背景

從SpringCloud 2020 版本之后，組件移除了除 Eureka 以外，所有 Netflix 的相關(guān)，包括最常用的 Ribbon Hystrix 等，所以 SpringCloud 在 spring-cloud-commons 提供了Loadbalancer 用來替代 Ribbon。本系列就來介紹Loadbalancer 的執(zhí)行流程

項(xiàng)目版本:

• Spring-Boot2.6.13
• Spring-Cloud 2021.0.5
• spring-cloud-alibaba 2021.0.5.0
• Loadbalancer 3.1.5

從項(xiàng)目中最常用的Feign來當(dāng)做入口，不了解Feign的原理也沒有關(guān)系，直接從Feign和Loadbalancer 的集成部分分析即可

分析

入口處

FeignBlockingLoadBalancerClient

public class FeignBlockingLoadBalancerClient implements Client {private static final Log LOG = LogFactory.getLog(FeignBlockingLoadBalancerClient.class);private final Client delegate;private final LoadBalancerClient loadBalancerClient;private final LoadBalancerClientFactory loadBalancerClientFactory;@Overridepublic Response execute(Request request, Request.Options options) throws IOException {//請求路徑final URI originalUri = URI.create(request.url());//獲取到要調(diào)用的服務(wù)idString serviceId = originalUri.getHost();DefaultRequest<RequestDataContext> lbRequest = new DefaultRequest<>(new RequestDataContext(buildRequestData(request), hint));	Set<LoadBalancerLifecycle> supportedLifecycleProcessors = LoadBalancerLifecycleValidator.getSupportedLifecycleProcessors(//在這步創(chuàng)建了每個(gè)服務(wù)的子容器	loadBalancerClientFactory.getInstances(serviceId, LoadBalancerLifecycle.class),RequestDataContext.class, ResponseData.class, ServiceInstance.class);supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onStart(lbRequest));		//執(zhí)行l(wèi)oadBalancer的負(fù)載均衡策略，返回將過濾后的服務(wù)，非常重要ServiceInstance instance = loadBalancerClient.choose(serviceId, lbRequest);org.springframework.cloud.client.loadbalancer.Response<ServiceInstance> lbResponse = new DefaultResponse(instance);//省略...//將ServiceInstance進(jìn)行解析后，轉(zhuǎn)換為真正的http方式進(jìn)行遠(yuǎn)程調(diào)用服務(wù)String reconstructedUrl = loadBalancerClient.reconstructURI(instance, originalUri).toString();Request newRequest = buildRequest(request, reconstructedUrl);LoadBalancerProperties loadBalancerProperties = loadBalancerClientFactory.getProperties(serviceId);return executeWithLoadBalancerLifecycleProcessing(delegate, options, newRequest, lbRequest, lbResponse,supportedLifecycleProcessors, loadBalancerProperties.isUseRawStatusCodeInResponseData());}protected Request buildRequest(Request request, String reconstructedUrl) {return Request.create(request.httpMethod(), reconstructedUrl, request.headers(), request.body(),request.charset(), request.requestTemplate());}private String getHint(String serviceId) {LoadBalancerProperties properties = loadBalancerClientFactory.getProperties(serviceId);String defaultHint = properties.getHint().getOrDefault("default", "default");String hintPropertyValue = properties.getHint().get(serviceId);return hintPropertyValue != null ? hintPropertyValue : defaultHint;}}

可以看到，在OpenFeign中調(diào)用了loadBalancerClient.choose(serviceId, lbRequest)來實(shí)現(xiàn)負(fù)載均衡策略，然后返回過濾后的服務(wù)ServiceInstance，也就是服務(wù)的對象。我們要重點(diǎn)分析此過程

首先分析loadBalancerClient是怎么注入進(jìn)來的呢，我們先看下其結(jié)構(gòu)

ServiceInstanceChooser

public interface ServiceInstanceChooser {ServiceInstance choose(String serviceId);<T> ServiceInstance choose(String serviceId, Request<T> request);}

LoadBalancerClient

public interface LoadBalancerClient extends ServiceInstanceChooser {<T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException;<T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request) throws IOException;URI reconstructURI(ServiceInstance instance, URI original);}

可以看到ServiceInstanceChooser定義了負(fù)載均衡的方法， LoadBalancerClient則繼承了ServiceInstanceChooser額外定義了execute執(zhí)行和reconstructURI構(gòu)建真正http請求的方法

那么LoadBalancerClient的實(shí)現(xiàn)是誰呢，剛才的疑問中又是怎么被注入的呢，其實(shí)LoadBalancerClient的實(shí)現(xiàn)是BlockingLoadBalancerClient，在配置類BlockingLoadBalancerClientAutoConfiguration中被注入

這里既然提到了自動(dòng)裝配配置類，那么我們就需要看下其結(jié)構(gòu)，來了解各個(gè)作用

# AutoConfiguration
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.cloud.loadbalancer.config.LoadBalancerAutoConfiguration,\
org.springframework.cloud.loadbalancer.config.BlockingLoadBalancerClientAutoConfiguration,\
org.springframework.cloud.loadbalancer.config.LoadBalancerCacheAutoConfiguration,\
org.springframework.cloud.loadbalancer.security.OAuth2LoadBalancerClientAutoConfiguration,\
org.springframework.cloud.loadbalancer.config.LoadBalancerStatsAutoConfiguration

