Feign

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。

简而言之：

Feign 采用的是基于接口的注解
        Feign 整合了ribbon

二、准备工作

继续用上一节的工程， 启动eureka-server，端口为9090; 启动service-hi 两次，端口分别为9091 、9092.
三、创建一个feign的服务

新建一个spring-boot工程，取名为serice-feign，在它的pom文件引入Feign的起步依赖spring-cloud-starter-feign、Eureka的起步依赖spring-cloud-starter-eureka、Web的起步依赖spring-boot-starter-web，代码如下：

<?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.jeffrey</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>1.5.12.RELEASE</version>
    	<relativePath/> 
    </parent>
    
    <properties>
    	<project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    	<project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding>
    	<java.version>1.8</java.version>
    	<spring-cloud.version>Edgware.SR3</spring-cloud.version>
    </properties>
    
    <dependencies>
    	<dependency>
    		<groupId>org.springframework.cloud</groupId>
    		<artifactId>spring-cloud-starter-eureka</artifactId>
    
    	</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.boot</groupId>
    		<artifactId>spring-boot-starter-test</artifactId>
    		<scope>test</scope>
    	</dependency>
    </dependencies>
    
    <dependencyManagement>
    	<dependencies>
    		<dependency>
    			<groupId>org.springframework.cloud</groupId>
    			<artifactId>spring-cloud-dependencies</artifactId>
    			<version>${spring-cloud.version}</version>
    			<type>pom</type>
    			<scope>import</scope>
    		</dependency>
    	</dependencies>
    </dependencyManagement>
    
    <build>
    	<plugins>
    		<plugin>
    			<groupId>org.springframework.boot</groupId>
    			<artifactId>spring-boot-maven-plugin</artifactId>
    		</plugin>
    	</plugins>
    </build>
    </project>

server:
  port: 10002
spring:
  application:
    name: eureka-consumer
eureka:
  client:
    service-url:
      defaultZone: http://localhost:8761/eureka
#设置feign熔断特性
feign:
  hystrix:
    enabled: true

在程序的启动类ServiceFeignApplication ，加上@EnableFeignClients注解开启Feign的功能：

@SpringBootApplication
    @EnableDiscoveryClient
    @EnableFeignClients
    public class ServiceFeignApplication {
    
        public static void main(String[] args) {
         SpringApplication.run(ServiceFeignApplication.class, args);
       }
    }

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service-provider服务的“/hello”接口，代码如下：

/**
     * Created by Jeffrey on 2018/4/21 0021.
     */
    @FeignClient(value="service-provider")  //通过@FeignClient(value="服务名") 定义该接口调用哪项服务
    public interface SchedualServiceProvider {
    /**
     * 调用service-provider服务的/hello接口，并传name参数
     * @param name  接口调用必须的参数
     * @return
     */
    @RequestMapping(value="/hello",method = RequestMethod.GET)
    public String sayHello(@RequestParam(value="name") String name);
    
    }

在Web层的controller层，对外暴露一个”/hello”的API接口，通过上面定义的Feign客户端SchedualServiceProvider 来消费服务。代码如下：

/**
     * Created by Jeffrey on 2018/4/21 0021.
     */
    @RestController
    public class HelloController {
        @Autowired
        private SchedualServiceProvider schedualServiceProvider;
        @RequestMapping(value="/hello",method= RequestMethod.GET)
        public String sayHelli(@RequestParam String name){
    
         return schedualServiceProvider.sayHello(name);
    
        }
    }

启动程序，多次访问http://localhost:9094/hello?name=jeffrey,浏览器交替显示：