前言

Spring Cloud OpenFeign 对 Netflix Feign 进行了封装，我们通常都使用Spring Cloud OpenFeign作为服务的负载均衡，本文章主要是探讨一下OpenFeign的初始化流程，以及生成代理类注入到Spring的过程

---

一.Feign的基本使用

Feign是一个声明式的http客户端，使用Feign可以实现声明式REST调用，它的目的就是让Web Service调用更加简单。Feign整合了Ribbon和SpringMvc注解，这让Feign的客户端接口看起来就像一个Controller，Feign提供了HTTP请求的模板，通过编写简单的接口和插入注解，就可以定义好HTTP请求的参数、格式、地址等信息。而Feign则会完全代理HTTP请求，我们只需要像调用方法一样调用它就可以完成服务请求及相关处理

1.导入依赖

这里使用的是spring-cloud-starter-openfeign:2.0.1.RELEASE 版本

<dependency>
  	<groupId>org.springframework.cloud</groupId>
    <artifactId>spring-cloud-starter-openfeign</artifactId>
</dependency>

2.开启Feign

通过@EnableFeignClients注解，他会去扫描指定包下的@FeignClient注解

/**
 * 支付的启动类
 * @EnableFeignClients :开启Feign支持
 */
@SpringBootApplication
@EnableEurekaClient
@EnableFeignClients(value="cn.itsource.springboot.feignclient")
public class PayServerApplication1040
{
    


    public static void main( String[] args )
    {
    
        SpringApplication.run(PayServerApplication1040.class);
    }
}

3.Feign的客户端接口

@FeignClient("user-server")
public interface UserFeignClient {
    

    //订单服务来调用这个方法      http://localhost:1020/user/10
    // @GetMapping(value = "/user/{id}" )
    @RequestMapping(value = "/user/{id}",method = RequestMethod.GET)
    User getById(@PathVariable("id")Long id);
}

通过主启动类上的@EnableFeignClients注解扫描@FeignClient标注的接口，这些接口会被代理，注入到Spring容器中

二.Feign的初始化

1.FeignAutoConfiguration Fegin的自动配置

同样是在SpringBoot启动的时候，执行自动装配流程，加载FeignAutoConfiguration自动配置类

看一下FeignAutoConfiguration源码

@Configuration
@ConditionalOnClass(Feign.class)
//加载Fiegn的相关配置
@EnableConfigurationProperties({
    FeignClientProperties.class, FeignHttpClientProperties.class})
public class FeignAutoConfiguration {
    
	//Feign的客户端配置，这里用的是@Autowired注入所有的FeignClientSpecification
	@Autowired(required = false)
	private List<FeignClientSpecification> configurations = new ArrayList<>();
	
	@Bean
	public HasFeatures feignFeature() {
    
		return HasFeatures.namedFeature("Feign", Feign.class);
	}
	//注册Feign的上下文对象，FeignContext会为每个Fiegn的客户端都创建一个Spring的ApplicationContext对象并从中提取所有的Bean
	@Bean
	public FeignContext feignContext() {
    
		FeignContext context = new FeignContext();
		context.setConfigurations(this.configurations);
		return context;
	}

FeignContext是Feign的上下文对象，FeignContext会为每个Fiegn的客户端都创建一个Spring的ApplicationContext上下文对象，并把相关的组件注册到容器中，并从中提取所有的Bean，这里的Fiegn的客户端指的是Fiegn的接口，会以@FeignClient(value=“服务名”) 注解指定的服务名作为客户端的名字。

思考一个问题，private List<FeignClientSpecification> configurations上面有个@Autowired，那FeignClientSpecification是在哪儿注册到Spring容器的呢？？？

FeignContext 继承了NamedContextFactory如下：

public class FeignContext extends NamedContextFactory<FeignClientSpecification> {
    

	public FeignContext() {
    
		super(FeignClientsConfiguration.class, "feign", "feign.client.name");
	}

}

继承关系如下：

2.Feign的上下创建

在分析Ribbon的时候我们已经探讨过NamedContextFactory，在FeignClientAutoConfiguration注册FeignContext时，通过NamedContextFactory构造器初始化，然后调用NamedContextFactory.setConfigurations(this.configurations);方法加载配置

public abstract class NamedContextFactory<C extends NamedContextFactory.Specification>
		implements DisposableBean, ApplicationContextAware {
    
