（三）spring-cloud入門學習：服務消費Feign

Feign是一個聲明式的僞Http客戶端，它使得寫Http客戶端變得更簡單。使用Feign，只需要創建一個接口並註解。它具有可插拔的註解特性，可使用Feign 註解和JAX-RS註解。Feign支持可插拔的編碼器和解碼器。Feign默認集成了Ribbon，並和Eureka結合，默認實現了負載均衡的效果。

1. 創建一個名爲eureka_feign的工程，pom.xml

<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.kevin</groupId>
  <artifactId>eureka_ribbon</artifactId>
  <version>0.0.1-SNAPSHOT</version>
  <packaging>jar</packaging>

  <name>eureka_ribbon</name>
  <url>http://maven.apache.org</url>

  <properties>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
  </properties>

  <parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>2.1.4.RELEASE</version>
  </parent>

  <dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-web</artifactId>
    </dependency>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
    <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>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-actuator</artifactId>
    </dependency>
  </dependencies>
  
  <dependencyManagement>
    <dependencies>
        <dependency>
           <groupId>org.springframework.cloud</groupId>
           <artifactId>spring-cloud-dependencies</artifactId>
           <version>Greenwich.SR1</version>
           <type>pom</type>
           <scope>import</scope>
        </dependency>
    </dependencies>
  </dependencyManagement>
</project>

2. application.properties

spring.application.name=eurekaFeign
server.port=9202

eureka.client.serviceUrl.defaultZone=http://localhost:9101/eureka/
eureka.client.healthcheck.enabled=true

3. Application.java，啓用Feign

package com.kevin.eureka_feign;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.openfeign.EnableFeignClients;

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

4. 使用feign申明service接口，HelloService.java

package com.kevin.eureka_feign.service;

import org.springframework.cloud.openfeign.FeignClient;
import org.springframework.web.bind.annotation.RequestMapping;

@FeignClient("eurekaClient")
public interface HelloService {

	@RequestMapping("/sayHello")
	String sayHello();

}

5. 對外開放接口

package com.kevin.eureka_feign.controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;

import com.kevin.eureka_feign.service.HelloService;

@RestController
public class HelloController {
	
	@Autowired
	private HelloService helloService;
	
	@RequestMapping("sayHello")
	public String sayHello() {
		return helloService.sayHello();
	}
	
}

6. 啓動測試http://127.0.0.1:9202/sayHello