Spring Cloud中使用ribbon（配合eureka）来访问service时，只需要使用http://service-name/xxx这样格式的URL即可访问对应的service的注册的服务，这具体是如何实现的呢？

Netflix ribbonH2

Netflix OSS中ribbon配合eureka来自动配置serverlist时，是需要做如下配置的

bash
myClient.ribbon.NIWSServerListClassName=com.netflix.niws.loadbalancer.DiscoveryEnabledNIWSServerList 
# the server must register itself with Eureka server with VipAddress "myservice"
myClient.ribbon.DeploymentContextBasedVipAddresses=myservice

DeploymentContextBasedVipAddresses 需要配置为需要访问的service的VipAddress的值。

Spring CloudH2

Spring Cloud中自动配置ribbon时会注册LoadBalancerInterceptor。LoadBalancerInterceptor会在拦截请求时，获取请求的Host来作为serviceName来获取ILoadBalancer的实例

bash
LoadBalancerInterceptor

public ClientHttpResponse intercept(final HttpRequest request, final byte[] body, final ClientHttpRequestExecution execution) throws IOException {
        URI originalUri = request.getURI();
        String serviceName = originalUri.getHost();
        return (ClientHttpResponse)this.loadBalancer.execute(serviceName, new LoadBalancerRequest<ClientHttpResponse>() {
            public ClientHttpResponse apply(ServiceInstance instance) throws Exception {
                HttpRequest serviceRequest = LoadBalancerInterceptor.this.new ServiceRequestWrapper(request, instance);
                return execution.execute(serviceRequest, body);
            }
        });
    }
    
RibbonLoadBalancerClient

public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
        ILoadBalancer loadBalancer = this.getLoadBalancer(serviceId);
        ...
}

而获取ILoadBalancer实例会单独的ApplicationContext中进行，这个ApplicationContext则会注册EurekaRibbonClientConfiguration类配置ribbonServerList等参数。

EurekaRibbonClientConfiguration返回ribbonServerList的逻辑是优先读取配置文件中xxx.ribbon.listOfServers设置的serverList。如果没有设置则返回DiscoveryEnabledNIWSServerList的包装类DomainExtractingServerList。

bash
EurekaRibbonClientConfiguration

    @Bean
    @ConditionalOnMissingBean
    public ServerList<?> ribbonServerList(IClientConfig config, Provider<EurekaClient> eurekaClientProvider) {
        if (this.propertiesFactory.isSet(ServerList.class, serviceId)) {
            return this.propertiesFactory.get(ServerList.class, config, serviceId);
        }
        DiscoveryEnabledNIWSServerList discoveryServerList = new DiscoveryEnabledNIWSServerList(
                config, eurekaClientProvider);
        DomainExtractingServerList serverList = new DomainExtractingServerList(
                discoveryServerList, config, this.approximateZoneFromHostname);
        return serverList;
    }

DiscoveryEnabledNIWSServerList则需要配合DeploymentContextBasedVipAddresses参数的 ，而EurekaRibbonClientConfiguration的@PostConstruct方法恰巧最终还是会配置DeploymentContextBasedVipAddresses参数

bash
EurekaRibbonClientConfiguration

@PostConstruct
public void preprocess() {
        setRibbonProperty(this.serviceId, DeploymentContextBasedVipAddresses.key(),
                this.serviceId);
        setRibbonProperty(this.serviceId, EnableZoneAffinity.key(), "true");
}