openfeign源码解析

registry -- 注册中心(当前采用eureka)
person -- 人员服务
  person-api -- 人员相关api提供, 包括 req, resp, service 等
  person-biz -- 人员相关服务接口具体实现, 依赖person-api
  person-provider -- 人员相关微服务启动类, 依赖person-biz
equipment -- 设备服务
  equipment-api -- 设备相关api提供, 包括 req, resp, service 等
  equipment-biz -- 设备相关服务接口具体实现, 依赖equipment-api
  equipment-provider -- 设备相关微服务启动类, 依赖equipment-biz

这里使用eureka作为注册中心,，person和equipment两个web服务作为业务中台，本例中会使用postman调用person服务，person服务中调用equipment服务

<parent>
    <artifactId>person</artifactId>
    <groupId>tca</groupId>
    <version>1.0.0</version>
</parent>
<modelVersion>4.0.0</modelVersion>

<artifactId>cloud-person-api</artifactId>

<dependencies>
    <!-- lombok -->
    <dependency>
        <groupId>org.projectlombok</groupId>
        <artifactId>lombok</artifactId>
    </dependency>

    <!-- 参数校验 -->
    <dependency>
        <groupId>javax.validation</groupId>
        <artifactId>validation-api</artifactId>
    </dependency>

    <!-- openFeign -->
    <!-- 注意： 这里仅仅是demo, 所以没有再引入feign-httpclient相关 -->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-openfeign</artifactId>
    </dependency>

</dependencies>

<dependencies>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-openfeign-core</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework</groupId>
        <artifactId>spring-web</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-commons</artifactId>
    </dependency>
    <dependency>
        <groupId>io.github.openfeign</groupId>
        <artifactId>feign-core</artifactId>
    </dependency>
    <dependency>
        <groupId>io.github.openfeign</groupId>
        <artifactId>feign-slf4j</artifactId>
    </dependency>
    <!-- hystrix -->
    <dependency>
        <groupId>io.github.openfeign</groupId>
        <artifactId>feign-hystrix</artifactId>
    </dependency>
    <dependency>
        <groupId>io.github.openfeign</groupId>
        <artifactId>feign-java8</artifactId>
    </dependency>
    <!-- ribbon -->
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-ribbon</artifactId>
        <optional>true</optional>
    </dependency>
    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-archaius</artifactId>
        <optional>true</optional>
    </dependency>
</dependencies>

<dependencies>
    <!-- 使用web -->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>

    <!-- person-api -->
    <dependency>
        <groupId>tca</groupId>
        <artifactId>cloud-person-api</artifactId>
        <version>1.0.0</version>
    </dependency>

    <!-- equipment-api -->
    <dependency>
        <groupId>tca</groupId>
        <artifactId>cloud-equipment-api</artifactId>
        <version>1.0.0</version>
    </dependency>

    <!-- log -->
    <dependency>
        <groupId>tca</groupId>
        <artifactId>common-log-springboot-logback</artifactId>
        <version>1.0.0</version>
    </dependency>
</dependencies>

<dependencies>
    <dependency>
        <groupId>tca</groupId>
        <artifactId>cloud-person-biz</artifactId>
        <version>1.0.0</version>
    </dependency>

    <dependency>
        <groupId>org.springframework.cloud</groupId>
        <artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
    </dependency>
</dependencies>

/**
 * @author zhouan
 * @Date 2021/7/29
 */
@EnableEurekaClient
@ComponentScan(basePackages = "com.tca")
@SpringBootApplication
@EnableFeignClients(basePackages = "com.tca")
public class PersonApplication {

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

package com.tca.cloud.standalone.person.api.service;

import com.tca.cloud.standalone.person.api.req.PersonReq;
import com.tca.cloud.standalone.person.api.resp.PersonResp;
import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PostMapping;

/**
 * @author zhouan
 * @Date 2021/7/29
 */
@FeignClient(value = "person-provider")
public interface PersonFeign {

    /**
     * 根据id获取人员
     * @param personReq
     * @return
     */
    @PostMapping("/person/get")
    PersonResp get(PersonReq personReq);
}

package com.tca.cloud.standalone.equipment.api.service;

import com.tca.cloud.standalone.equipment.api.req.EquipmentReq;
import com.tca.cloud.standalone.equipment.api.resp.EquipmentResp;
import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.Mapping;
import org.springframework.web.bind.annotation.PostMapping;
import org.springframework.web.bind.annotation.RequestBody;

/**
 * @author zhouan
 * @Date 2021/7/29
 */
@FeignClient(value = "equipment-provider")
public interface EquipmentFeign {

    /**
     * 根据id获取人员
     * @param equipmentReq
     * @return
     */
    @PostMapping("/equipment/get")
    EquipmentResp get(EquipmentReq equipmentReq);
}

package com.tca.cloud.standalone.person.biz.controller;

import com.tca.cloud.standalone.equipment.api.req.EquipmentReq;
import com.tca.cloud.standalone.equipment.api.resp.EquipmentResp;
import com.tca.cloud.standalone.equipment.api.service.EquipmentFeign;
import com.tca.cloud.standalone.person.api.req.PersonReq;
import com.tca.cloud.standalone.person.api.resp.PersonResp;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.validation.annotation.Validated;
import org.springframework.web.bind.annotation.*;

/**
 * @author zhouan
 * @Date 2021/7/29
 */
@RestController
@RequestMapping(value = "/person")
@Slf4j
public class PersonController {

    @Autowired
    private EquipmentFeign equipmentFeign;