	//上下文对象集合
	private Map<String, AnnotationConfigApplicationContext> contexts = new ConcurrentHashMap<>();
	//配置列表
	private Map<String, C> configurations = new ConcurrentHashMap<>();
	...省略...
	//初始化	
	public NamedContextFactory(Class<?> defaultConfigType, String propertySourceName,
			String propertyName) {
    
		//默认配置类，默认使用的是FeignClientsConfiguration
		this.defaultConfigType = defaultConfigType;
		//配置名，feign
		this.propertySourceName = propertySourceName;
		//feign.client.name
		this.propertyName = propertyName;
	}
	//设置配置，每个Feign的客户端接口都会对应一个配置
	public void setConfigurations(List<C> configurations) {
    
		for (C client : configurations) {
    
			this.configurations.put(client.getName(), client);
		}
	}
	//销毁工作，清除上下文对象
	public void destroy() {
    
		Collection<AnnotationConfigApplicationContext> values = this.contexts.values();
		for (AnnotationConfigApplicationContext context : values) {
    
			// This can fail, but it never throws an exception (you see stack traces
			// logged as WARN).
			context.close();
		}
		this.contexts.clear();
	}
	//根据名字获取上下文对象，如：传入服务名“user-server”获取上下文对象
	protected AnnotationConfigApplicationContext getContext(String name) {
    
		if (!this.contexts.containsKey(name)) {
    
		//如果容器集合中还没有当前客户端的上下文对象，就调用createContext方法创建客户端上下文对象
		//然后存储到contexts集合中
			synchronized (this.contexts) {
    
				if (!this.contexts.containsKey(name)) {
    
					this.contexts.put(name, createContext(name));
				}
			}
		}
		//根据客户端名字获取上下文对象
		return this.contexts.get(name);
	}
	//根据客户端名字，创建上下文
	protected AnnotationConfigApplicationContext createContext(String name) {
    
		//new一个 基于注解的ApplicationContext对象，用来装载Feign的客户端
		AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext();
		//如果 configurations 中包含了客户端的配置，把配置设置到创建的上下文中
		if (this.configurations.containsKey(name)) {
    
			for (Class<?> configuration : this.configurations.get(name)
					.getConfiguration()) {
    
				context.register(configuration);
			}
		}
		//注册默认的配置“default.”到上下文中
		for (Map.Entry<String, C> entry : this.configurations.entrySet()) {
    
			if (entry.getKey().startsWith("default.")) {
    
				for (Class<?> configuration : entry.getValue().getConfiguration()) {
    
					context.register(configuration);
				}
			}
		}
		//注册了默认的配置类，defaultConfigType就是FeignClientConfiguration
		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) {
    
			// Uses Environment from parent as well as beans
			context.setParent(this.parent);
		}
		context.setDisplayName(generateDisplayName(name));
		//刷新容器
		context.refresh();
		return context;
	}

	protected String generateDisplayName(String name) {
    
		return this.getClass().getSimpleName() + "-" + name;
	}
	
	//根据名字和类型从上下文中获取实例
	public <T> T getInstance(String name, Class<T> type) {
    
		//获取上下文
		AnnotationConfigApplicationContext context = getContext(name);
		if (BeanFactoryUtils.beanNamesForTypeIncludingAncestors(context,
				type).length > 0) {
    
				//从上下文中获取实例
			return context.getBean(type);
		}
		return null;
	}

NamedContextFactory主要提供了

• getContext(String name) : 根据客户端名字从 Map<String, AnnotationConfigApplicationContext> contexts 中获取上下文对象的方法，如果contexts中没有上下文会调用createContext(name) 创建上下文，然后放入contexts中再返回
• createContext(String name) ：根据客户端名字创建上下文对象，方法中分别注册了RibbonClient客户端配置类，default开头的所有默认配置，以及FeignClientsConfiguration默认配置，然后刷新容器，返回上下文对象
• getInstance(String name, Class type) ：根据名字和类型返回实例，方法先根据名字得到上下文对象然后从上下文对象中获取Bean