• LoadBalancerAutoConfiguration 是最核心最重要的配置，隨后會(huì)詳細(xì)的分析
• BlockingLoadBalancerClientAutoConfiguration 上文中提高的LoadBalancerClient的實(shí)現(xiàn)類BlockingLoadBalancerClient就是在此裝配的

BlockingLoadBalancerClientAutoConfiguration

@Configuration(proxyBeanMethods = false)
@LoadBalancerClients
@AutoConfigureAfter(LoadBalancerAutoConfiguration.class)
@AutoConfigureBefore({ org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration.class,AsyncLoadBalancerAutoConfiguration.class })
@ConditionalOnClass(RestTemplate.class)
public class BlockingLoadBalancerClientAutoConfiguration {@Bean@ConditionalOnBean(LoadBalancerClientFactory.class)@ConditionalOnMissingBeanpublic LoadBalancerClient blockingLoadBalancerClient(LoadBalancerClientFactory loadBalancerClientFactory) {return new BlockingLoadBalancerClient(loadBalancerClientFactory);}//省略}

可以看到注入了LoadBalancerClient的實(shí)現(xiàn)類BlockingLoadBalancerClient。但此自動(dòng)配置類的裝配有很多的規(guī)則：

1. @LoadBalancerClients 此注解非常重要，實(shí)現(xiàn)每個(gè)服務(wù)間的負(fù)載均衡配置隔離就是通過此注解，后面會(huì)詳細(xì)的分析
2. @AutoConfigureAfter(LoadBalancerAutoConfiguration.class)在LoadBalancerAutoConfiguration之后進(jìn)行裝配，LoadBalancerAutoConfiguration也很重要，后面也會(huì)詳細(xì)的分析
3. @AutoConfigureBefore({ org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration.class, AsyncLoadBalancerAutoConfiguration.class }) 在org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration和AsyncLoadBalancerAutoConfiguration之前進(jìn)行裝配。注意：2和3中的LoadBalancerAutoConfiguration不是一個(gè)對象，2中的是在spring-cloud-loadbalancer模塊中，3中的是在spring-cloud-commons模塊中，這也是有步驟2的原因，畢竟肯定是要先裝配本模塊下的
4. @ConditionalOnClass(RestTemplate.class)這個(gè)不是重點(diǎn)，可以略過
5. @ConditionalOnBean(LoadBalancerClientFactory.class) 在注入時(shí)依賴了LoadBalancerClientFactory，LoadBalancerClientFactory也非常的重要，后面也會(huì)進(jìn)行詳細(xì)的分析

上面多次提到了LoadBalancerAutoConfiguration，自動(dòng)裝配中也有它，那么現(xiàn)在就來分析下其流程

LoadBalancerAutoConfiguration

@Configuration(proxyBeanMethods = false)
@LoadBalancerClients
@EnableConfigurationProperties(LoadBalancerClientsProperties.class)
@AutoConfigureBefore({ ReactorLoadBalancerClientAutoConfiguration.class,LoadBalancerBeanPostProcessorAutoConfiguration.class })
@ConditionalOnProperty(value = "spring.cloud.loadbalancer.enabled", havingValue = "true", matchIfMissing = true)
public class LoadBalancerAutoConfiguration {//依賴了LoadBalancerClientSpecification類型的對象集合private final ObjectProvider<List<LoadBalancerClientSpecification>> configurations;public LoadBalancerAutoConfiguration(ObjectProvider<List<LoadBalancerClientSpecification>> configurations) {this.configurations = configurations;}@Bean@ConditionalOnMissingBeanpublic LoadBalancerZoneConfig zoneConfig(Environment environment) {return new LoadBalancerZoneConfig(environment.getProperty("spring.cloud.loadbalancer.zone"));}@ConditionalOnMissingBean@Beanpublic LoadBalancerClientFactory loadBalancerClientFactory(LoadBalancerClientsProperties properties) {LoadBalancerClientFactory clientFactory = new LoadBalancerClientFactory(properties);//LoadBalancerClientSpecification類型的配置類集合對象注入到NamedContextFactory，實(shí)現(xiàn)個(gè)性化配置clientFactory.setConfigurations(this.configurations.getIfAvailable(Collections::emptyList));return clientFactory;}}

可以看到此配置類的裝配規(guī)則也比較復(fù)雜，但大部分都和本系列要分析的內(nèi)容關(guān)聯(lián)性不大，直接略過即可，我們只關(guān)心兩個(gè)地方

• @LoadBalancerClients 這個(gè)注解出現(xiàn)多次，我們會(huì)做詳細(xì)的分析
• 注入了loadBalancerClientFactory，這個(gè)也非常的重要，隨后會(huì)做詳細(xì)的分析

到這里我們要解決的三個(gè)重點(diǎn)：

• @LoadBalancerClients的作用
• ObjectProvider<List<LoadBalancerClientSpecification>> configurations的作用
• LoadBalancerClientFactory的作用，別忘了負(fù)載均衡的執(zhí)行對象BlockingLoadBalancerClient在生成時(shí)，將此對象注入了進(jìn)去

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