    /**
     * 获取人员
     * @param personReq
     * @return
     */
    @GetMapping("/get")
    public PersonResp get(@Validated @RequestBody PersonReq personReq) {
        PersonResp personResp = new PersonResp();
        personResp.setId(personReq.getId());
        personResp.setAge(30);
        personResp.setName("Messi");

        EquipmentReq equipmentReq = new EquipmentReq();
        equipmentReq.setId(personReq.getId());
        EquipmentResp equipmentResp = equipmentFeign.get(equipmentReq);
        log.info("equipmentResp = {}", equipmentResp);
        return personResp;
    }
}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(FeignClientsRegistrar.class) // 引入FeignClientsRegistrar, @Import作用在spring和springboot源码中已经描述过, 这里不作赘述
public @interface EnableFeignClients {

    // 等用于 basePackages, 即扫描的包
	String[] value() default {};

	String[] basePackages() default {};

	Class<?>[] basePackageClasses() default {};

    // FeignClients的全局配置类, 可以定义一些bean提供给FeignClient使用, 比如: Decoder, Encoder, Contract
	Class<?>[] defaultConfiguration() default {};

    // 通过该属性直接指定要加载哪些@FeignClient接口, 使用这个属性之后, basePackages就不再起作用了
	Class<?>[] clients() default {};

}

@EnableFeignClients 注解用于向 spring 容器中导入 FeignClientsRegistrar, 他有两个核心属性: 1.需要扫描的包或者指定的Class 2.提供FeignClients的全局配置类

/*
 * Copyright 2013-2019 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.openfeign;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.springframework.core.annotation.AliasFor;

/**
 * Annotation for interfaces declaring that a REST client with that interface should be
 * created (e.g. for autowiring into another component). If ribbon is available it will be
 * used to load balance the backend requests, and the load balancer can be configured
 * using a <code>@RibbonClient</code> with the same name (i.e. value) as the feign client.
 *
 * @author Spencer Gibb
 * @author Venil Noronha
 */
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface FeignClient {

	// 指定服务名称, 一般与 ${spring.application.name} 相同
	@AliasFor("name")
	String value() default "";

	// 跟value相同
	@Deprecated
	String serviceId() default "";

	// 如果存在，它将被用作bean的名称，代替name属性，但不会用作服务id
	String contextId() default "";

	// 跟value相同
	@AliasFor("value")
	String name() default "";

	// 根据Class Type注入，如果存在多个会报错，就需要根据名称注入@Qualifier可以指定名称
	String qualifier() default "";

	// 配置这个相当于直连方式，例如url="localhost:8080", 这样就不会走负载均衡了
	String url() default "";

	// 如果发生FeignExceptions是否需要返回404
	boolean decode404() default false;

	// 这个配置相当于是局部配置，某一个feign client专门的配置
	Class<?>[] configuration() default {};

	// 指定Feign client接口的服务降级类。服务降级类必须实现由该注解注释的接口, 并且必须是一个有效的spring bean
	Class<?> fallback() default void.class;

	// 为指定的Feign client接口定义一个服务降级工厂。服务降级工厂必须生成服务降级类的实例, 这些实例实现了由{@link FeignClient}注释的接口。服务降级工厂必须是一个有效的spring bean
	Class<?> fallbackFactory() default void.class;

	// 所有方法级映射使用的路径前缀。可以@RibbonClient一起使用。
	String path() default "";

    // 是否将feign代理标记为主bean。默认值为true。和@Primary注解类似，同一个接口多个实现类中根据类型注入时候会首选被@Primary标记的实现类
    // 每个feign client接口都会为其生成一个代理类，这里是为这个代理类标记为Primary
	boolean primary() default true;

}

@FeignClient提供了一些属性, 用于FeignClientFactoryBean创建FeignClient代理对象

// 在类声明中可以看到 它 实现了 ImportBeanDefinitionRegistrar接口，这个在spring源码中有介绍，在ConfigurationClassPostProcessor中会委托ConfigurationClassParser解析 @Import 注解获取到 FeignClientsRegistrar, 判断其实现了 ImportBeanDefinitionRegistrar 接口, 此时会使用反射创建FeignClientsRegistrar实例, 回调其Aware接口, 之后, 会统一回调 FeignClientsRegistrar#registerBeanDefinitions方法
class FeignClientsRegistrar implements ImportBeanDefinitionRegistrar, ResourceLoaderAware, EnvironmentAware

@Override
public void registerBeanDefinitions(AnnotationMetadata metadata,
                                    BeanDefinitionRegistry registry) {
    // 注册默认配置 见@3.2.1
    registerDefaultConfiguration(metadata, registry);
    // 注册FeignClient 见@3.2.2
    registerFeignClients(metadata, registry);
}

private void registerDefaultConfiguration(AnnotationMetadata metadata,
                                          BeanDefinitionRegistry registry) {
    // 1.获取 EnableFeignClients 的属性
    // 该方法第二个参数true，表示将注解中class类型的属性转换为字符串类名暴露到返回到map中
    Map<String, Object> defaultAttrs = metadata
        .getAnnotationAttributes(EnableFeignClients.class.getName(), true);
    // 2.处理defaultConfiguration属性
    if (defaultAttrs != null && defaultAttrs.containsKey("defaultConfiguration")) {
        String name;
        if (metadata.hasEnclosingClass()) {
            name = "default." + metadata.getEnclosingClassName();
        }
        else {
            // 3.name == default+类名, 这里 name == default.PersonApplication
            name = "default." + metadata.getClassName();
        }
        // 4.向spring容器中注册FeignClient相关配置类, 见下
        registerClientConfiguration(registry, name,
                                    defaultAttrs.get("defaultConfiguration"));
    }
}