三.Feign的客户端注册

我们接着来看下Feign是如何对客户端接口进行注册到Spring容器中的，从@EnableFeignClients标签入手

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

该注解导入了FeignClientsRegistrar，见名知意，它对Feign的客户端的注册器

class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar,
		ResourceLoaderAware, EnvironmentAware {
    

	// patterned after Spring Integration IntegrationComponentScanRegistrar
	// and RibbonClientsConfigurationRegistgrar

	private ResourceLoader resourceLoader;

	private Environment environment;

	public FeignClientsRegistrar() {
    
	}

	@Override
	public void setResourceLoader(ResourceLoader resourceLoader) {
    
		this.resourceLoader = resourceLoader;
	}
	//registerBeanDefinitions方法是ImportBeanDefinitionRegistrar接口中的用来注册Bean到Spring容器的方法
	
	@Override
	public void registerBeanDefinitions(AnnotationMetadata metadata,
			BeanDefinitionRegistry registry) {
    
		//注册默认的配置
		registerDefaultConfiguration(metadata, registry);
		//注册Feign的客户端
		registerFeignClients(metadata, registry);
	}

registerBeanDefinitions调用两个方法，

• registerDefaultConfiguration：注册默认配置，
• registerFeignClients注册Feign的客户端

1.Feign的默认配置注册

我们先看registerDefaultConfiguration方法

 private void registerDefaultConfiguration(AnnotationMetadata metadata,
            BeanDefinitionRegistry registry) {
    
            //得到@EnableFeignClients注解的原始信息
        Map<String, Object> defaultAttrs = metadata
                .getAnnotationAttributes(EnableFeignClients.class.getName(), true);
		//是否配置了@EnableFeignClients(defaultConfiguration=)默认配置
		//使用的类型是FeignClientsConfiguration
        if (defaultAttrs != null && defaultAttrs.containsKey("defaultConfiguration")) {
    
            String name;
            //处理类型，以default开头
            if (metadata.hasEnclosingClass()) {
    
                name = "default." + metadata.getEnclosingClassName();
            }
            else {
    
                name = "default." + metadata.getClassName();
            }
            //注册默认配置，name 默认以 default 开头，后续会根据名称选择配置
            registerClientConfiguration(registry, name,
                    defaultAttrs.get("defaultConfiguration"));
        }
    }

上面这个方法在为@EnableFeignClients(defaultConfiguration=)默认配置类做注册，一般可以不配置该属性

 private void registerClientConfiguration(BeanDefinitionRegistry registry, Object name,
            Object configuration) {
    
            //创建一个BeanDefinitionBuilder
        BeanDefinitionBuilder builder = BeanDefinitionBuilder
                .genericBeanDefinition(FeignClientSpecification.class);
                //添加参数，把配置类交给builder
        builder.addConstructorArgValue(name);
        builder.addConstructorArgValue(configuration);
        //注册了Bean到Spring容器
        registry.registerBeanDefinition(
                name + "." + FeignClientSpecification.class.getSimpleName(),
                builder.getBeanDefinition());
    }

这里是把配置封装成FeignClientSpecification类型，然后通过BeanDefinitionRegistry注册到Spring容器中，该配置类包含了FeignClient的重试，超时，日志等配置，如服务定义配置会使用默认的配置。

还记得在FeignAutoConfiguration配置类中的private List<FeignClientSpecification> configurations吗？这是不是前后呼应起来了呢？

2.Feign的客户端注册

我们在看一下 registerFeignClients 方法

public void registerFeignClients(AnnotationMetadata metadata,
			BeanDefinitionRegistry registry) {
    
