爲什麼@LoadBalanced可以讓RestTemplate具有負載均衡的能力

核心類LoadBalancerAutoConfiguration

LoadBalancerAutoConfiguration主要是給容器中所有的加了@LoadBalanced註解的RestTemplate注入一個
LoadBalancerInterceptor，RestTemplate的負載均衡功能就是LoadBalancerInterceptor實現的，

/*
 * 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告訴Spring不代理當前配置類，如果配置類裏的Bean之間沒有依賴，可以設置，能提高性能
@Configuration(proxyBeanMethods = false)
//當容器中有RestTemplate和LoadBalancerClient的時候此配置類纔會生效
@ConditionalOnClass(RestTemplate.class)
@ConditionalOnBean(LoadBalancerClient.class)
@EnableConfigurationProperties(LoadBalancerRetryProperties.class)
public class LoadBalancerAutoConfiguration {

	//將容器中所有標註了@LoadBalanced註解的RestTemplate注入到restTemplates裏
	@LoadBalanced
	@Autowired(required = false)
	private List<RestTemplate> restTemplates = Collections.emptyList();
	
	//獲取容器中所有的LoadBalancerRequestTransformer
	@Autowired(required = false)
	private List<LoadBalancerRequestTransformer> transformers = Collections.emptyList();

	//向容器中注入一個匿名的SmartInitializingSingleton對象，這個對象的afterSingletonsInstantiated方法會在Spring初始化的倒數第二步調用(AbstractApplicationContext.refresh(){finishBeanFactoryInitialization(beanFactory)})，詳情見DefaultListableBeanFactory.preInstantiateSingletons
	@Bean
	public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(
			final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
		//返回一個實現了SmartInitializingSingleton.afterSingletonsInstantiated方法的對象
		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);
	}

	//這個靜態內部類主要是爲初始化LoadBalancerAutoConfiguration做一些準備工作
	@Configuration(proxyBeanMethods = false)
	@ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
	static class LoadBalancerInterceptorConfig {

		//往容器中注入一個LoadBalancerInterceptor，LoadBalancerClient只有一個實現RibbonLoadBalancerClient
		@Bean
		public LoadBalancerInterceptor ribbonInterceptor(
				LoadBalancerClient loadBalancerClient,
				LoadBalancerRequestFactory requestFactory) {
			return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
		}

		//當容器中沒有RestTemplateCustomizer的時候，會注入一個
		@Bean
		@ConditionalOnMissingBean
		public RestTemplateCustomizer restTemplateCustomizer(
				final LoadBalancerInterceptor loadBalancerInterceptor) {
			//這一段是返回一個實現了RestTemplateCustomizer.customize方法的對象	
			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);
			};
		}

	}

}

對LoadBalancerInterceptor的理解

LoadBalancerInterceptor持有的RibbonLoadBalancerClient是客戶端負載均衡的入口，提供了以下幾個功能

1：根據spring.application.name獲取註冊中心裏可用的服務器列表，getLoadBalancer()
2：根據IRule裏的算法，選擇一個服務器，或隨機，或輪詢，由容器中IRule的實現類是哪一個決定，默認輪詢，getServer()
3：:將http://order-application/order/createOrder替換成http://ip:port/order/createOrder，reconstructURI
4：發起請求，execute()