前面一篇介绍了LoadBalancerClient来实现负载均衡， 这里介绍Spring cloud ribbon
1、ribbon
Spring Cloud Ribbon 是一个基于Http和TCP的客服端负载均衡工具，它是基于Netflix Ribbon实现的。它不像服务注册中心、配置中心、API网关那样独立部署，但是它几乎存在于每个微服务的基础设施中理解Ribbon对于我们使用Spring Cloud来讲非常的重要，因为负载均衡是对系统的高可用、网络压力的缓解和处理能力扩容的重要手段之一。
它内部提供了一个叫做ILoadBalance的接口代表负载均衡器的操作，比如有添加服务器操作、选择服务器操作、获取所有的服务器列表、获取可用的服务器列表等等。
2、这里我们复制上一章consumer工程，修改pom.xml,增加ribbon依赖
<?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.example</groupId> <artifactId>eurekaRibbon</artifactId> <version>0.0.1-SNAPSHOT</version> <packaging>jar</packaging> <name>eurekaRibbon</name> <description>Demo project for Spring Boot</description> <parent> <groupId>org.springframework.boot</groupId> <artifactId>spring-boot-starter-parent</artifactId> <version>1.4.0.RELEASE</version> <relativePath /> <!-- lookup parent from repository --> </parent> <properties> <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> <project.reporting.outputEncoding>UTF-8</project.reporting.outputEncoding> <java.version>1.8</java.version> </properties> <dependencies> <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-starter-eureka</artifactId> </dependency> <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-starter-ribbon</artifactId> </dependency> <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> </dependencies> <dependencyManagement> <dependencies> <dependency> <groupId>org.springframework.cloud</groupId> <artifactId>spring-cloud-dependencies</artifactId> <version>Dalston.SR3</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>
3、修改主类，为RestTemplate增加@LoadBalanced注解
package com.example.demo;
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.web.client.RestTemplate;
@EnableDiscoveryClient
@SpringBootApplication
public class EurekaRibbonApplication {
@Bean
@LoadBalanced
public RestTemplate restTemplate() {
return new RestTemplate();
}
public static void main(String[] args) {
SpringApplication.run(EurekaRibbonApplication.class, args);
}
}
4、接口
package com.example.demo.controller;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;
@RestController
public class ClientRibbonController {
@Autowired
RestTemplate restTemplate;
@GetMapping("/consumer")
public String all() {
// 发起REST请求
return restTemplate.getForObject("http://eurekaClient/all", String.class);
}
}
这里请求的host位置并没有使用一个具体的IP地址和端口的形式，而是采用了服务名的方式组成。Spring Cloud Ribbon有一个拦截器，它能够在这里进行实际调用的时候，自动的去选取服务实例，并将实际要请求的IP地址和端口替换这里的服务名，从而完成服务接口的调用，进而实现自动化：
然而Ribbon要实现负载均衡自动化配置需要满足如下两个条件：
1、ConditionalOnClass(RestTemplate.class)：RestTemplate类必须存在于当前工程的环境中
2、ConditionalOnBean(LoadBalancerClient.class)：使用@LoadBalanced注解用来给RestTemplate做标记，以使用负载均衡的客户端（LoadBalancerClient）来配置RestTemplate。
在自动化配置中主要做三件事：
- 创建一个LoadBalancerInterceptor的Bean，用于实现对客户端发起请求时进行拦截，以实现客户端负载均衡。
- 创建一个RestTemplateCustomizer的Bean，用于给RestTemplate增加LoadBalancerInterceptor拦截器。
- 维护了一个被@LoadBalanced注解修饰的RestTemplate对象列表，并在这里进行初始化，通过调用RestTemplateCustomizer的实例来给需要客户端负载均衡的RestTemplate增加LoadBalancerInterceptor拦截器。
1、条件二LoadBalancerClient
/** * Represents a client side load balancer * @author Spencer Gibb */ public interface LoadBalancerClient extends ServiceInstanceChooser { /** * execute request using a ServiceInstance from the LoadBalancer for the specified * service * @param serviceId the service id to look up the LoadBalancer * @param request allows implementations to execute pre and post actions such as * incrementing metrics * @return the result of the LoadBalancerRequest callback on the selected * ServiceInstance */ <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException; /** * execute request using a ServiceInstance from the LoadBalancer for the specified * service * @param serviceId the service id to look up the LoadBalancer * @param serviceInstance the service to execute the request to * @param request allows implementations to execute pre and post actions such as * incrementing metrics * @return the result of the LoadBalancerRequest callback on the selected * ServiceInstance */ <T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request) throws IOException; /** * Create a proper URI with a real host and port for systems to utilize. * Some systems use a URI with the logical serivce name as the host, * such as http://myservice/path/to/service. This will replace the * service name with the host:port from the ServiceInstance. * @param instance * @param original a URI with the host as a logical service name * @return a reconstructed URI */ URI reconstructURI(ServiceInstance instance, URI original); }