		
		//ClassPathScanningCandidateComponentProvider是用来扫Feign描客户端接口的
		ClassPathScanningCandidateComponentProvider scanner = getScanner();
		//resourceLoader是用来加载类资源的
		scanner.setResourceLoader(this.resourceLoader);

		Set<String> basePackages;
		//获取@EnableFeignClients注解元数据
		Map<String, Object> attrs = metadata
				.getAnnotationAttributes(EnableFeignClients.class.getName());
		//类型过滤器，过滤FeignClient
		AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(
				FeignClient.class);
		//检查是否配置clients属性
		final Class<?>[] clients = attrs == null ? null
				: (Class<?>[]) attrs.get("clients");
		if (clients == null || clients.length == 0) {
    
		//默认走这里
		//如果没有配置@EnableFeignClient(clients={})属性，就获取basePackages属性
		//给扫描添加includeFilters过滤器，目的是为了扫描@FeignClient注解
			scanner.addIncludeFilter(annotationTypeFilter);
			basePackages = getBasePackages(metadata);
		}
		else {
    
			//如果配置了clients就把 clients的包添加到扫描器中
			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) {
    
		//把basePackage下的注解了@FeignClient的接口封装成BeanDefinition
			Set<BeanDefinition> candidateComponents = scanner
					.findCandidateComponents(basePackage);
					
			for (BeanDefinition candidateComponent : candidateComponents) {
    
				if (candidateComponent instanceof AnnotatedBeanDefinition) {
    
					// verify annotated class is an interface
					AnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition) candidateComponent;
					//获取每个接口的注解元数据
					AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();
					Assert.isTrue(annotationMetadata.isInterface(),
							"@FeignClient can only be specified on an interface");
					//获取@FeignClient注解上的属性
					Map<String, Object> attributes = annotationMetadata
							.getAnnotationAttributes(
									FeignClient.class.getCanonicalName());
					//获取客户端名字
					String name = getClientName(attributes);
					//获取@FeignClient注解的configuration配置，对配置进行注册
					registerClientConfiguration(registry, name,
							attributes.get("configuration"));
					//注册客户端接口feignClient
					registerFeignClient(registry, annotationMetadata, attributes);
				}
			}
		}
	}

上面代码主要扫描了注解了@FeignClient的接口，会扫描到很多的接口，然后做2个事情

• 将其配置类通过registerClientConfiguration注册到容器中，这个方法在刚才已经分析过了，这里是第二次调用
• 通过registerFeignClient注册FeignClient接口到容器，内部会生成接口的代理类

继续跟进registerFeignClient

private void registerFeignClient(BeanDefinitionRegistry registry,
			AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
    
			//获取到组合的Feign接口类名
		String className = annotationMetadata.getClassName();
		//FeignClientFactoryBean是用用来创建FeignClient接口的，
		//这里要对Feign接口进行代理，使用的是FeignClientFactoryBean
		BeanDefinitionBuilder definition = BeanDefinitionBuilder
				.genericBeanDefinition(FeignClientFactoryBean.class);
		//校验@FeignClient相关属性
		validate(attributes);
		//封装相关的属性到definition
		definition.addPropertyValue("url", getUrl(attributes));
		definition.addPropertyValue("path", getPath(attributes));
		String name = getName(attributes);
		definition.addPropertyValue("name", name);
		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 = name + "FeignClient";
		//得到BeanDefinition对象，它是对Feign客户端的Bean的定义对象，基于FeignClientFactoryBean
		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 });
        //将BeanDefinition加入到spring容器
		BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
	}

从上面方法可以看出，Feign的客户端是通过Spring的工厂生成代理类，Feign接口被封装成BeanDefinition，类型是FeignClientFactoryBean，也就是说Feign把所有的接口都包装成FeignClientFactoryBean

思考一个问题，这里虽然对Feign接口进行了描述，封装成FeignClientFactoryBean,那么接口实例到底在什么时候创建呢？？？

3.FeignClientFactoryBean 工厂

这个工厂是用来生成Feign接口的代理类的，我们看一下他是如何做的

class FeignClientFactoryBean implements FactoryBean<Object>, InitializingBean,
		ApplicationContextAware {
    
	/***********************************
	 * WARNING! Nothing in this class should be @Autowired. It causes NPEs because of some lifecycle race condition.
	 ***********************************/
	//Feign接口类型
	private Class<?> type;
	//Fiegn客户端名字/请求的服务名
	private String name;
	//请求的url
	private String url;
	
	private String path;

	private boolean decode404;

	private ApplicationContext applicationContext;

	private Class<?> fallback = void.class;
	//降级工厂
	private Class<?> fallbackFactory = void.class;
	....省略....
	
