Ribbon源码分析之@LoadBalanced与LoadBalancerClient

Ribbon源码分析之@LoadBalanced与LoadBalancerClient

@LoadBalanced注解

@LoadBalanced注解用来给RestTemplate做标记，方便使用负载均衡的客户端LoadBalancerClient来配置它。

LoadBalancerClient：

public interface LoadBalancerClient {
   /**
    * Choose a ServiceInstance from the LoadBalancer for the specified service
    * @param serviceId the service id to look up the LoadBalancer
    * @return a ServiceInstance that matches the serviceId
    */
   ServiceInstance choose(String serviceId);

   /**
    * 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;

   /**
    * 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);

}

接口中，我们通过定义抽象方法来了解客户端负载均衡器中应具备的能力

• ServiceInstance choose(String serviceId); 根据传入的服务名serviceId从负载均衡器中挑选一个对应服务的实例

• T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException; 从负载均衡器中挑选出的服务实例来执行请求内容

• URI reconstructURI(ServiceInstance instance, URI original); 为系统构建一个合适的host:port 形式的URI

  在分布式系统中，我们使用逻辑上的服务名称作为host来构建URI进行请求。在操作定义中，ServiceInstance对象带有host和port具体服务实例，后者URI对象使用逻辑服务名定义为host的URI，返回的URI内容是通过ServiceInstance 的服务实例详情拼接出来的host：port形式的请求地址。

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LoadBalancerClient

LoadBalancerAutoConfiguration实现客户端负载均衡器的自动化配置类。

LoadBalancerAutoConfiguration：

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

import org.springframework.beans.factory.SmartInitializingSingleton;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.client.ClientHttpRequestInterceptor;
import org.springframework.web.client.RestTemplate;

/**
 * Auto configuration for Ribbon (client side load balancing).
 *
 * @author Spencer Gibb
 * @author Dave Syer
 */
@Configuration
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
public class LoadBalancerAutoConfiguration {

	@LoadBalanced
	@Autowired(required = false)
	private List<RestTemplate> restTemplates = Collections.emptyList();

	@Bean
	public SmartInitializingSingleton loadBalancedRestTemplateInitializer(
			final List<RestTemplateCustomizer> customizers) {
		return new SmartInitializingSingleton() {
			@Override
			public void afterSingletonsInstantiated() {
				for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
					for (RestTemplateCustomizer customizer : customizers) {
						customizer.customize(restTemplate);
					}
				}
			}
		};
	}

	@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);
			}
		};
	}

	@Bean
	public LoadBalancerInterceptor ribbonInterceptor(
			LoadBalancerClient loadBalancerClient) {
		return new LoadBalancerInterceptor(loadBalancerClient);
	}

}

LoadBalancerAutoConfiguration类头的注解表示Ribbon实现的负载均衡自动化配置需要满足两个条件

• @ConditionalOnClass(RestTemplate.class)：RestTemplate必须在当前工程的环境中
• @ConditionalOnBean(LoadBalancerClient.class)：在Spring的Bean工厂中必须有LoadBalancerClient的实现Bean

自动化配置类中主要做三件事：

• 创建一个LoadBalancerInterceptor的Bean，用于实现对客户端发起请求时进行拦截，实现客户端负载均衡
• 创建RestTemplateCustomizer的Bean，用于RestTemplate增加LoadBalancerInterceptor拦截器
• 维护被LoadBalanced注解修饰的RestTemplate对象列表，并在这里进行初始化，通过RestTemplateCustomizer的实例给客户端负载均衡的RestTemplate增加LoadBalancerInterceptor拦截器

总结

这就是LoadBalancerAutoConfiguration的源码分析，它可以实现客户端负载均衡器的自动化配置类。这下一篇文章中我们要分析拦截器LoadBalancerInterceptor做了什么功能，我们下一篇文章不见不散咯。让我们一起深入Ribbon源码，共同学习