一起來學Spring Cloud（F版） | 第二篇：Ribbon軟負載

<dependencies>
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-web</artifactId>
</dependency>
<dependency>
<groupId>org.springframework.cloud</groupId>
<artifactId>spring-cloud-starter-netflix-eureka-client</artifactId>
</dependency>
</dependencies>

server:
port: 7072
spring:
application:
name: order-server
eureka:
instance:
prefer-ip-address: true
instance-id: ${spring.cloud.client.ip-address}:${spring.application.instance_id:${server.port}}
client:
service-url:
defaultZone: http://localhost:7071/eureka/

package com.battcn;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.cloud.client.discovery.EnableDiscoveryClient;
import org.springframework.cloud.client.loadbalancer.LoadBalanced;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.client.RestTemplate;

/**
 * @author Levin
 */
@EnableDiscoveryClient
@SpringBootApplication
public class OrderApplication {

@Configuration
class MyConfiguration {
@LoadBalanced
@Bean
RestTemplate restTemplate() {
return new RestTemplate();
}
}

public static void main(String[] args) {
SpringApplication.run(OrderApplication.class, args);
}
}

package com.battcn.controller;

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.cloud.client.ServiceInstance;
import org.springframework.cloud.client.discovery.DiscoveryClient;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;

import java.util.List;

/**
 * @author Levin
 * @since 2018/9/28 0028
 */
@RestController
@RequestMapping("/orders")
public class OrderController {

private final RestTemplate restTemplate;
private final DiscoveryClient discoveryClient;

@Autowired
public OrderController(RestTemplate restTemplate, DiscoveryClient discoveryClient) {
this.restTemplate = restTemplate;
this.discoveryClient = discoveryClient;
}

@GetMapping
public String query() {
final List<String> services = discoveryClient.getServices();
for (String service : services) {
List<ServiceInstance> list = discoveryClient.getInstances(service);
for (ServiceInstance instance : list) {
System.out.println(instance.getUri() + "/" + service + " - " + instance.getServiceId());
}
}
return restTemplate.getForObject("http://PRODUCT-SERVER/products", String.class);
}
}

用通俗的概念來說，它就是編碼與解碼操作，還記得 Eureka Server UI 中 Application 嗎？ 
編碼：根據 spring.application.name 設定 serviceId 
Server ID：PRODUCT-SERVER
Client A ：http://localhost:7073/products
Client B ：http://localhost:7074/products

解碼：通過 serviceId 找到對應的客戶端，
然後根據客戶端配置的負載演算法從對應集合中
找出符合當前演算法條件的結果，最後拼接出相應的 http 地址即可

解碼前：http://PRODUCT-SERVER/products 
那麼將 http://PRODUCT-SERVER 替換成 instance.getUri() 內容是不是就出來了
http://localhost:7073/products 和 http://localhost:7074/products

package com.battcn.config;

import com.netflix.loadbalancer.IRule;
import com.netflix.loadbalancer.RandomRule;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

/**
 * @author Levin
 * @since 2018/9/28 0028
 */
@Configuration
public class RibbonRuleConfiguration {

@Bean
public IRule ribbonRule() {
return new RandomRule();
}
}

@RibbonClient(name = "ribbonRule",configuration = RibbonRuleConfiguration.class)
public class OrderController {

}