十六.SpringCloud源码剖析-Feign源码分析

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 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