spring cloud alibaba 学习（六）OpenFeign 初始化流程

前言一、使用示例二、注册FeignClients三、实例化FeignClients四、FeignContext总结

在spring cloud alibaba 中使用OpenFeign，可以用来使用HTTP请求访问远程服务。

1、引入依赖

		
			org.springframework.cloud
			spring-cloud-starter-openfeign
			3.0.0
		

2、启动类

加上注解@EnableFeignClients

@SpringBootApplication
@EnableFeignClients
public class CloudApplication {

	public static void main(String[] args) {
		SpringApplication.run(CloudApplication.class, args);
		System.out.println("started...");
	}

}

3、创建服务api接口

用@FeignClient 注解标识，value = “server” 表示要访问的服务提供者的名称是server

@FeignClient(value = "server")
@Component
public interface ServerApi {

    @GetMapping("/server/test/{id}")
    Map test(@PathVariable String id);
}

4、Controller接口

@RestController
@RequestMapping("/client")
public class ClientController {

    @Autowired
    private ServerApi serverApi;

    @GetMapping("/test/{id}")
    public Map test(@PathVariable String id){
        return serverApi.test(id);
    }
}

1、@EnableFeignClients

在启动类上加上了注解@EnableFeignClients，该注解导入了配置类@import(FeignClientsRegistrar.class)

2、FeignClientsRegistrar

实现了 importBeanDefinitionRegistrar 接口，重写了 registerBeanDefinitions( ) 方法，
在配置类加载过程中，会回调该方法

loadBeanDefinitionsFromRegistrars(configClass.getimportBeanDefinitionRegistrars());

3、回调方法

FeignClientsRegistrar.java

@Override
	public void registerBeanDefinitions(Annotationmetadata metadata, BeanDefinitionRegistry registry) {
		//注册默认的配置类
		registerDefaultConfiguration(metadata, registry);
		//注册feign客户端 FeignClients
		registerFeignClients(metadata, registry);
	}

4、注册默认的配置类

private void registerDefaultConfiguration(Annotationmetadata metadata, BeanDefinitionRegistry registry) {
		//获取 @EnableFeignClients 注解信息
		Map defaultAttrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName(), true);

		if (defaultAttrs != null && defaultAttrs.containsKey("defaultConfiguration")) {
			String name;
			if (metadata.hasEnclosingClass()) {
				name = "default." + metadata.getEnclosingClassName();
			}
			else {
				name = "default." + metadata.getClassName();
			}
			//注册客户端配置类
			registerClientConfiguration(registry, name, defaultAttrs.get("defaultConfiguration"));
		}
	}

注册 FeignClientSpecification 类

private void registerClientConfiguration(BeanDefinitionRegistry registry, Object name, Object configuration) {
		BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition(FeignClientSpecification.class);
		builder.addConstructorArgValue(name);
		builder.addConstructorArgValue(configuration);
		registry.registerBeanDefinition(name + "." + FeignClientSpecification.class.getSimpleName(),
				builder.getBeanDefinition());
	}

5、注册feign客户端

（1）registerFeignClients( )

public void registerFeignClients(Annotationmetadata metadata, BeanDefinitionRegistry registry) {

		linkedHashSet candidateComponents = new linkedHashSet<>();
		//1、获取 @EnableFeignClients 注解信息
		Map attrs = metadata.getAnnotationAttributes(EnableFeignClients.class.getName());
		//@FeignClient 注解类型过滤器
		AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(FeignClient.class);
		final Class[] clients = attrs == null ? null : (Class[]) attrs.get("clients");
		//2、获取包含@FeignClient 注解的接口
		if (clients == null || clients.length == 0) {
			//注解中没有指定要加载的 FeignClient 接口类，则进行扫描指定的包或当前处理的配置类所在的包
			//获取扫描器
			ClassPathScanningCandidateComponentProvider scanner = getScanner();
			scanner.setResourceLoader(this.resourceLoader);
			//设置注解类型过滤器
			scanner.addIncludeFilter(new AnnotationTypeFilter(FeignClient.class));
			//获取@FeignClient注解上的value/basePackages/basePackageClasses 或当前处理的配置类所在的包
			Set basePackages = getbasePackages(metadata);
			for (String basePackage : basePackages) {
				//找出包含@FeignClient 注解的接口
				candidateComponents.addAll(scanner.findCandidateComponents(basePackage));
			}
		}
		else {
			//注解中指定了要加载的 FeignClient 接口类
			for (Class clazz : clients) {
				candidateComponents.add(new AnnotatedGenericBeanDefinition(clazz));
			}
		}

		//3、解析@FeignClient接口
		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());

				//@FeignClient制定的服务端的名称
				String name = getClientName(attributes);
				//根据服务端的名称注册FeignClientSpecification类
				registerClientConfiguration(registry, name, attributes.get("configuration"));

				//4、注册FeignClient
				registerFeignClient(registry, annotationmetadata, attributes);
			}
		}
	}

6、注册FeignClient

private void registerFeignClient(BeanDefinitionRegistry registry, Annotationmetadata annotationmetadata,
			Map attributes) {
		String className = annotationmetadata.getClassName();
		//设置类型为 FeignClientFactoryBean，实现了FactoryBean