	@Override
	public Object getObject() throws Exception {
    
	//获取Feign的上下文，在Feign的初始化一节有说道
		FeignContext context = applicationContext.getBean(FeignContext.class);
		//调用feign方法得到Builder构造器，用来生成feign
		//开启Hystrix使用的是HystrixFeign.Builder 
		//否则使用的是feign.Feign.Builder
		Feign.Builder builder = feign(context);
		//如果没有url，即没有指定@FeignClient(url=""),就根据服务名拼接url
		if (!StringUtils.hasText(this.url)) {
    
			String url;
			//拼接请求的url如：http://user-server
			if (!this.name.startsWith("http")) {
    
				url = "http://" + this.name;
			}
			else {
    
				url = this.name;
			}
			url += cleanPath();
			//生成负载均衡代理类
			return loadBalance(builder, context, new HardCodedTarget<>(this.type,
					this.name, url));
		}
		//如果配置了@FeignClient(url="")，生成默认代理
		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 targeter.target(this, builder, context, new HardCodedTarget<>(
				this.type, this.name, url));
	}

FeignClientFactoryBean实现了FactoryBean，getObject方法即是用来创建FeignClient实例的

• 该方法授权通过ApplicationContext得到FeignContext上下文

• 然后在从FeignContext上下文中得到Feign.Builder，它是用来构建Feign客户端代理类的

• 然后根据Feign接口 的URL(要么通过@FeignClient(url=“”)指定，如果没指定，就根据@FeignClient(value=“服务名”)拼接) 生成负载均衡代理类

  为什么这里要判断URL呢？如果我们不指定URL会默认走Ribbon的负载均衡

  @FeignClient(value="user-server")
  

  如果指定了URL会走默认的负载均衡,相当于不走负载均衡

  @FeignClient(value="user-server",url="localhost:8989")
  

Feign.Builder

这里的Feign.Builder 是从FeignContext中得到的，我们看一下它的源码

 public static class Builder {
    
 		//请求拦截器
        private final List<RequestInterceptor> requestInterceptors = new ArrayList();
      	...省略...

        public Builder() {
    
        	//Feign的日志打印级别，默认不打印
            this.logLevel = Level.NONE;
            this.contract = new Default();
            this.client = new feign.Client.Default((SSLSocketFactory)null, (HostnameVerifier)null);
            //重试策略
            this.retryer = new feign.Retryer.Default();
            this.logger = new NoOpLogger();
            //编码器
            this.encoder = new feign.codec.Encoder.Default();
            //解码器
            this.decoder = new feign.codec.Decoder.Default();
            //错误解码器
            this.errorDecoder = new feign.codec.ErrorDecoder.Default();
            this.options = new Options();
            //方法调用工厂
            this.invocationHandlerFactory = new feign.InvocationHandlerFactory.Default();
        }
		//设置日志级别
        public Feign.Builder logLevel(Level logLevel) {
    
            this.logLevel = logLevel;
            return this;
        }

        public Feign.Builder contract(Contract contract) {
    
            this.contract = contract;
            return this;
        }

        public Feign.Builder client(Client client) {
    
            this.client = client;
            return this;
        }

        public Feign.Builder retryer(Retryer retryer) {
    
            this.retryer = retryer;
            return this;
        }

        public Feign.Builder logger(Logger logger) {
    
            this.logger = logger;
            return this;
        }

        public Feign.Builder encoder(Encoder encoder) {
    
            this.encoder = encoder;
            return this;
        }

        public Feign.Builder decoder(Decoder decoder) {
    
            this.decoder = decoder;
            return this;
        }

        public Feign.Builder mapAndDecode(ResponseMapper mapper, Decoder decoder) {
    
            this.decoder = new Feign.ResponseMappingDecoder(mapper, decoder);
            return this;
        }

