一、前言

学习Feign之前最好先阅读一下Ribbon源码，因为Feign依赖于Ribbon。

Ribbon源码阅读：

SpringClientFactory命名容器工厂：juejin.cn/post/687554…
Ribbon如何实现服务定制化配置：juejin.cn/post/687592…
Ribbon请求流程解读及实现：juejin.cn/post/687672…

二、FeignContext命名容器工厂

FeignClient使用案例

@SpringBootApplication
@EnableFeignClients
public class Application {
	public static void main(String[] args) {
		SpringApplication.run(Application.class, args);
	}
}
@FeignClient(name = "trade-service")
public interface StockClient {
    @RequestMapping(value = "/getStockByMpId/{mpId}", method = RequestMethod.GET)
    ApiResponse<Stock> getStock(@PathVariable("mpId") Long mpId);
}
@RestController
public class ExampleController {
    @Autowired
    private StockClient stockClient;
    @RequestMapping("/seckill/{mpId}")
    @ResponseBody
    public ApiResponse<Stock> getStock(@PathVariable("mpId") Long mpId) {
        return stockClient.getStock(mpId);
    }
 }

FeignContext

FeignContext继承了NamedContextFactory，和Ribbon的SpringClientFactory一样，都是为了创建Spring子容器。

public class FeignContext extends NamedContextFactory<FeignClientSpecification> {

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

	@Nullable
	public <T> T getInstanceWithoutAncestors(String name, Class<T> type) {
		return BeanFactoryUtils.beanOfType(getContext(name), type);
	}

	@Nullable
	public <T> Map<String, T> getInstancesWithoutAncestors(String name, Class<T> type) {
		return getContext(name).getBeansOfType(type);
	}

}

重要构造函数，之前已经学习过SpringClientFactory，这里通过一个单元测试复习一下，不过多阐述

@Autowired
private FeignContext feignContext;

private static final String serviceName = "trade-service";
@Test
public void test01() {
    // 构造参数一：默认注入子容器的配置类
    FeignClientsConfiguration defaultConfig = feignContext.getInstance(serviceName, FeignClientsConfiguration.class);
    System.out.println(defaultConfig);// org.springframework.cloud.openfeign.FeignClientsConfiguration@296e281a
    // 构造参数二：PropertySource的name
    AbstractEnvironment env = feignContext.getInstance(serviceName, AbstractEnvironment.class);
    Iterator<PropertySource<?>> iterator = env.getPropertySources().iterator();
    if (iterator.hasNext()) {
        System.out.println(iterator.next().getName()); // feign
    }
    // 构造参数三：PropertySource里的配置的key，value是子容器名（一般是服务名称，如果设置FeignClient注解里的contextId，就是contextId）
    String property = env.getProperty("feign.client.name");
    System.out.println(property); // trade-service
}

getInstanceWithoutAncestors：只从子容器寻找bean，返回bean实例
getInstancesWithoutAncestors：只从子容器寻找bean，返回Map<bean名称，bean实例>

三、FeignClientsConfiguration子容器默认注入的配置

重点看一下FeignClientsConfiguration，给子容器默认注入了哪些bean

Encoder、Decoder编码解码器

@Bean
@ConditionalOnMissingBean
public Decoder feignDecoder() {
	return new OptionalDecoder(
			new ResponseEntityDecoder(new SpringDecoder(this.messageConverters)));
}
@Bean
@ConditionalOnMissingBean
@ConditionalOnMissingClass("org.springframework.data.domain.Pageable")
public Encoder feignEncoder(ObjectProvider<AbstractFormWriter> formWriterProvider) {
	return springEncoder(formWriterProvider);
}

Contract：默认实现类SpringMvcContract，解析SpringMVC的注解如：@RequestMapping。如果要使用Feign自己的注解如@RequestLine，那就要注入feign.Contract.Default实例。

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

FormattingConversionService：参数转换，比如时间类型转换等等

@Bean
public FormattingConversionService feignConversionService() {
	FormattingConversionService conversionService = new DefaultFormattingConversionService();
	for (FeignFormatterRegistrar feignFormatterRegistrar : this.feignFormatterRegistrars) {
		feignFormatterRegistrar.registerFormatters(conversionService);
	}
	return conversionService;
}

Retryer：Feign自己的重试机制，默认不重试

@Bean
@ConditionalOnMissingBean
public Retryer feignRetryer() {
	return Retryer.NEVER_RETRY;
}

FeignLoggerFactory：Feign自己的日志工厂

@Bean
@ConditionalOnMissingBean(FeignLoggerFactory.class)
public FeignLoggerFactory feignLoggerFactory() {
	return new DefaultFeignLoggerFactory(this.logger);
}

FeignClientConfigurer：用于配置"是否可以从配置文件定制化FeignClient"，默认是

@Bean
@ConditionalOnMissingBean(FeignClientConfigurer.class)
public FeignClientConfigurer feignClientConfigurer() {
	return new FeignClientConfigurer() {
	};
}

Feign.Builder：关键，用于构建ReflectiveFeign，这里是个原型对象，非单例

@Bean
@Scope("prototype")
@ConditionalOnMissingBean
public Feign.Builder feignBuilder(Retryer retryer) {
	return Feign.builder().retryer(retryer);
}

当集成hystrix时，Feign.Builder是实现会是HystrixFeign，虽然同样是@ConditionalOnMissingBean，但是@ConditionalOnProperty(name = "feign.hystrix.enabled")的优先级较高，会优先取HystrixFeign.builder()

@Configuration(proxyBeanMethods = false)
@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();
	}

}

四、总结

通过FeignContext命名容器工厂的构造函数和默认注入的配置类，对Feign先有个大致的概念。现在会有几个疑问：

为什么IRule、ServerList、ILoadBalancer这些Ribbon里的抽象在FeignContext子容器的默认配置里没有找到？
为什么Feign.Builder在子容器里是非单例的？ ...

Feign源码阅读（一）FeignContext命名容器工厂