private void registerClientConfiguration(BeanDefinitionRegistry registry, Object name,
                                         Object configuration) {
    // 1.创建 BeanDefinitionBuilder, beanClass == FeignClientSpecification, 用于创建 FeignClientSpecification 对应的 BeanDefinition, 
    BeanDefinitionBuilder builder = BeanDefinitionBuilder
        .genericBeanDefinition(FeignClientSpecification.class);
    // 2.为FeignClientSpecification添加两个构造器参数:
    // name == default.PersonApplication
    // configuration == {}, 因为EnableFeignClients中的 defaultConfiguration 属性为 {}
    builder.addConstructorArgValue(name);
    builder.addConstructorArgValue(configuration);
    // 3.向spring容器中注册bd
    // name == default.PersonApplication.FeignClientSpecification
    // value == FeignClientSpecification对应的bd
    registry.registerBeanDefinition(
        name + "." + FeignClientSpecification.class.getSimpleName(),
        builder.getBeanDefinition());
}

FeignClientsRegistrar#registerDefaultConfiguration的作用：
1.解析 EnableFeignClients注解, 获取 defaultConfiguration 属性
2.创建 主配置类 PersonApplication 对应的 FeignClient全局配置类FeignClientSpecification的bd, 并注册到spring容器中
3.FeignClientSpecification 有两个重要的属性, name 以及 configuration, 这里的configuration == defaultConfiguration, 默认为 {}

public void registerFeignClients(AnnotationMetadata metadata,
                                 BeanDefinitionRegistry registry) {
    // 1.获取包扫描器, 这里跟 mybatis-spring 集成不同, mybatis-spring 是定义一个扫描器继承 ClasspathBeanDefinitionScanner, 重写isCandidateComponent方法
    // 在这里, 它重新定义了一个scanner --> ClassPathScanningCandidateComponentProvider, 原理差不多
    ClassPathScanningCandidateComponentProvider scanner = getScanner();
    // 设置资源加载器
    scanner.setResourceLoader(this.resourceLoader);

    Set<String> basePackages;
	// 2.获取@FeignClient注解属性
    Map<String, Object> attrs = metadata
        .getAnnotationAttributes(EnableFeignClients.class.getName());
    AnnotationTypeFilter annotationTypeFilter = new AnnotationTypeFilter(
        FeignClient.class);
    // 获取 clients 属性, 默认为{}, 这里可以看出 如果配置了 clients 属性就不会走basePackage了
    final Class<?>[] clients = attrs == null ? null
        : (Class<?>[]) attrs.get("clients");
    if (clients == null || clients.length == 0) {
        // 3.添加扫描过滤器, 即只有@FeignClient注解的才会被扫描
        scanner.addIncludeFilter(annotationTypeFilter);
        // 从属性中获取需要扫描的包
        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)));
    }
	// 4.扫描包中的有@FeignClient注解的接口
    for (String basePackage : basePackages) {
        // 5.扫描出对应的接口对应并创建bd
        // 这里可以获取 equipmentFeign 和 personFeign 对应的bd
        Set<BeanDefinition> candidateComponents = scanner
            .findCandidateComponents(basePackage);
        for (BeanDefinition candidateComponent : candidateComponents) {
            // 6.处理所有的bd
            if (candidateComponent instanceof AnnotatedBeanDefinition) {
                // verify annotated class is an interface
                // 7.获取bd
                AnnotatedBeanDefinition beanDefinition = (AnnotatedBeanDefinition) candidateComponent;
                AnnotationMetadata annotationMetadata = beanDefinition.getMetadata();
                Assert.isTrue(annotationMetadata.isInterface(),
                              "@FeignClient can only be specified on an interface");
			   // 8.获取bd上@FeignClient注解的属性
                Map<String, Object> attributes = annotationMetadata
                    .getAnnotationAttributes(
                    FeignClient.class.getCanonicalName());
			   // 9.获取name属性
                String name = getClientName(attributes);
                // 10.这里跟@3.2.1相同, 都是注册FeignClientSpecification的bd
                // 但是这里注册的是 每一个 FeignClient 的私有配置 FeignClientSpecification的bd, 并注册到spring容器中
                // name == person-provider.FeignClientSpecification
                // value == FeignClientSpecification对应bd
                registerClientConfiguration(registry, name,
                                            attributes.get("configuration"));
			   // 11.【重点】注册FeignClient, 见下方
                registerFeignClient(registry, annotationMetadata, attributes);
            }
        }
    }
}

// 这里就是注册FeignClient对应bd, 这里以 personFeign 为例
private void registerFeignClient(BeanDefinitionRegistry registry,
                                 AnnotationMetadata annotationMetadata, Map<String, Object> attributes) {
    // 1.className == com.tca.cloud.standalone.person.api.service.PersonFeign
    String className = annotationMetadata.getClassName();
    // 2.创建BeanDefinitionBuilder, 可以看到, 这里的beanClass == FeignClientFactoryBean, FeignClientFactoryBean是FactoryBean, 这里的原理和mybatis-spring整合一致, 使用FactoryBean来创建每一个代理对象！！！
    BeanDefinitionBuilder definition = BeanDefinitionBuilder
        .genericBeanDefinition(FeignClientFactoryBean.class);
    validate(attributes);
    // 3.给bd中的beanClass添加相关属性: url, path, name, contextId 等
    // 查看 FeignClientFactoryBean 代码可以看到, 其确实有这些对应的属性 
    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 });
    // 4.注册到spring容器中
    // name == com.tca.cloud.standalone.person.api.service.PersonFeign
    // value == FeignClientFactoryBean对应的bd
    BeanDefinitionReaderUtils.registerBeanDefinition(holder, registry);
}