        public Feign.Builder decode404() {
    
            this.decode404 = true;
            return this;
        }

        public Feign.Builder errorDecoder(ErrorDecoder errorDecoder) {
    
            this.errorDecoder = errorDecoder;
            return this;
        }

        public Feign.Builder options(Options options) {
    
            this.options = options;
            return this;
        }

        public Feign.Builder requestInterceptor(RequestInterceptor requestInterceptor) {
    
            this.requestInterceptors.add(requestInterceptor);
            return this;
        }
		//添加Feign的拦截器
        public Feign.Builder requestInterceptors(Iterable<RequestInterceptor> requestInterceptors) {
    
            this.requestInterceptors.clear();
            Iterator var2 = requestInterceptors.iterator();

            while(var2.hasNext()) {
    
                RequestInterceptor requestInterceptor = (RequestInterceptor)var2.next();
                this.requestInterceptors.add(requestInterceptor);
            }

            return this;
        }

        public Feign.Builder invocationHandlerFactory(InvocationHandlerFactory invocationHandlerFactory) {
    
            this.invocationHandlerFactory = invocationHandlerFactory;
            return this;
        }

        public <T> T target(Class<T> apiType, String url) {
    
            return this.target(new HardCodedTarget(apiType, url));
        }
		//这个方法挺重要的，它是用来构建FeignClient的代理类的
        public <T> T target(Target<T> target) {
    
            return this.build().newInstance(target);
        }
        
		//注意：这里使用的是ReflectiveFeign作为Feign的默认实现
		//创建Feign接口的代理就是ReflectiveFeign搞定的
        public Feign build() {
    
            Factory synchronousMethodHandlerFactory = new Factory(this.client, this.retryer, this.requestInterceptors, this.logger, this.logLevel, this.decode404);
            ParseHandlersByName handlersByName = new ParseHandlersByName(this.contract, this.options, this.encoder, this.decoder, this.errorDecoder, synchronousMethodHandlerFactory);
            return new ReflectiveFeign(handlersByName, this.invocationHandlerFactory);
        }
    }

Feign.Builder中封装了Feign的日志注解，编码器，解码器，拦截器等等，这些东西是在FeignClientsConfiguration中被创建的

FeignClientsConfiguration

@Configuration
public class FeignClientsConfiguration {
    
	//Feign的消息转换器
	@Autowired
	private ObjectFactory<HttpMessageConverters> messageConverters;

	@Autowired(required = false)
	private List<AnnotatedParameterProcessor> parameterProcessors = new ArrayList<>();

	@Autowired(required = false)
	private List<FeignFormatterRegistrar> feignFormatterRegistrars = new ArrayList<>();

	@Autowired(required = false)
	private Logger logger;

	//解码器
	@Bean
	@ConditionalOnMissingBean
	public Decoder feignDecoder() {
    
		return new OptionalDecoder(new ResponseEntityDecoder(new SpringDecoder(this.messageConverters)));
	}
	//编码器
	@Bean
	@ConditionalOnMissingBean
	public Encoder feignEncoder() {
    
		return new SpringEncoder(this.messageConverters);
	}

	@Bean
	@ConditionalOnMissingBean
	public Contract feignContract(ConversionService feignConversionService) {
    
		return new SpringMvcContract(this.parameterProcessors, feignConversionService);
	}

	@Bean
	public FormattingConversionService feignConversionService() {
    
		FormattingConversionService conversionService = new DefaultFormattingConversionService();
		for (FeignFormatterRegistrar feignFormatterRegistrar : feignFormatterRegistrars) {
    
			feignFormatterRegistrar.registerFormatters(conversionService);
		}
		return conversionService;
	}
	//如果开启了Hystrix,那么使用的是HystrixFeign.builder();
	//而不是Feign.Builder
	@Configuration
	@ConditionalOnClass({
     HystrixCommand.class, HystrixFeign.class })
	protected static class HystrixFeignConfiguration {
    
