Feign受Retrofix、JAXRS-2.0和WebSocket影响，采用了声明式API接口的风格，将Java Http客户端绑定到他的内部。Feign的首要目标是将Java Http客户端调用过程变得简单。
源码地址：https://github.com/OpenFeign/feign
本章案例基于上一章，可参考：
spring boot 2.0.3+spring cloud （Finchley）1、搭建Eureka 以及构建高可用Eureka Server集群
spring boot 2.0.3+spring cloud （Finchley）2、搭建负载均衡Ribbon （Eureka+Ribbon+RestTemplate）
新建module工程eureka-feign-client。pom文件引入相关依赖，包括主maven工程的pom文件，eureka client起步依赖，feign的起步依赖，web的起步依赖（已在主maven工程pom中配置）
<?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">
<modelVersion>4.0.0</modelVersion><span style="color: #0000ff;"><</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>com.cralor<span style="color: #0000ff;"></</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>eureka-feign-client<span style="color: #0000ff;"></</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">version</span><span style="color: #0000ff;">></span>0.0.1-SNAPSHOT<span style="color: #0000ff;"></</span><span style="color: #800000;">version</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">packaging</span><span style="color: #0000ff;">></span>jar<span style="color: #0000ff;"></</span><span style="color: #800000;">packaging</span><span style="color: #0000ff;">></span> <span style="color: #0000ff;"><</span><span style="color: #800000;">name</span><span style="color: #0000ff;">></span>eureka-feign-client<span style="color: #0000ff;"></</span><span style="color: #800000;">name</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">description</span><span style="color: #0000ff;">></span>Demo project for Spring Boot<span style="color: #0000ff;"></</span><span style="color: #800000;">description</span><span style="color: #0000ff;">></span> <span style="color: #0000ff;"><</span><span style="color: #800000;">parent</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>com.cralor<span style="color: #0000ff;"></</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>chap7-feign<span style="color: #0000ff;"></</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">version</span><span style="color: #0000ff;">></span>0.0.1-SNAPSHOT<span style="color: #0000ff;"></</span><span style="color: #800000;">version</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">relativePath</span><span style="color: #0000ff;">/></span> <span style="color: #008000;"><!--</span><span style="color: #008000;"> lookup parent from repository </span><span style="color: #008000;">--></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">parent</span><span style="color: #0000ff;">></span> <span style="color: #0000ff;"><</span><span style="color: #800000;">properties</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">project.build.sourceEncoding</span><span style="color: #0000ff;">></span>UTF-8<span style="color: #0000ff;"></</span><span style="color: #800000;">project.build.sourceEncoding</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">project.reporting.outputEncoding</span><span style="color: #0000ff;">></span>UTF-8<span style="color: #0000ff;"></</span><span style="color: #800000;">project.reporting.outputEncoding</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">java.version</span><span style="color: #0000ff;">></span>1.8<span style="color: #0000ff;"></</span><span style="color: #800000;">java.version</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">properties</span><span style="color: #0000ff;">></span> <span style="color: #0000ff;"><</span><span style="color: #800000;">dependencies</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">dependency</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>org.springframework.cloud<span style="color: #0000ff;"></</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>spring-cloud-starter-netflix-eureka-client<span style="color: #0000ff;"></</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">dependency</span><span style="color: #0000ff;">></span> <span style="color: #0000ff;"><</span><span style="color: #800000;">dependency</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>org.springframework.cloud<span style="color: #0000ff;"></</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>spring-cloud-starter-openfeign<span style="color: #0000ff;"></</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">dependency</span><span style="color: #0000ff;">></span> <span style="color: #0000ff;"><</span><span style="color: #800000;">dependency</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>org.springframework.boot<span