FeignClientsRegistrar#registerFeignClients的作用:
1.扫描@FeignClient注解的接口
2.对于每一个 FeignClient, 创建两个bd, 并分别注册到spring容器中:
  a. FeignClient对应的FeignClientSpecification, name == ${value}.FeignClientSpecification, value == FeignClientSpecification, 这里是当前FeignClient的局部配置类
  b. FeignClient对应的FeignClientFactoryBean, name == 接口全限定名, value == FeignClientFactoryBean

在springboot源码解析中, 我们知道自动配置的原理, 其核心在于spring会读取所有jar包中的META-INF/spring.factories配置文件, 获取配置文件中的所有配置类, 再根据配置类中的@ConditionOnxxx等方式判断是否需要将相关配置类注册到spring容器中！
可以看到spring-cloud-starter-openfeign中依赖了spring-cloud-openfeign-core，spring-cloud-openfeign-core的jar包中包含了spring.factories配置文件！其中有一个重要的配置类：FeignAutoConfiguration

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.cloud.openfeign.ribbon.FeignRibbonClientAutoConfiguration,\
org.springframework.cloud.openfeign.FeignAutoConfiguration,\
org.springframework.cloud.openfeign.encoding.FeignAcceptGzipEncodingAutoConfiguration,\
org.springframework.cloud.openfeign.encoding.FeignContentGzipEncodingAutoConfiguration

/**
 * @author Spencer Gibb
 * @author Julien Roy
 */
@Configuration
@ConditionalOnClass(Feign.class)
@EnableConfigurationProperties({ FeignClientProperties.class,
                                FeignHttpClientProperties.class })
public class FeignAutoConfiguration {
	
    // 这里会获取容器中所有的FeignClientSpecification, 包括当前应用的全局FeignClientSpecification 以及每个 FeignClient 对应的局部的FeignClientSpecification 
    @Autowired(required = false)
    private List<FeignClientSpecification> configurations = new ArrayList<>();

	// 向容器中注册FeignContext【核心组件】
    @Bean
    public FeignContext feignContext() {
        FeignContext context = new FeignContext();
        context.setConfigurations(this.configurations);
        return context;
    }

   // ......
}

// FeignContext 是feign的全局上下文, 它组合了所有 FeignClientSpecification
public class FeignContext extends NamedContextFactory<FeignClientSpecification> {

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

}

public abstract class NamedContextFactory<C extends NamedContextFactory.Specification>
		implements DisposableBean, ApplicationContextAware {

	private final String propertySourceName;

	private final String propertyName;
    
	// 【重要】
    // key == 微服务的名称, 即@FeignClient的value属性值
    // value == spring容器, 它是服务容器的子容器, 即创建FeignClient代理对象时的组件都放在子容器中
	private Map<String, AnnotationConfigApplicationContext> contexts = new ConcurrentHashMap<>();
    
    // configurations即为之前封装的所有FeignClientSpecification
	private Map<String, C> configurations = new ConcurrentHashMap<>();
    
    // 父容器
	private ApplicationContext parent;

	private Class<?> defaultConfigType;
    
    //......
}

1.根据上面学习知道, 使用 @EnableFeignClients 和 @FeignClient 注解, springboot容器启动时会解析这两个核心注解, 根据 @EnableFeignClients 的属性创建全局 FeignClientSpecification (这个是全局默认配置类), 根据每一个 @FeignClient 的属性创建每一个 @FeignClient 的 FeignClientSpecification 配置类, 全部都注册到spring容器中, 对于全局配置, key == default.***, 对于局部配置, key == FeignClient的value属性, 一般为微服务的name！！！
2.根据自动配置, 在容器中创建了 FeignContext, 它继承了 NamedContextFactory, NamedContextFactory很重要, 它有两个重要的属性:
private Map<String, C> configurations = new ConcurrentHashMap<>(); // 这个用来存储上述说的默认配置和全局配置类
private Map<String, AnnotationConfigApplicationContext> contexts = new ConcurrentHashMap<>(); // 这个会为每个FeignClient创建一个对应的spring子容器, 实现FeignClient的隔离！！！
3.通过阅读@ribbon源码, ribbon中和openfeign的设计如出一辙, ribbon中有@RibbonClients 和 @RibbonClient, 对应 EnableFeignClients 和 FeignClient, 也会创建全局配置类 RibbonClientSpecification, ribbon中有 SpringClientFactory 对应 openfeign中的 FeignContext, 同样继承了 NamedContextFactory, 也存储了每个RibbonClient的配置以及为每个 RibbonClient 创建 spring子容器, 实现客户端之间的配置隔离！！！

前期我们向spring容器中注册了feignClient相关bd, 对应的bd为FactoryBean的实现类 --> FeignClientFactoryBean, 因此代理对象是通过 FeignClientFactoryBean#getObject方法创建的

