1.前言
Feign是一个声明式的web service客户端，它使得编写web service客户端更为容易。创建接口，为接口添加注解，即可使用Feign。Feign可以使用Feign注解或者JAX-RS注解，还支持热插拔的编码器和解码器。

Spring Cloud为Feign添加了Spring MVC的注解支持，并整合了Ribbon和Eureka来为使用Feign时提供负载均衡。使用@EnableFeignClients即可开启Feign。

架构技术选型

核心框架 Spring Boot
SOA Spring Cloud
安全框架 Spring Security Oauth2
注册中心/配置中心 Nacos 集群部署
网关 Spring Cloud Gateway 部署多套,使用nginx负载
链路监控 Skywalking
流量控制、熔断降级 Sentinel
数据库 Mysql
分布式事务 Seata
定时调度 xxl-job
缓存 Redis
MQ RocketMq
持久框架 Mybatis Plus
搜索引擎 ElasticSearch
需要架构源码的朋友可以看我个人简介联系我

简而言之：

Feign采用的是基于接口的注解
Feign默认整合了ribbon，具有负载均衡的能力
Feign默认整合了Hystrix，具有熔断的能力

OpenFeign属性配置列表文档： https://cloud.spring.io/spring-cloud-static/spring-cloud-openfeign/2.2.2.RELEASE/reference/html/appendix.html

OpenFeign官方教程文档： https://cloud.spring.io/spring-cloud-static/spring-cloud-openfeign/2.2.2.RELEASE/reference/html/#configuration-properties

2.初探Feign
微服务间的通讯是基于HTTP的，而Feign就是一个伪HTTP请求客户端，通过声明式的Feign客户端可以调用远程微服务，在实现远程微服务调用的过程中，多个实例存在的时候，Ribbon负载均衡器会默认的进行本地负载均衡，默认通过轮询机制进行微服务的调用，Feign默认则是集成了并且支持Ribbon负载均衡。

下面以订单服务（Order）调用商品服务（Product）为例，搭建环境使用Feign远程调用消费服务。

3.商品(Product)工程
pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>springcloud-alibaba-nacos</artifactId>
        <groupId>com.bruce.springcloud-alibaba-nacos</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>
    <artifactId>springcloud-alibaba-provider</artifactId>
    <dependencies>
        <!-- web组件 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <exclusions>
                <!--排除tomcat依赖 -->
                <exclusion>
                    <artifactId>spring-boot-starter-tomcat</artifactId>
                    <groupId>org.springframework.boot</groupId>
                </exclusion>
            </exclusions>
        </dependency>
        <!--undertow容器 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-undertow</artifactId>
        </dependency>
        <!--nacos服务注册与发现 -->
        <dependency>
            <groupId>com.alibaba.cloud</groupId>
            <artifactId>spring-cloud-starter-alibaba-nacos-discovery</artifactId>
        </dependency>
        <!-- lombok插件 -->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
</project>

application.yml

server:
  port: 8715
spring:
  cloud:
    nacos:
      discovery:
        #服务注册与发现地址
        server-addr: 127.0.0.1:8848
        #开启服务注册与发现功能
        enabled: true
  application:
    name: nacos-product

商品服务接口ProviderController

@Slf4j
@RestController
public class ProductController {
    @Value("${server.port}")
    private String port;
    @GetMapping(value = "/getProductInfo/{productId}")
    public String getProductInfo(@PathVariable("productId") String productId) {
        log.info("请求进来啦");
        return "Hello Nacos Discovery " + productId;
    }
}

启动类ProductProviderApp

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
/**
 * @BelongsProject: springcloud-alibaba-nacos
 * @BelongsPackage: com.bruce
 * @CreateTime: 2021-02-18 13:08
 * @Description: TODO
 */
@SpringBootApplication
public class AppProvider {
    public static void main(String[] args) {
        SpringApplication.run(AppProvider.class);
    }
}

