SpringCloud FeignClient底层实现原理(一)

SpringCloud FeignClient底层实现原理(一)

在springcloud架构的微服中，服务提供者与服务消费之间主要以HTTP方式进行调用，这与dubbo之间通过tcp方式调用存在一定差异，dubbo中通过接口方式进行远程调用SpringCloud中通过FeignClient定义接口实现远端调用
FeignClient主要针对服务消费者使用服务消费者通过 @EnableFeignClient，@FeignClient 来启勇和声明FeignClient 其中@EnableFeignClient作用于服务启动主类，而@FeignClient用于定义服务提供方服务名和参数

@EnableFeignClients
@EnableDiscoveryClient
@SpringBootApplication
public class ConsumerApplication {
	public static void main(String[] args) {
		SpringApplication.run(ConsumerApplication.class, args);
	}

}

在@FeignClient中如果定义url而不是服务提供者的名字在调用时将不会使用loadbalance

@FeignClient("service-provider")
public interface UserFeignClient {
    @GetMapping("/user")
    String getUser();
}

@EnableFeignClient 分析

首先看下该注解的定义其中@Import(FeignClientsRegistrar.class)将FeignClient注解的接口变成springBoot中的BeanDefinition

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

接下来看下FeignClientsRegistrar类该类实现了ImportBeanDefinitionRegistrar接口，该接口用于将beanDefinition注册到springBoot的DefaultListableBeanFactory中registerBeanDefinitions方法的作用
是用来加载@EnableFeignClients中的defaultConfiguration和@FeignClient中的configuration配置文件

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

我们重点分析registerFeignClients方法该方法是扫描带有@FeignClient的接口,而担任本次扫描工作的类为ClassPathScanningCandidateComponentProvider,通过配置annotationTypeFilter需要扫描的注解为@FeignClient
扫描前需要定义其实扫描路径，如果@EnableCleint中设置了参数clients则根据设置类进行扫描，否则使用主类包名
AnnotatedBeanDefinition后判断@Feignclient接口是否作用在interface上，如果否抛出异常接下来就是注册configuration和FeignClient，我们主要分析registerFeignClient

public void registerFeignClients(AnnotationMetadata metadata,BeanDefinitionRegistry registry) {
	ClassPathScanningCandidateComponentProvider scanner = getScanner();
	scanner.setResourceLoader(this.resourceLoader);

	Set basePackages;

	Map 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 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 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 attributes = annotationMetadata.getAnnotationAttributes(FeignClient.class.getCanonicalName());
				String name = getClientName(attributes);
				registerClientConfiguration(registry, name,attributes.get("configuration"));
				registerFeignClient(registry, annotationMetadata, attributes);
			}
		}
	}
}

registerFeignClient中我们真正的看到了注册到spring Beanfactory中beandefinition为FeignClientFactoryBean,registerFeignClient中主要对@FeignClient中的配置进行解析并放到beanDefinition中

BeanDefinitionBuilder definition = BeanDefinitionBuilder.genericBeanDefinition(FeignClientFactoryBean.class);

private void registerFeignClient(BeanDefinitionRegistry registry,AnnotationMetadata annotationMetadata, Map 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);
}

继续分析FeignClientFactoryBean，该类并不会直接将将FeigClient Bean暴露，而是通过getObject获取FeigClient实例这也是 spring中FactoryBean与普通bean的区别，最终我们终于到了获取FeignClient实例的核心code了
在通过配置中是url参数是否配置来确定是否使用带有loadbalacne的实例，至此 FeignClient的第一部分分析完成，第二部分的分析将在下一篇文章中继续

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

 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));
}

接下来将会用两篇文章具体分析Feign+Ribbon的流程，分析过程设计的方法比较多没有逐一分析，如果感兴趣可以debug分析方法细节