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

<T> T getTarget() {
    // 1.由@4自动配置可以看到, 通过自动配置已经向spring容器中注册了FeignContext
    FeignContext context = this.applicationContext.getBean(FeignContext.class);
    // 2.通过 FeignContext 创建 Feign.Builder, 见@5.2
    Feign.Builder builder = feign(context);
    // 3.判断@FeignClient的url属性, 为空时, 走负载均衡策略, 不能空时, 则通过直连调用
    // 这里的分支是负载均衡策略(默认的方式)
    if (!StringUtils.hasText(this.url)) {
        if (!this.name.startsWith("http")) {
            this.url = "http://" + this.name;
        }
        else {
            this.url = this.name;
        }
        this.url += cleanPath();
        // 3.负载均衡策略, 见@5.3
        return (T) loadBalance(builder, context,
                               new HardCodedTarget<>(this.type, this.name, this.url));
    }
    // 这里的分支是直连方式
    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));
}

protected Feign.Builder feign(FeignContext context) {
    // 1.从FeignContext中获取FeignLoggerFactory
    // 由 @4.3 可以看到, FeignContext的父类中有一个重要的属性: 
    // private Map<String, AnnotationConfigApplicationContext> contexts = new ConcurrentHashMap<>();
    // key == 微服务的名称, 即@FeignClient的value属性值
    // value == spring容器, 它是服务容器的子容器, 即创建FeignClient代理对象时的组件都放在子容器中
    // 实例化FeignContext时, 当前属性为空, 当我们调用 get(context, FeignLoggerFactory.class) 时, 会根据当前FeignClient的value, 即微服务的名称(person-provider/equipment-provider)从 contexts 属性中获取对应的spring子容器, 第一次获取时, 会创建子容器, 并存储到context属性中！！！
    // 创建子容器时, 会从局部配置类和全局配置类 FeignClientSpecification 中读取相关配置bean, 并注册到子容器中！！！
    // 【综上】这里会创建 子容器, 并以key-value格式存储到 FeignContext 的 contexts属性中, 
    FeignLoggerFactory loggerFactory = get(context, FeignLoggerFactory.class);
    Logger logger = loggerFactory.create(this.type);

    // 2.从子容器中获取 Feign.Builder 对象, Encoder对象, Decoder对象, Contract对象, 封装到 Feign.Builder 中
    Feign.Builder builder = get(context, Feign.Builder.class)
        .logger(logger)
        // encoder --> SpringEncoder
        .encoder(get(context, Encoder.class))
        // decoder --> OptionalDecoder
        .decoder(get(context, Decoder.class))
        // contract --> SpringMVCContract
        .contract(get(context, Contract.class));
	// 3.根据 FeignClientProperties 继续配置 Feign.Builder
    configureFeign(context, builder);

    return builder;
}

这里主要用于构建Feign.Builder对象, 其中一个核心的步骤是创建每个FeignClient对应的spring子容器, 将FeignClient对应的局部配置类和全局配置类中的相关bean注册到子容器中, 并将其存储在 FeignContext 中！！！并用于构建Feign.Builder对象！！！

protected <T> T loadBalance(Feign.Builder builder, FeignContext context,
                            HardCodedTarget<T> target) {
    // 1.从 FeignContext 中获取 Client 对象, 这里的client == LoadBalanceFeignClient
  	// 这里的Client是哪个注入的, 又是如何集成ribbon的, 见@7.1 
    Client client = getOptional(context, Client.class);
    if (client != null) {
        // 2.继续配置 Feign.Builder 
        builder.client(client);
        // 3.从 FeignContext 中获取 Targeter 对象, 这里的targeter == HystrixTargeter
        Targeter targeter = get(context, Targeter.class);
        // 4.创建代理对象 见@5.4
        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?");
}

这里从 FeignContext 中的子容器中继续获取 Client, Targeter对象, 用于构建Feign.Builder对象

// 这里会创建代理对象
@Override
public <T> T target(FeignClientFactoryBean factory, Feign.Builder feign,
                    FeignContext context, Target.HardCodedTarget<T> target) {
    // 默认进入 feign instanceof feign.hystrix.HystrixFeign.Builder == false
    // 没有开启熔断功能话就不是熔断的Builder
    if (!(feign instanceof feign.hystrix.HystrixFeign.Builder)) {
        // 见下
        return feign.target(target);
    }
    //如果开启了熔断，就会处理一些服务降级的配置
    feign.hystrix.HystrixFeign.Builder builder = (feign.hystrix.HystrixFeign.Builder) feign;
    SetterFactory setterFactory = getOptional(factory.getName(), context,
                                              SetterFactory.class);
    if (setterFactory != null) {
        builder.setterFactory(setterFactory);
    }
    Class<?> fallback = factory.getFallback();
    if (fallback != void.class) {
        return targetWithFallback(factory.getName(), context, target, builder,
                                  fallback);
    }
    Class<?> fallbackFactory = factory.getFallbackFactory();
    if (fallbackFactory != void.class) {
        return targetWithFallbackFactory(factory.getName(), context, target, builder,
                                         fallbackFactory);
    }

    return feign.target(target);
}

// target == Target.HardCodedTarget
public <T> T target(Target<T> target) {
    // 1.build --> 创建 Feign 对象, 这里的 Feign == ReflectiveFeign
    // 2.newInstance --> 创建代理对象
    return build().newInstance(target);
}

public Feign build() {
    SynchronousMethodHandler.Factory synchronousMethodHandlerFactory =
        new SynchronousMethodHandler.Factory(client, retryer, requestInterceptors, logger,
                                             logLevel, decode404, closeAfterDecode, propagationPolicy);
    ParseHandlersByName handlersByName =
        new ParseHandlersByName(contract, options, encoder, decoder, queryMapEncoder,
                                errorDecoder, synchronousMethodHandlerFactory);
    return new ReflectiveFeign(handlersByName, invocationHandlerFactory, queryMapEncoder);
}

