一、 feign简介
Feign是一个声明式的伪Http客户端，它使得写Http客户端变得更简单。使用Feign，只需要创建一个接口并注解。它具有可插拔的注解特性，可使用Feign注解和JAX-RS注解。Feign支持可插拔的编码器和解码器。Feign默认集成了Ribbon，并和Eureka结合，默认实现了负载均衡的效果。
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简而言之：
Feign 采用的是基于接口的注解
Feign 整合了ribbon

二、创建一个feign的服务
1）新建一个Springboot项目service-feign，它的pom.xml文件如下

<?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>
 
    <groupId>com.example</groupId>
    <artifactId>service-feign</artifactId>
    <version>0.0.1-SNAPSHOT</version>
    <packaging>jar</packaging>
 
    <name>service-feign</name>
    <description>Demo project for Spring Boot</description>
 
    <parent>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-parent</artifactId>
        <version>1.4.0.RELEASE</version>
        <relativePath /> <!-- lookup parent from repository -->
    </parent>
 
    <properties>
        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
        <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
        <java.version>1.8</java.version>
    </properties>
 
    <dependencies>
        
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-test</artifactId>
            <scope>test</scope>
        </dependency>
        
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-web</artifactId>
        </dependency>
        
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-feign</artifactId>
        </dependency>
 
        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-eureka</artifactId>
        </dependency>
    </dependencies>
 
    <build>
        <plugins>
            <plugin>
                <groupId>org.springframework.boot</groupId>
                <artifactId>spring-boot-maven-plugin</artifactId>
            </plugin>
        </plugins>
    </build>
    <repositories>
        <repository>
            <id>spring-milestones</id>
            <name>Spring Milestones</name>
            <url>https://repo.spring.io/milestone</url>
            <snapshots>
                <enabled>false</enabled>
            </snapshots>
        </repository>
    </repositories>
</project>

2）配置application.yml

eureka:
  client:
    serviceUrl:
      defaultZone: http://localhost:8761/eureka/
server:
  port: 8765
spring:
  application:
    name: service-feign

3）feign-service的启动类，在启动类上添加@EnableDiscoveryClient @EnableFeignClients 注解，表示用注解开启feign功能。如果标注了@FeignClient的接口和启动类不在一个包下，应该在@EnableFeignClient()中添加所在包

package com.example.demo;
 
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.netflix.feign.EnableFeignClients;
@EnableFeignClients
@EnableDiscoveryClient
@SpringBootApplication
public class ServiceFeignApplication {
 
    public static void main(String[] args) {
        SpringApplication.run(ServiceFeignApplication.class, args);
    }
}

4）定义一个feign接口，通过@FeignClient（“服务名”），来指定调用哪个服务。比如在代码中调用了hello-service服务的“/hello”接口，代码如下，在启动该项目时，@EnableFeignClients注解就会让Feign在指定包下扫描所有标注了@FeignClient的接口

package com.liantong.service;
 
import org.springframework.cloud.netflix.feign.FeignClient;
import org.springframework.web.bind.annotation.RequestMapping;
@FeignClient("service-helloworld") //向注册中心申请使用service-helloworld客户端
public interface HelloService {
    @RequestMapping("/hello") //表明使用service-helloworld中的“/hello方法”
    String sayHelloFromClient();
}

5）controller层

package com.liantong.controller;
 
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Controller;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.ResponseBody;
 
import com.liantong.service.HelloService;
 
@Controller
public class HelloController {
    @Autowired
    private HelloService hs;
    @RequestMapping("/hello")
    @ResponseBody
    public String hello() {
        return hs.sayHelloFromClient();
    }
}

6）依次启动eureka-server，两个service-helloworld，再启动本项目，完成！
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