我们可以知道 choose()方法根据传入的serviceId服务Id，从负载均衡器选择一个一个对应的服务实例。execute()方法根据serviceId服务ID和请求request来执行请求内容。reconstructURI()方法构建出一个合适的Host:Port的URI。而 RibbonLoadBalancerClient就是LoadBalancerClient的具体实现
2、Ribbon自动化配置：LoadBalancerAutoConfiguration
/** * Auto configuration for Ribbon (client side load balancing). * * @author Spencer Gibb * @author Dave Syer * @author Will Tran */ @Configuration @ConditionalOnClass(RestTemplate.class) //条件 : RestTemplate必须在工程的类路径下 @ConditionalOnBean(LoadBalancerClient.class) //条件: Spring 容器中必须包含LoadBalancerClient的实现，即RibbonLoadBalancerClient @EnableConfigurationProperties(LoadBalancerRetryProperties.class) //启动重试功能，可以spring.cloud.loadbalancer.retry=false,取消重试，默认参数为true public class LoadBalancerAutoConfiguration { @LoadBalanced @Autowired(required = false) private List<RestTemplate> restTemplates = Collections.emptyList(); //维护一个RestTemplate列表，通过LoadBalanced来注解。 @Bean public SmartInitializingSingleton loadBalancedRestTemplateInitializer( //加载初始话自定义的restTeplate,实质是初始化InterceptingHttpAccessor具体调用 final List<RestTemplateCustomizer> customizers) { return new SmartInitializingSingleton() { @Override public void afterSingletonsInstantiated() { for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) { for (RestTemplateCustomizer customizer : customizers) { customizer.customize(restTemplate); } } } }; } @Autowired(required = false) private List<LoadBalancerRequestTransformer> transformers = Collections.emptyList(); @Bean @ConditionalOnMissingBean public LoadBalancerRequestFactory loadBalancerRequestFactory( LoadBalancerClient loadBalancerClient) { return new LoadBalancerRequestFactory(loadBalancerClient, transformers); } @Configuration @ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate") static class LoadBalancerInterceptorConfig { @Bean public LoadBalancerInterceptor ribbonInterceptor( LoadBalancerClient loadBalancerClient, LoadBalancerRequestFactory requestFactory) { return new LoadBalancerInterceptor(loadBalancerClient, requestFactory); } @Bean @ConditionalOnMissingBean public RestTemplateCustomizer restTemplateCustomizer( final LoadBalancerInterceptor loadBalancerInterceptor) { return new RestTemplateCustomizer() { @Override public void customize(RestTemplate restTemplate) { List<ClientHttpRequestInterceptor> list = new ArrayList<>( restTemplate.getInterceptors()); list.add(loadBalancerInterceptor); restTemplate.setInterceptors(list); } }; } } @Configuration @ConditionalOnClass(RetryTemplate.class) static class RetryAutoConfiguration { @Bean public RetryTemplate retryTemplate() { RetryTemplate template = new RetryTemplate(); template.setThrowLastExceptionOnExhausted(true); return template; } @Bean @ConditionalOnMissingBean public LoadBalancedRetryPolicyFactory loadBalancedRetryPolicyFactory() { return new LoadBalancedRetryPolicyFactory.NeverRetryFactory(); } @Bean public RetryLoadBalancerInterceptor ribbonInterceptor( LoadBalancerClient loadBalancerClient, LoadBalancerRetryProperties properties, LoadBalancedRetryPolicyFactory lbRetryPolicyFactory, LoadBalancerRequestFactory requestFactory) { return new RetryLoadBalancerInterceptor(loadBalancerClient, retryTemplate(), properties, lbRetryPolicyFactory, requestFactory); } @Bean @ConditionalOnMissingBean public RestTemplateCustomizer restTemplateCustomizer( //自定义RestTemplate ,实质是初始化InterceptingHttpAccessor final RetryLoadBalancerInterceptor loadBalancerInterceptor) { return new RestTemplateCustomizer() { @Override public void customize(RestTemplate restTemplate) { List<ClientHttpRequestInterceptor> list = new ArrayList<>( restTemplate.getInterceptors()); list.add(loadBalancerInterceptor); restTemplate.setInterceptors(list); } }; } } }