		@Bean
		@Scope("prototype")
		@ConditionalOnMissingBean
		@ConditionalOnProperty(name = "feign.hystrix.enabled")
		public Feign.Builder feignHystrixBuilder() {
    
			return HystrixFeign.builder();
		}
	}
	//重试策略
	@Bean
	@ConditionalOnMissingBean
	public Retryer feignRetryer() {
    
		return Retryer.NEVER_RETRY;
	}
	//注册Feign.Builder
	@Bean
	@Scope("prototype")
	@ConditionalOnMissingBean
	public Feign.Builder feignBuilder(Retryer retryer) {
    
		return Feign.builder().retryer(retryer);
	}
	//日志工厂
	@Bean
	@ConditionalOnMissingBean(FeignLoggerFactory.class)
	public FeignLoggerFactory feignLoggerFactory() {
    
		return new DefaultFeignLoggerFactory(logger);
	}

}

loadBalance方法

回到getObject方法看一下loadBalance方法是如何生成负载均衡代理类的

protected <T> T loadBalance(Feign.Builder builder, FeignContext context,
			HardCodedTarget<T> target) {
    
			//从上下文对象FeignContext中获取Client,默认使用的是LoadBalancerFeignClient
		Client client = getOptional(context, Client.class);
		if (client != null) {
    
			//client设置给builder
			builder.client(client);
			
			Targeter targeter = get(context, Targeter.class);
			//targeter的target方法调用了Feign的target方法
			//Feign的默认实现是ReflectiveFeign类，最终调用了ReflectiveFeign.newInstance方法创建实例
			return targeter.target(this, builder, context, target);
		}

		throw new IllegalStateException(
				"No Feign Client for loadBalancing defined. Did you forget to include spring-cloud-starter-netflix-ribbon?");
	}

这里会从FeignContext上下文中获取到LoadBalancerFeignClient负载客户端设置给Feign.Builder ，然后从FeignContext上下文得到Targeter,通过Targeter.target方法创建实例，底层通过Feign.newInstance方法完成实例创建，默认实现是ReflectiveFeign完成创建

4.ReflectiveFeign创建Feign代理

ReflectiveFeign通过Proxy.newProxyInstance为Feign接口生成代理

public class ReflectiveFeign extends Feign {
    
	...省略...
	//target是HardCodedTarget，封装了客户端的type,name,url
	public <T> T newInstance(Target<T> target) {
    
	//得到Feign接口中的所有的方法MethodHandler
        Map<String, MethodHandler> nameToHandler = this.targetToHandlersByName.apply(target);
        //把 Map<String, MethodHandler>变成Map<Method, MethodHandler> 
        Map<Method, MethodHandler> methodToHandler = new LinkedHashMap();
        List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList();
        Method[] var5 = target.type().getMethods();
        int var6 = var5.length;

        for(int var7 = 0; var7 < var6; ++var7) {
    
            Method method = var5[var7];
            if (method.getDeclaringClass() != Object.class) {
    
                if (Util.isDefault(method)) {
    
                	//这里在添加默认的方法
                    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 = this.factory.create(target, methodToHandler);
        //生成代理
        T proxy = Proxy.newProxyInstance(target.type().getClassLoader(), new Class[]{
    target.type()}, handler);
        Iterator var12 = defaultMethodHandlers.iterator();

        while(var12.hasNext()) {
    
        	//代理类绑定默认的方法

            DefaultMethodHandler defaultMethodHandler = (DefaultMethodHandler)var12.next();
            defaultMethodHandler.bindTo(proxy);
        }
		//返回代理
        return proxy;
    }

到这里，Feign的客户端的注册流程就结束了，那注册是在什么时候发生的呢？是在Spring启动过程中会调用refresh()方法，在该方法中会触发FeignClientFactoryBean.getObject()得到实例注册到Spring容器中

四.总结