@Override
public <T> T newInstance(Target<T> target) {
    // 1.生成 name -- methodHandler , 以EquipmentFeign为例
    // name == EquipmentFeign#get(EquipmentReq)
    // value == SynchronousMethodHandler
    Map<String, MethodHandler> nameToHandler = targetToHandlersByName.apply(target);
    // 2.声明 method - MethodHandler
    Map<Method, MethodHandler> methodToHandler = new LinkedHashMap<Method, MethodHandler>();
    List<DefaultMethodHandler> defaultMethodHandlers = new LinkedList<DefaultMethodHandler>();
	// 3.获取 EquipmentFeign 的所有方法 
    for (Method method : target.type().getMethods()) {
        if (method.getDeclaringClass() == Object.class) {
            continue;
        } else if (Util.isDefault(method)) {
            DefaultMethodHandler handler = new DefaultMethodHandler(method);
            defaultMethodHandlers.add(handler);
            methodToHandler.put(method, handler);
        } else {
            // 添加 method - MethodHandler 到 methodToHandler中
            methodToHandler.put(method, nameToHandler.get(Feign.configKey(target.type(), method)));
        }
    }
    // 4.创建 InvocationHandler, 这里为 ReflectiveFeign.FeignInvocationHandler
    InvocationHandler handler = factory.create(target, methodToHandler);
    // 5.创建代理对象
    T proxy = (T) Proxy.newProxyInstance(target.type().getClassLoader(),
                                         new Class<?>[] {target.type()}, handler);

    for (DefaultMethodHandler defaultMethodHandler : defaultMethodHandlers) {
        defaultMethodHandler.bindTo(proxy);
    }
    return proxy;
}

从上述可以看到, 通过jdk动态代理创建 clientfeign 代理对象, jdk的动态代理有一个核心类 -- InvocationHandler, 这里默认实现是: ReflectiveFeign.FeignInvocationHandler, 因此方法调用时, 调用其invoke方法

@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    if ("equals".equals(method.getName())) {
        try {
            Object otherHandler =
                args.length > 0 && args[0] != null ? Proxy.getInvocationHandler(args[0]) : null;
            return equals(otherHandler);
        } catch (IllegalArgumentException e) {
            return false;
        }
    } else if ("hashCode".equals(method.getName())) {
        return hashCode();
    } else if ("toString".equals(method.getName())) {
        return toString();
    }
    // dispatch存储的是 method -- MethodHandler 关联关系, 调用某个method 就是会调用对应的 MethodHandler#invoke方法
    //如果执行的方法是接口中的抽象方法，方法处理器实现类是SynchronousMethodHandler【重点】, 见 @6.2
    //如果执行的方法是接口中的默认方法，方法处理器实现类是DefaultMethodHandler
    return dispatch.get(method).invoke(args);
}

@Override
public Object invoke(Object[] argv) throws Throwable {
    // 1.构建请求模板, 包括: 请求头, 请求体, 请求方法, 请求url等
    RequestTemplate template = buildTemplateFromArgs.create(argv);
    Retryer retryer = this.retryer.clone();
    while (true) {
        try {
            // 2.执行并解码, 见@6.3
            return executeAndDecode(template);
        } catch (RetryableException e) {
            try {
                retryer.continueOrPropagate(e);
            } catch (RetryableException th) {
                Throwable cause = th.getCause();
                if (propagationPolicy == UNWRAP && cause != null) {
                    throw cause;
                } else {
                    throw th;
                }
            }
            if (logLevel != Logger.Level.NONE) {
                logger.logRetry(metadata.configKey(), logLevel);
            }
            continue;
        }
    }
}

Object executeAndDecode(RequestTemplate template) throws Throwable {
    // 1.target中包含请求url的前段 -- http://equipment-provider/ , RequestTemplate中包含 /equipment/get , 拼在一起: http://equipment-provider/equipment/get
    Request request = targetRequest(template);

    if (logLevel != Logger.Level.NONE) {
        logger.logRequest(metadata.configKey(), logLevel, request);
    }

    Response response;
    long start = System.nanoTime();
    try {
        // 这个Client就是发起远程调用的客户端
        // 如果导入了Ribbon负载均衡依赖，这个client就是 LoadBalancerFeignClient, openfeign的包中默认集成了ribbon!
        // client默认为负载均衡client --> LoadBalancerFeignClient
        // 见@6.4
        response = client.execute(request, options);
    } catch (IOException e) {
        if (logLevel != Logger.Level.NONE) {
            logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime(start));
        }
        throw errorExecuting(request, e);
    }
    long elapsedTime = TimeUnit.NANOSECONDS.toMillis(System.nanoTime() - start);

    boolean shouldClose = true;
    try {
        if (logLevel != Logger.Level.NONE) {
            response =
                logger.logAndRebufferResponse(metadata.configKey(), logLevel, response, elapsedTime);
        }
        if (Response.class == metadata.returnType()) {
            if (response.body() == null) {
                return response;
            }
            if (response.body().length() == null ||
                response.body().length() > MAX_RESPONSE_BUFFER_SIZE) {
                shouldClose = false;
                return response;
            }
            // Ensure the response body is disconnected
            byte[] bodyData = Util.toByteArray(response.body().asInputStream());
            return response.toBuilder().body(bodyData).build();
        }
        if (response.status() >= 200 && response.status() < 300) {
            if (void.class == metadata.returnType()) {
                return null;
            } else {
                Object result = decode(response);
                shouldClose = closeAfterDecode;
                return result;
            }
        } else if (decode404 && response.status() == 404 && void.class != metadata.returnType()) {
            Object result = decode(response);
            shouldClose = closeAfterDecode;
            return result;
        } else {
            throw errorDecoder.decode(metadata.configKey(), response);
        }
    } catch (IOException e) {
        if (logLevel != Logger.Level.NONE) {
            logger.logIOException(metadata.configKey(), logLevel, e, elapsedTime);
        }
        throw errorReading(request, response, e);
    } finally {
        if (shouldClose) {
            ensureClosed(response.body());
        }
    }
}

