一、原理

1. 声明式Rest Client
2. 是对Ribbon的封装（间接对httpclient或okhttp的封装）
3. 采用动态代理机制，针对接口直接生成远程代理对象
4. 同时支持：Feign annotations and JAX-RS annotations（或SpringMVC annotations）
5. 支持较简单的熔断和降级的规则

二、开发过程:

1. pom.xml

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

2. 主启动类：

@SpringBootApplication
@EnableEurekaClient
@EnableFeignClients
public class OrderApp {
    public static void main(String[] args) {
      ...

3. 配置服务接口，用以生成代理对象，并向Controller进行注入：
   service

@Component
@FeignClient(value = "payment") //payment为注册在Eureka中的服务名称
public interface PaymentService {
    //此处也可以使用javax.rs.Path进行注解
    @RequestMapping(value = "/payments/{id}",method = RequestMethod.GET)
    CommonResult<Payment> loadPayment(@PathVariable(value = "id") int id);
}

controller

//controller的代码片段
    @Resource
    private PaymentService paymentService;
    @GetMapping("/payments/{id}")
    public CommonResult<Payment> loadPaymentWithFeign(@PathVariable(value = "id") int id){
        return paymentService.loadPayment(id);
    }

三、负载均衡规则：

由于其是对Ribbon的的封装，所以原有的Ribbon规则的配置方法，仍然适用，如：

//配置多个规则的方式
@RibbonClients(
            value = {
                    @RibbonClient(value = "payment",configuration = MyRuleConf.class)
                    ,@RibbonClient(value = "other",configuration = YourRuleConf.class)
            }
        )

此时，对payment服务的规则，将由MyRuleConf.java确定，详见的我的简书：Ribbon使用备忘

四、配置open feign的一般方法：

application.yml
全局配置：

feign:
    client:
        config:
            default:
                connectTimeout: 5000
                readTimeout: 5000
                loggerLevel: basic

针对feignName(payment服务名)的配置：

feign:
    client:
        config:
            payment:
                connectTimeout: 5000
                readTimeout: 5000

--- 参考： https://docs.spring.io/spring-cloud-openfeign/docs/current/reference/html/#spring-cloud-feign
--- 参考：
https://docs.spring.io/spring-cloud-openfeign/docs/current/reference/html/appendix.html