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本例是在springboot整合H2內存數據庫，實現單元測試與數據庫無關性和使用RestTemplate消費spring boot的Restful服務兩個示例的基礎上改造而來
在使用RestTemplate來消費spring boot的Restful服務示例中，我們提到，調用spring boot服務的時候，需要將服務的URL寫死或者是寫在配置文件中，但這兩種方式，無論哪一種，一旦ip地址發生了變化，都需要改動程序，并重新部署服務，使用Ribbon的時候，可以有效的避免這個問題。
前言：
軟負載均衡的實現方式有兩種，分別是服務端的負載均衡和客戶端的負載均衡
服務端負載均衡：當瀏覽器向后臺發出請求的時候，會首先向反向代理服務器發送請求，反向代理服務器會根據客戶端部署的ip：port映射表以及負載均衡策略，來決定向哪臺服務器發送請求，一般會使用到nginx反向代理技術。
客戶端負載均衡：當瀏覽器向后臺發出請求的時候，客戶端會向服務注冊器(例如：Eureka Server)，拉取注冊到服務器的可用服務信息，然后根據負載均衡策略，直接命中哪臺服務器發送請求。這整個過程都是在客戶端完成的，并不需要反向代理服務器的參與。
一、啟動Eureka Server
請參考該例：spring cloud中啟動Eureka Server
二、啟動微服務，并注冊到Eureka Server上
spring cloud-將spring boot服務注冊到Eureka Server上
為了演示負載均衡的效果，再啟動一個為服務，注意需要將端口號改成不一致
三、添加Ribbon支持
1、添加Ribbon的依賴
2、添加負載均衡支持
package com.chhliu.springboot.restful;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.cloud.client.loadbalancer.LoadBalanced;
import org.springframework.cloud.netflix.eureka.EnableEurekaClient;
import org.springframework.context.annotation.Bean;
import org.springframework.web.client.RestTemplate;
@SpringBootApplication
@EnableEurekaClient
public class SpringbootRestTemplateApplication {
@Autowired
private RestTemplateBuilder builder;
@Bean
@LoadBalanced // 添加負載均衡支持，很簡單，只需要在RestTemplate上添加@LoadBalanced注解，那么RestTemplate即具有負載均衡的功能,如果不加@LoadBalanced注解的話，會報java.net.UnknownHostException:springboot-h3異常，此時無法通過注冊到Eureka Server上的服務名來調用服務，因為RestTemplate是無法從服務名映射到ip:port的，映射的功能是由LoadBalancerClient來實現的。
public RestTemplate restTemplate() {
return builder.build();
}
public static void main(String[] args) {
SpringApplication.run(SpringbootRestTemplateApplication.class, args);
}
}
3、修改調用微服務的URL
package com.chhliu.springboot.restful.controller;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;
import com.chhliu.springboot.restful.vo.User;
@RestController
public class RestTemplateController {
@Autowired
private RestTemplate restTemplate;
@GetMapping("/template/{id}")
public User findById(@PathVariable Long id) {// 將原來的ip:port的形式，改成注冊到Eureka Server上的應用名即可
User u = this.restTemplate.getForObject("http://springboot-h3/user/" + id, User.class);
System.out.println(u);
return u;
}
}
四、查看Eureka Server狀態
五，在瀏覽器中，多次刷新http://localhost:7904/template/2地址
六、測試結果
7900端口服務：
Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=? Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=? Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=? Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=?
7901端口服務：
Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=? Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=? Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=? Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=? Hibernate: select user0_.id as id1_0_0_, user0_.age as age2_0_0_, user0_.balance as balance3_0_0_, user0_.name as name4_0_0_, user0_.username as username5_0_0_ from user user0_ where user0_.id=?
7904端口服務：
User [id=2, username=user2, name=李四, age=20, balance=100.00] 2017-01-23 09:58:05.682 INFO 7436 --- [erListUpdater-0] c.netflix.config.ChainedDynamicProperty : Flipping property: springboot-h3.ribbon.ActiveConnectionsLimit to use NEXT property: niws.loadbalancer.availabilityFilteringRule.activeConnectionsLimit = 2147483647 User [id=2, username=user2, name=李四, age=20, balance=100.00] User [id=2, username=user2, name=李四, age=20, balance=100.00] User [id=2, username=user2, name=李四, age=20, balance=100.00] User [id=2, username=user2, name=李四, age=20, balance=100.00] User [id=2, username=user2, name=李四, age=20, balance=100.00] User [id=2, username=user2, name=李四, age=20, balance=100.00] User [id=2, username=user2, name=李四, age=20, balance=100.00] User [id=2, username=user2, name=李四, age=20, balance=100.00]
從上面的測試結果可以看出，總共調了7904端口服務9次，其中7904端口服務調7900端口服務4次，調7901端口5次，剛好是9次
經過上面的幾個步驟，就基本使用Ribbon實現了客戶端負載均衡的功能
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