4.订单(Order)工程
pom.xml

<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
    <parent>
        <artifactId>springcloud-alibaba-nacos</artifactId>
        <groupId>com.bruce.springcloud-alibaba-nacos</groupId>
        <version>1.0-SNAPSHOT</version>
    </parent>
    <modelVersion>4.0.0</modelVersion>
    <artifactId>springcloud-alibaba-consumer</artifactId>
    <dependencies>
        <!-- Alibaba-nacos服务发现-->
        <dependency>
            <groupId>com.alibaba.cloud</groupId>
            <artifactId>spring-cloud-starter-alibaba-nacos-discovery</artifactId>
            <exclusions>
                <exclusion>
                    <artifactId>nacos-client</artifactId>
                    <groupId>com.alibaba.nacos</groupId>
                </exclusion>
            </exclusions>
        </dependency>
        <!-- nacos-client -->
        <dependency>
            <groupId>com.alibaba.nacos</groupId>
            <artifactId>nacos-client</artifactId>
            <version>1.2.0</version>
        </dependency>
        <!-- hystrix断路器 -->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-netflix-hystrix</artifactId>
        </dependency>
        <!-- openfeign客户端 ,默认集成并开启了ribbon负载均衡-->
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-openfeign</artifactId>
        </dependency>
        <!-- web组件 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
            <exclusions>
                <!--排除tomcat依赖 -->
                <exclusion>
                    <artifactId>spring-boot-starter-tomcat</artifactId>
                    <groupId>org.springframework.boot</groupId>
                </exclusion>
            </exclusions>
        </dependency>
        <!--undertow容器 -->
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-undertow</artifactId>
        </dependency>
        <!-- lombok插件 -->
        <dependency>
            <groupId>org.projectlombok</groupId>
            <artifactId>lombok</artifactId>
        </dependency>
    </dependencies>
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
            <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-compiler-plugin</artifactId>
                <configuration>
                    <source>6</source>
                    <target>6</target>
                </configuration>
            </plugin>
        </plugins>
    </build>
</project>

application.yml

server:
  port: 8710
spring:
  cloud:
    nacos:
      discovery:
        server-addr: 127.0.0.1:8848
        enabled: true
  application:
    name: nacos-order

定义Feign客户端OrderFeign

OrderFeign 中引入了Hystrix熔断器组件起到服务的保护降级作用，@FeignClient注解当中使用断路器支持的有两种方式fallback 和fallbackFactory

关于Feign Hystrix Fallbacks：

第一种：Hystrix支持回退的概念：当它们的电路断开或出现错误时，将执行默认代码路径。要为给定@FeignClient集启用后备，该fallback属性应为实现后备的类名称。您还需要将实现声明为Spring bean。

import com.thinkingcao.order.feign.callback.OrderFeignFallbackFactory;
import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
@FeignClient(name = "nacos-product",fallback = HystrixClientFallback.class)
public interface OrderFeign {
    @GetMapping(value = "/getProductInfo/{productId}")
    public String getProductInfo(@PathVariable("productId") String productId);
    
    @Component
    @Slf4j
    static class HystrixClientFallback implements OrderFeign {
        @Override
        public String getProductInfo(String productId) {
            return "fallback; reason was:  服务忙，稍后重试" ;
        }
    }
}

第二种：如果需要访问引起后备触发器的原因，则可以使用fallbackFactory inside的属性@FeignClient。说白了就是如果你想知道服务调用为什么失败了，可以使用下面这种断路器方式。

import com.thinkingcao.order.feign.callback.OrderFeignFallbackFactory;
import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
@FeignClient(name = "nacos-product",fallbackFactory = OrderFeignFallbackFactory.class)
public interface OrderFeign {
    @GetMapping(value = "/getProductInfo/{productId}")
    public String getProductInfo(@PathVariable("productId") String productId);
    @Component
    @Slf4j
    static class OrderFeignFallbackFactory implements FallbackFactory<OrderFeign> {
      @Override
      public OrderFeign create(Throwable throwable) {
         log.error("调用异常："+ throwable.toString());
         return new OrderFeign() {
            @Override
            public String getProductInfo(String string) {
                return "开启断路-fallback; reason was: "+ throwable;
            }
        };
      }
}

服务消费OrderController

package com.bruce.controller;

import com.bruce.client.OrderFeign;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;
/**
 * @BelongsProject: springcloud-alibaba-nacos
 * @BelongsPackage: com.bruce.controller
 * @CreateTime: 2021-02-18 15:19
 * @Description: TODO
 */
@RestController
@Slf4j
public class OrderController {
    @Autowired
    private OrderFeign orderFeign;
    @GetMapping("/getProduct/{productId}")
    public String getProduct(@PathVariable("productId") String id){
        String response = orderFeign.getProductInfo(id);
        log.info("调用服务结束： "+ response);
        return response;
    }
}

启动类OrderConsumerApp

1.@SpringCloudApplication：组合注解：

@SpringBootApplication(启动SPringBoot程序)、
@EnableDiscoveryClient(开启服务注册与发现)、
@EnableCircuitBreaker(开启断路器)

2.@EnableFeignClients：开启Feign客户端

package com.bruce;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.SpringCloudApplication;
import org.springframework.cloud.openfeign.EnableFeignClients;
/**
 * @BelongsProject: springcloud-alibaba-nacos
 * @BelongsPackage: com.bruce
 * @CreateTime: 2021-02-18 13:16
 * @Description: TODO
 */
//@SpringBootApplication
@SpringCloudApplication
@EnableFeignClients
public class AppConsumer {
    public static void main(String[] args) {
        SpringApplication.run(AppConsumer.class);
    }
}

5.测试
地址：http://127.0.0.1:8710/getProduct/10

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