（二）spring-cloud入门学习：服务消费rest+ribbon

前面讲了如何将服务注册到Eureka服务端，为了保证服务的高可用，同一个服务都会采用集群形式注册多个节点，那如何去消费这些服务？通过IP+端口的形式明显不合理，那就需要通过服务应用名来进行访问，同时需要能够将请求均衡转发到各节点。而Ribbon就很好地解决了这个问题，Ribbon是一个负载均衡客户端，可以很好的控制htt和tcp的一些行为。

1. 创建一个服务消费者工程eureka_ribbon， pom.xml文件如下：

<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-netflix-ribbon</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=eurekaRibbon
server.port=9201

#注册到eureka
eureka.client.serviceUrl.defaultZone=http://localhost:9101/eureka/

3. Application.java
使用@EnableDiscoveryClient开启服务发现，同时需要申明RestTemplate。

package com.kevin.eureka_ribbon;

import org.springframework.boot.WebApplicationType;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.client.loadbalancer.LoadBalanced;
import org.springframework.context.annotation.Bean;
import org.springframework.web.client.RestTemplate;

@SpringBootApplication
@EnableDiscoveryClient
public class Application {
	
    public static void main( String[] args ) {
    	new SpringApplicationBuilder(Application.class)
    		.web(WebApplicationType.SERVLET).run(args);
    }
    
    @Bean
    @LoadBalanced
    RestTemplate restTemplate() {
        return new RestTemplate();
    }
}

4. 创建一个service，HelloService.java
使用RestTemplate进行服务调用

package com.kevin.eureka_ribbon.service;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
import org.springframework.web.client.RestTemplate;

@Service
public class HelloService {
	
	@Autowired
	private RestTemplate restTemplate;

	public String sayHello() {
		return restTemplate.getForObject("http://eurekaClient/sayHello", String.class);
	}
}

备注：通过服务注册的name来访问api，该name不能包含下划线，否则报错Request URI does not contain a valid hostname.

5. 创建一个api对外访问，HelloController.java

package com.kevin.eureka_ribbon.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_ribbon.service.HelloService;

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

6. 启动项目并访问http://127.0.0.1:9201/sayHello

能够正常访问结果，同时查看两个服务client的后台，请求7次，eureka_client9102收到4次请求，eureka_client9103收到3次请求，说明ribbon会将请求负载均衡到各服务端。