为什么@LoadBalanced可以让RestTemplate具有负载均衡的能力

核心类LoadBalancerAutoConfiguration

LoadBalancerAutoConfiguration主要是给容器中所有的加了@LoadBalanced注解的RestTemplate注入一个
LoadBalancerInterceptor，RestTemplate的负载均衡功能就是LoadBalancerInterceptor实现的，

/*
 * Copyright 2012-2019 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.cloud.client.loadbalancer;

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

import org.springframework.beans.factory.ObjectProvider;
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.boot.autoconfigure.condition.ConditionalOnMissingClass;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.client.ClientHttpRequestInterceptor;
import org.springframework.retry.support.RetryTemplate;
import org.springframework.web.client.RestTemplate;

/**
 * Auto-configuration for Ribbon (client-side load balancing).
 *
 * @author Spencer Gibb
 * @author Dave Syer
 * @author Will Tran
 * @author Gang Li
 */
 //proxyBeanMethods = false告诉Spring不代理当前配置类，如果配置类里的Bean之间没有依赖，可以设置，能提高性能
@Configuration(proxyBeanMethods = false)
//当容器中有RestTemplate和LoadBalancerClient的时候此配置类才会生效
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {

	//将容器中所有标注了@LoadBalanced注解的RestTemplate注入到restTemplates里
	@LoadBalanced
	@Autowired(required = false)
	private List<RestTemplate> restTemplates = Collections.emptyList();
	
	//获取容器中所有的LoadBalancerRequestTransformer
	@Autowired(required = false)
	private List<LoadBalancerRequestTransformer> transformers = Collections.emptyList();

	//向容器中注入一个匿名的SmartInitializingSingleton对象，这个对象的afterSingletonsInstantiated方法会在Spring初始化的倒数第二步调用(AbstractApplicationContext.refresh(){finishBeanFactoryInitialization(beanFactory)})，详情见DefaultListableBeanFactory.preInstantiateSingletons
	@Bean
	public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(
			final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
		//返回一个实现了SmartInitializingSingleton.afterSingletonsInstantiated方法的对象
		return () -> restTemplateCustomizers.ifAvailable(customizers -> {
			for (RestTemplate restTemplate : LoadBalancerAutoConfiguration.this.restTemplates) {
				for (RestTemplateCustomizer customizer : customizers) {
					customizer.customize(restTemplate);
				}
			}
		});
	}

	@Bean
	@ConditionalOnMissingBean
	public LoadBalancerRequestFactory loadBalancerRequestFactory(
			LoadBalancerClient loadBalancerClient) {
		return new LoadBalancerRequestFactory(loadBalancerClient, this.transformers);
	}

	//这个静态内部类主要是为初始化LoadBalancerAutoConfiguration做一些准备工作
	@Configuration(proxyBeanMethods = false)
	@ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
	static class LoadBalancerInterceptorConfig {

		//往容器中注入一个LoadBalancerInterceptor，LoadBalancerClient只有一个实现RibbonLoadBalancerClient
		@Bean
		public LoadBalancerInterceptor ribbonInterceptor(
				LoadBalancerClient loadBalancerClient,
				LoadBalancerRequestFactory requestFactory) {
			return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
		}

		//当容器中没有RestTemplateCustomizer的时候，会注入一个
		@Bean
		@ConditionalOnMissingBean
		public RestTemplateCustomizer restTemplateCustomizer(
				final LoadBalancerInterceptor loadBalancerInterceptor) {
			//这一段是返回一个实现了RestTemplateCustomizer.customize方法的对象	
			return restTemplate -> {
				List<ClientHttpRequestInterceptor> list = new ArrayList<>(
						restTemplate.getInterceptors());
				list.add(loadBalancerInterceptor);
				restTemplate.setInterceptors(list);
			};
		}

	}

	/**
	 * Auto configuration for retry mechanism.
	 */
	@Configuration(proxyBeanMethods = false)
	@ConditionalOnClass(RetryTemplate.class)
	public static class RetryAutoConfiguration {

		@Bean
		@ConditionalOnMissingBean
		public LoadBalancedRetryFactory loadBalancedRetryFactory() {
			return new LoadBalancedRetryFactory() {
			};
		}

	}

	/**
	 * Auto configuration for retry intercepting mechanism.
	 */
	@Configuration(proxyBeanMethods = false)
	@ConditionalOnClass(RetryTemplate.class)
	public static class RetryInterceptorAutoConfiguration {

		@Bean
		@ConditionalOnMissingBean
		public RetryLoadBalancerInterceptor ribbonInterceptor(
				LoadBalancerClient loadBalancerClient,
				LoadBalancerRetryProperties properties,
				LoadBalancerRequestFactory requestFactory,
				LoadBalancedRetryFactory loadBalancedRetryFactory) {
			return new RetryLoadBalancerInterceptor(loadBalancerClient, properties,
					requestFactory, loadBalancedRetryFactory);
		}

		@Bean
		@ConditionalOnMissingBean
		public RestTemplateCustomizer restTemplateCustomizer(
				final RetryLoadBalancerInterceptor loadBalancerInterceptor) {
			return restTemplate -> {
				List<ClientHttpRequestInterceptor> list = new ArrayList<>(
						restTemplate.getInterceptors());
				list.add(loadBalancerInterceptor);
				restTemplate.setInterceptors(list);
			};
		}

	}

}

对LoadBalancerInterceptor的理解

LoadBalancerInterceptor持有的RibbonLoadBalancerClient是客户端负载均衡的入口，提供了以下几个功能

1：根据spring.application.name获取注册中心里可用的服务器列表，getLoadBalancer()
2：根据IRule里的算法，选择一个服务器，或随机，或轮询，由容器中IRule的实现类是哪一个决定，默认轮询，getServer()
3：将http://order-application/order/createOrder替换成http://ip:port/order/createOrder，reconstructURI
4：发起请求，execute()