style="color: #0000ff;"></</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>spring-boot-starter-test<span style="color: #0000ff;"></</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">scope</span><span style="color: #0000ff;">></span>test<span style="color: #0000ff;"></</span><span style="color: #800000;">scope</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">dependency</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">dependencies</span><span style="color: #0000ff;">></span> <span style="color: #0000ff;"><</span><span style="color: #800000;">build</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">plugins</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">plugin</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>org.springframework.boot<span style="color: #0000ff;"></</span><span style="color: #800000;">groupId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"><</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>spring-boot-maven-plugin<span style="color: #0000ff;"></</span><span style="color: #800000;">artifactId</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">plugin</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">plugins</span><span style="color: #0000ff;">></span>
<span style="color: #0000ff;"></</span><span style="color: #800000;">build</span><span style="color: #0000ff;">></span></project>
配置文件做相关配置
server:
port: 8765
spring:
application:
name: eureka-feign-client
eureka:
client:
serviceUrl:
defaultZone: http://localhost:8761/eureka/
在程序的启动类加上注解@EnableFeignClients开启Feign Client功能
@EnableFeignClients
@SpringBootApplication
public class EurekaFeignClientApplication {</span><span style="color: #0000ff;">public</span> <span style="color: #0000ff;">static</span> <span style="color: #0000ff;">void</span><span style="color: #000000;"> main(String[] args) {
SpringApplication.run(EurekaFeignClientApplication.</span><span style="color: #0000ff;">class</span><span style="color: #000000;">, args);
}}
通过以上步骤，该程序已经具备了Feign的功能，现在来实现一个简单的feign client。新建一个EurekaClientFeign的接口，在接口上加@FeignClient注解来声明一个Feign Client。value为远程调用其他服务的服务名，FeignConfig.class为配置类，在EurekaClientFeign内部有一个sayHiFromClientEureka()的方法，该方法通过Feign来调用eureka-client服务的“/hi”的aipi接口。
@Component
@FeignClient(value = "eureka-client",configuration = FeignConfig.class)
public interface EurekaClientFeign {
@GetMapping(value = "/hi")
String sayHiFromClientEureka(@RequestParam(value = "name")String name);
}
在FeignConfig类上加上@Configuration注解，表明这是一个配置类，并注入一个BeanName为feignRetryer的Retryer的Bean。可使feign在远程调用失败后会进行重试。
@Configuration
public class FeignConfig {
@Bean
public Retryer feignRetryer(){
return new Retryer.Default(100,TimeUnit.SECONDS.toMillis(1),5);
}
}
service层HiService类注入EurekaClient的Bean
@Service
public class HiService {
@Autowired
EurekaClientFeign eurekaClientFeign;</span><span style="color: #0000ff;">public</span><span style="color: #000000;"> String sayHi(String name){
</span><span style="color: #0000ff;">return</span><span style="color: #000000;"> eurekaClientFeign.sayHiFromClientEureka(name);
}}
新建HiController类，调用HiService的sayHi()方法，HiService通过EurekaClientFeign远程调用eureka-client服务的API接口“/hi”。
@RestController
public class HiController {
@Autowired
HiService hiService;@GetMapping(</span>"/hi"<span style="color: #000000;">)
</span><span style="color: #0000ff;">public</span> String sayHi(@RequestParam(defaultValue = "cralor",required = <span style="color: #0000ff;">false</span><span style="color: #000000;">)String name){
</span><span style="color: #0000ff;">return</span><span style="color: #000000;"> hiService.sayHi(name);
}}
启动eureka-server工程，端口号8761，启动两个eureka-client工程的实例，端口号8762、8763，启动eureka-feign-client工程，端口号8765。在浏览器多次访问http://localhost:8765/hi，浏览器会轮流显示
可见Feign Client远程调用了eureka-client的服务（存在端口号为8762和8763的两个实例）的“/hi”API接口，feign client有负载均衡的能力。
总结：feign源码实现过程如下
1.首先通过@EnableFeignClients注解开启FeignClient功能。只有这个注解的存在，程序启动时才会开启对@FeignConfig注解的包的扫描，
2.根据Feig的规则实现接口，并在接口上面加上@FeignClient注解。
3.程序启动后会进行包扫描，扫描所有@FeignClient注解的类，并将这些信息注入IOC容器中。
4.当接口的方法被调用时，通过JDK的代理来生成具体的RequestTemplate模板对象。
5.根据RequestTemplate再生成Http请求的Request对象。
6.Request对象交给Client去处理，其中Client的网络请求框架可以是HttpURLConnection、HttpClient和OkHttp。
7.最后Client被封装到LoadBalanceClient类，这个类结合类Ribbon做到了负载均衡。
参考：https://windmt.com/2018/04/15/spring-cloud-3-service-producer-and-consumer/#Spring-Cloud-Feign
案例代码地址：https://github.com/cralor7/springcloud
原文地址：http://www.cnblogs.com/cralor/p/9228678.html