spring负载均衡自动配置类LoadBalancerAutoConfiguration

/*
 * Copyright 2013-2015 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
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
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 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
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package org.springframework.cloud.client.loadbalancer;

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
 */
//表示这是一个配置类
@Configuration
//必须存在RestTemplate类
@ConditionalOnClass(RestTemplate.class)
//必须存在LoadBalancerClient类型的bean
@ConditionalOnBean(LoadBalancerClient.class)
public class LoadBalancerAutoConfiguration {

    //所有被@LoadBalanced注解修饰的RestTemplate
	@LoadBalanced
	@Autowired(required = false)
	private List<RestTemplate> restTemplates = Collections.emptyList();

    //对于所有被@LoadBalanced注解修饰的RestTemplate，调用SmartInitializingSingleton的
    //customize方法
	@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);
					}
				}
			}
		};
	}
    
    //对于所有被@LoadBalanced注解修饰的RestTemplate，增加loadBalancerInterceptor属性
	@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);
			}
		};
	}

   //产生一个LoadBalancerInterceptor类型的bean，包含loadBalancerClient类型的bean
	@Bean
	public LoadBalancerInterceptor ribbonInterceptor(
			LoadBalancerClient loadBalancerClient) {
		return new LoadBalancerInterceptor(loadBalancerClient);
	}

}

LoadBalancerAutoConfiguration配置类的作用是将所有被@LoadBalanced注解修饰的RestTemplate bean增LoadBalancerInterceptor拦截器bean，而LoadBalancerInterceptor又包含loadBalancerClient，这样当用RestTemplate调用时，会首先调用拦截器方法，在拦截器方法里使用loadBalancerClient真正实现负载均衡以及url转换，达到服务名到真正的host之间的转换和负载均衡；