@EnableFeignClients原理剖析

今天闲来无事，临时突然想看看@EnableFeignClients的实现原理，现就个人的理解，整理一下记录下来，有不对的欢迎指正，共同进步，不喜勿喷！

首先我们来看看@EnableFeignClients的定义：

@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 {};

}

看到@import注解，相信大家对这个哥们并不陌生，面试被问及springboot自动装配原理的时候必须提到这哥们，否则就只能回家等消息了（通常@EnableXXX注解基本上都是通过@import来实现的，除了@EnableEurekaClient这个意外，由此可见import注解的重要性，关于import注解是怎么解析的，可以看看之前的一片文章：Springboot自动转配原理之@Import注解处理过程_@importautoconfiguration-CSDN博客）

我们继续看FeignClientsRegistrar这个类的定义：

class FeignClientsRegistrar
		implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware 

它实现了ImportBeanDefinitionRegistrar：

public interface ImportBeanDefinitionRegistrar {

	/**
	 * Register bean definitions as necessary based on the given annotation metadata of
	 * the importing {@code @Configuration} class.
	 * <p>Note that {@link BeanDefinitionRegistryPostProcessor} types may <em>not</em> be
	 * registered here, due to lifecycle constraints related to {@code @Configuration}
	 * class processing.
	 * @param importingClassMetadata annotation metadata of the importing class
	 * @param registry current bean definition registry
	 */
	void registerBeanDefinitions(AnnotationMetadata importingClassMetadata, BeanDefinitionRegistry registry);

}

我们对spring要有一个最基本的认识，就是在spring真正初始化一个bean的时候都会事先把bean的定义信息转换成一个BeanDefinition存起来，到了后面在需要初始化bean的时候直接去拿到对应的BeanDefinition进行初始化工作，而ImportBeanDefinitionRegistrar这个接口看名称就知道是用来注册BeanDefinition的，注册BeanDefinition其实就是把bean的定义信息转换成一个BeanDefinition存起来，接下来我们看看FeignClientsRegistrar中的registerBeanDefinitions方法：

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

看到registerFeignClients就知道这个方法是注册rFeignClient了：

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() {
				@Override
				protected 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 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);
				}
			}
		}
	}

Map<String, Object> attrs = metadata
      .getAnnotationAttributes(EnableFeignClients.class.getName());

其中这一行看名字就知道了就是拿到被@EnableFeignClients修饰的类，但是更重要的是后面的registerFeignClient方法：

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
																// null

		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);
	}

其中最重要的一行：BeanDefinitionBuilder definition = BeanDefinitionBuilder .genericBeanDefinition(FeignClientFactoryBean.class);

这个方法的逻辑就是把FeignClientFactoryBean对应的beandefinition注册到spring容器中去，我们看看它的声明：

class FeignClientFactoryBean
		implements FactoryBean<Object>, InitializingBean, ApplicationContextAware

它实现了FactoryBean<Object>, InitializingBean, ApplicationContextAware，这三个接口，用spring开发的小伙伴对这三个接口应该是滚瓜烂熟了吧，到此我们也就明白了其实@EnableFeignClients最终是在FeignClientFactoryBean中来初始化客户端的（FactoryBean的getObject方法）：

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

	/**
	 * @param <T> the target type of the Feign client
	 * @return a {@link Feign} client created with the specified data and the context
	 * information
	 */
	<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();
			}
			builder.client(client);
		}
		Targeter targeter = get(context, Targeter.class);
		return (T) targeter.target(this, builder, context,
				new HardCodedTarget<>(this.type, this.name, url));
	}

总结：@EnableFeignClients其本质是通过FeignClientFactoryBean来初始化http客户端