@Override
public Response execute(Request request, Request.Options options) throws IOException {
    try {
        URI asUri = URI.create(request.url());
        String clientName = asUri.getHost();
        URI uriWithoutHost = cleanUrl(request.url(), clientName);
        // 1.Ribbon整合就在这！！！
		// this.delegate --> ApacheHttpClient
		// 包装了一下先进行负载均衡，再进行远程调用，最终远程调用还是依靠this.delegate
        FeignLoadBalancer.RibbonRequest ribbonRequest = new FeignLoadBalancer.RibbonRequest(
            this.delegate, request, uriWithoutHost);
        IClientConfig requestConfig = getClientConfig(options, clientName);
        // 2.lbClient：获取一个负载均衡器, 见@6.5
        // 这里的clientName 即为微服务名称, 如 equipment-provider
        // executeWithLoadBalancer: 负载均衡执行
        return lbClient(clientName)
            .executeWithLoadBalancer(ribbonRequest, requestConfig).toResponse();
    }
    catch (ClientException e) {
        IOException io = findIOException(e);
        if (io != null) {
            throw io;
        }
        throw new RuntimeException(e);
    }
}

private FeignLoadBalancer lbClient(String clientName) {
    // lbClientFactory --> CachingSpringLoadBalancerFactory
    return this.lbClientFactory.create(clientName);
}

public FeignLoadBalancer create(String clientName) {
    // 1.先从缓存中获取
    FeignLoadBalancer client = this.cache.get(clientName);
    if (client != null) {
        return client;
    }
    // 2.获取客户端配置
    IClientConfig config = this.factory.getClientConfig(clientName);
    // 3.获取负载均衡器【重点】
    // this.factory == SpringClientFactory
    ILoadBalancer lb = this.factory.getLoadBalancer(clientName);
    ServerIntrospector serverIntrospector = this.factory.getInstance(clientName,
    // 包装成具有重试功能的负载均衡器
	// 同时注意ILoadBalancer是Ribbon的接口
	// RetryableFeignLoadBalancer和FeignLoadBalancer都是OpenFeign的
	// 也可以认为是适配了一下                                                              
    client = this.loadBalancedRetryFactory != null
        ? new RetryableFeignLoadBalancer(lb, config, serverIntrospector,
                                         this.loadBalancedRetryFactory)
        : new FeignLoadBalancer(lb, config, serverIntrospector);
    this.cache.put(clientName, client);
    return client;
}

public ILoadBalancer getLoadBalancer(String name) {
    // 根据微服务名, 从子容器中获取ILoadBalancer实例, 取不到就从父容器中获取
    // 这里获取的是 ZoneAwareLoadBalancer
    return getInstance(name, ILoadBalancer.class);
}

@Override
public <C> C getInstance(String name, Class<C> type) {
    C instance = super.getInstance(name, type);
    if (instance != null) {
        return instance;
    }
    IClientConfig config = getInstance(name, IClientConfig.class);
    return instantiateWithConfig(getContext(name), type, config);
}

/**
 * 当调用者希望将请求分派到负载平衡器选择的服务器, 而不是在请求的URI中指定服务器时, 应该使用此方法。
 * 它通过调用reconstructURIWithServer来计算最终的URI
 */
public T executeWithLoadBalancer(final S request, final IClientConfig requestConfig) throws ClientException {
    LoadBalancerCommand<T> command = buildLoadBalancerCommand(request, requestConfig);

    try {
        // 1.command提交了一个服务器操作, 当负载均衡选到了要访问的服务器后, 就会执行这个ServerOperation, 所以负载均衡触发点主要在sumbit中
        return command.submit(
            new ServerOperation<T>() {
                @Override
                public Observable<T> call(Server server) {
                    // 通过选择的Server计算最终的URI
                    URI finalUri = reconstructURIWithServer(server, request.getUri());
                    S requestForServer = (S) request.replaceUri(finalUri);
                    try {
                        // 调用execute发起请求
                        return Observable.just(AbstractLoadBalancerAwareClient.this.execute(requestForServer, requestConfig));
                    } 
                    catch (Exception e) {
                        return Observable.error(e);
                    }
                }
            })
            .toBlocking()
            .single();
    } catch (Exception e) {
        Throwable t = e.getCause();
        if (t instanceof ClientException) {
            throw (ClientException) t;
        } else {
            throw new ClientException(e);
        }
    }

}

public Observable<T> submit(final ServerOperation<T> operation) {
    // ......
    // 通过 selectServer 选择服务器
    Observable<T> o = 
        (server == null ? selectServer() : Observable.just(server))
        .concatMap(new Func1<Server, Observable<T>>() {
            
                    });

                if (maxRetrysSame > 0) 
                    o = o.retry(retryPolicy(maxRetrysSame, true));
                return o;
            }
        });
	// ......
    
