloud LoadBalancer is on the classpath, so unwrap
			client = ((FeignBlockingLoadBalancerClient) client).getDelegate();
		}
		builder.client(client);
	}
	Targeter targeter = get(context, Targeter.class);
	return (T) targeter.target(this, builder, context,
			new HardCodedTarget<>(this.type, this.name, url));
}

在这个 getTarget 方法中，可以看到明显的封装请求 url 地址等相关参数的代码。

可以看到最后通过 target.target 方法返回了实例，跟踪到默认实现 DefaultTargeter 里面查看一下

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@Override
public <T> T target(FeignClientFactoryBean factory, Feign.Builder feign,
		FeignContext context, Target.HardCodedTarget<T> target) {
	return feign.target(target);
}

 public <T> T target(Target<T> target) {
    return build().newInstance(target);
 }

进入 build 的 newinstance 方法

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public <T> T newInstance(Target<T> target) {
   Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
   Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();
   List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();

   for (Method method : target.type().getMethods()) {
     if (method.getDeclaringClass() == Object.class) {
       continue;
     } else if (Util.isDefault(method)) {
       DefaultMethodHandler handler = new DefaultMethodHandler(method);
       defaultMethodHandlers.add(handler);
       methodToHandler.put(method, handler);
     } else {
       methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));
     }
   }
   InvocationHandler handler = factory.create(target, methodToHandler);
   T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),
       new Class<?>[] {target.type()}, handler);

   for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
     defaultMethodHandler.bindTo(proxy);
   }
   return proxy;
 }

可以看到明显的动态代理的代码，可以知道调用的时候是通过动态代理生成的代理对象来调用的