}

private Observable<Server> selectServer() {
    return Observable.create(new OnSubscribe<Server>() {
        @Override
        public void call(Subscriber<? super Server> next) {
            try {
                // 选择服务器
                Server server = loadBalancerContext.getServerFromLoadBalancer(loadBalancerURI, loadBalancerKey);   
                next.onNext(server);
                next.onCompleted();
            } catch (Exception e) {
                next.onError(e);
            }
        }
    });
}

public Server getServerFromLoadBalancer(@Nullable URI original, @Nullable Object loadBalancerKey) throws ClientException {
    // ......
    
    ILoadBalancer lb = getLoadBalancer();
    if (host == null) {
        // Partial URI or no URI Case
        // well we have to just get the right instances from lb - or we fall back
        if (lb != null){
            // 选择服务器！
            Server svc = lb.chooseServer(loadBalancerKey);
            if (svc == null){
                throw new ClientException(ClientException.ErrorType.GENERAL,
                                          "Load balancer does not have available server for client: "
                                          + clientName);
            }
            host = svc.getHost();
            if (host == null){
                throw new ClientException(ClientException.ErrorType.GENERAL,
                                          "Invalid Server for :" + svc);
            }
            logger.debug("{} using LB returned Server: {} for request {}", new Object[]{clientName, svc, original});
            return svc;
        } else {
        }
    }
    
    // ...... 省略很多

    return new Server(host, port);
}

public Server chooseServer(Object key) {
    if (counter == null) {
        counter = createCounter();
    }
    counter.increment();
    if (rule == null) {
        return null;
    } else {
        try {
            // 借助 负载均衡策略 选择
            return rule.choose(key);
            logger.warn("LoadBalancer [{}]:  Error choosing server for key {}", name, key, e);
            return null;
        }
    }
}

Feign的构建涉及到一些核心组件: contract, encoder, decoder, interceptor、logger、httpclient、retryer。
使用springcloud-openfeign时, 我们可以将组件通过@EnableFeignClients或@FeignClient的configuration配置, 将自定义组件注册到spring容器中, 在构建Feign时, 可以使用spring容器中的相关组件！

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.cloud.openfeign.ribbon.FeignRibbonClientAutoConfiguration,\
org.springframework.cloud.openfeign.FeignAutoConfiguration,\
org.springframework.cloud.openfeign.encoding.FeignAcceptGzipEncodingAutoConfiguration,\
org.springframework.cloud.openfeign.encoding.FeignContentGzipEncodingAutoConfiguration

@ConditionalOnClass({ ILoadBalancer.class, Feign.class })
@ConditionalOnProperty(value = "spring.cloud.loadbalancer.ribbon.enabled",
		matchIfMissing = true)
@Configuration(proxyBeanMethods = false)
@AutoConfigureBefore(FeignAutoConfiguration.class)
@EnableConfigurationProperties({ FeignHttpClientProperties.class })
@Import({ HttpClientFeignLoadBalancedConfiguration.class,
		OkHttpFeignLoadBalancedConfiguration.class,
		DefaultFeignLoadBalancedConfiguration.class })
public class FeignRibbonClientAutoConfiguration {
  
  // ......
}

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(OkHttpClient.class)
@ConditionalOnProperty("feign.okhttp.enabled")
@Import(OkHttpFeignConfiguration.class)
class OkHttpFeignLoadBalancedConfiguration {

    @Bean
    @ConditionalOnMissingBean(Client.class)
    public Client feignClient(CachingSpringLoadBalancerFactory cachingFactory,
            SpringClientFactory clientFactory, okhttp3.OkHttpClient okHttpClient) {
        OkHttpClient delegate = new OkHttpClient(okHttpClient);
        return new LoadBalancerFeignClient(delegate, cachingFactory, clientFactory);
    }

}

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(ApacheHttpClient.class)
@ConditionalOnProperty(value = "feign.httpclient.enabled", matchIfMissing = true)
@Import(HttpClientFeignConfiguration.class)
class HttpClientFeignLoadBalancedConfiguration {

    @Bean
    @ConditionalOnMissingBean(Client.class)
    public Client feignClient(CachingSpringLoadBalancerFactory cachingFactory,
            SpringClientFactory clientFactory, HttpClient httpClient) {
        ApacheHttpClient delegate = new ApacheHttpClient(httpClient);
        return new LoadBalancerFeignClient(delegate, cachingFactory, clientFactory);
    }

}

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass(OkHttpClient.class)
@ConditionalOnProperty("feign.okhttp.enabled")
@Import(OkHttpFeignConfiguration.class)
class OkHttpFeignLoadBalancedConfiguration {

    @Bean
    @ConditionalOnMissingBean(Client.class)
    public Client feignClient(CachingSpringLoadBalancerFactory cachingFactory,
            SpringClientFactory clientFactory, okhttp3.OkHttpClient okHttpClient) {
        OkHttpClient delegate = new OkHttpClient(okHttpClient);
        return new LoadBalancerFeignClient(delegate, cachingFactory, clientFactory);
    }

}

@Configuration(proxyBeanMethods = false)
class DefaultFeignLoadBalancedConfiguration {

    @Bean
    @ConditionalOnMissingBean
    public Client feignClient(CachingSpringLoadBalancerFactory cachingFactory,
            SpringClientFactory clientFactory) {
        return new LoadBalancerFeignClient(new Client.Default(null, null), cachingFactory,
                clientFactory);
    }

}