1：ribbon 是提供通过 serviceId 拜访服务的能力，而且提供了负载平衡能力。在 springcloud 外面，有 2 个主动配置加载类 LoadBalancerAutoConfiguration 和 RibbonAutoConfiguration。LoadBalancerAutoConfiguration 次要实现对 RestTemplate 对象的负载平衡拜访，RibbonAutoConfiguration 次要是对 Ribbon 所需组件的实例化。别离在 springcloud-common 和 netflix-core 的 spring.foctories 文件里。
org.springframework.boot.autoconfigure.EnableAutoConfiguration=
org.springframework.cloud.netflix.hystrix.HystrixAutoConfiguration,
org.springframework.cloud.netflix.hystrix.security.HystrixSecurityAutoConfiguration,
org.springframework.cloud.netflix.ribbon.RibbonAutoConfiguration,
org.springframework.cloud.netflix.rx.RxJavaAutoConfiguration,
org.springframework.boot.autoconfigure.EnableAutoConfiguration=
org.springframework.cloud.client.CommonsClientAutoConfiguration,
org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration,
org.springframework.cloud.client.serviceregistry.ServiceRegistryAutoConfiguration, 2：RibbonAutoConfiguration 次要是对 Ribbon 组件的实例化，其对应关系是：
IClientConfig –> DefaultClientConfigImpl
IRule –> ZoneAvoidanceRule
IPing –> DummyPing
ServerList –> ConfigurationBasedServerList
ServerListUpdater –> PollingServerListUpdater
ILoadBalancer –> ZoneAwareLoadBalancer
LoadBalancerClient –> RibbonLoadBalancerClient
同时，也会创立：SpringClientFactory 对象。
3：LoadBalancerAutoConfiguration 次要是对 RestTemplate 进行负载平衡配置。其次要实现是创立拦截器 LoadBalancerInterceptor。而后对 RestTemplate 进行自定义配置，代码如下：
@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);
}
}
}
};
}
@Configuration
@ConditionalOnMissingClass("org.springframework.retry.support.RetryTemplate")
static class LoadBalancerInterceptorConfig {
@Bean
public LoadBalancerInterceptor ribbonInterceptor(
LoadBalancerClient loadBalancerClient,
LoadBalancerRequestFactory requestFactory) {return new LoadBalancerInterceptor(loadBalancerClient, requestFactory);
}
@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);
}
};
}
} 在下面的代码中，减少的拦截器类是：LoadBalancerInterceptor。LoadBalancerClient 接口的实现类是：RibbonLoadBalancerClient。
4：在应用 RestTemplate 进行 POST，GET 申请的时候，最终执行的是 办法：LoadBalancerInterceptor#intercept。在该办法里，调用办法：RibbonLoadBalancerClient#execute(String, LoadBalancerRequest<T>)
@Override
public ClientHttpResponse intercept(final HttpRequest request, final byte[] body,
final ClientHttpRequestExecution execution) throws IOException {final URI originalUri = request.getURI();
String serviceName = originalUri.getHost();
Assert.state(serviceName != null, "Request URI does not contain a valid hostname:" + originalUri);
return this.loadBalancer.execute(serviceName, requestFactory.createRequest(request, body, execution));
} 5：在 execute 办法里，次要是 获取到负载均衡器，获取服务提供者的拜访门路，拜访
@Override
public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {ILoadBalancer loadBalancer = getLoadBalancer(serviceId); // 获取负载均衡器
Server server = getServer(loadBalancer); // 获取服务拜访门路，包含 IP 端口
if (server == null) {throw new IllegalStateException("No instances available for" + serviceId);
}
RibbonServer ribbonServer = new RibbonServer(serviceId, server, isSecure(server,
serviceId), serverIntrospector(serviceId).getMetadata(server));
return execute(serviceId, ribbonServer, request); // 拜访服务
} 6：在 SpringClientFactory#getLoadBalancer 里，获取的是接口 ILoadBalancer 的实例。其实现是 结构 Spring 的上下文对象 AnnotationConfigApplicationContext，而后加载配置类:RibbonClientConfiguration.
在 RibbonClientConfiguration 类里，会实例化 ZoneAwareLoadBalancer 对象。获取到 ILoadBalancer 的单例模式后，会缓存在内存外面。
public ILoadBalancer getLoadBalancer(String name) {return getInstance(name, ILoadBalancer.class);
}
protected AnnotationConfigApplicationContext getContext(String name) {if (!this.contexts.containsKey(name)) {synchronized (this.contexts) {if (!this.contexts.containsKey(name)) {this.contexts.put(name, createContext(name));
}
}
}
return this.contexts.get(name);
}
@Bean
@ConditionalOnMissingBean
public ILoadBalancer ribbonLoadBalancer(IClientConfig config,
ServerList<Server> serverList, ServerListFilter<Server> serverListFilter,
IRule rule, IPing ping, ServerListUpdater serverListUpdater) {if (this.propertiesFactory.isSet(ILoadBalancer.class, name)) {return this.propertiesFactory.get(ILoadBalancer.class, config, name);
}
return new ZoneAwareLoadBalancer<>(config, rule, ping, serverList,
serverListFilter, serverListUpdater);
} 7：ZoneAwareLoadBalancer 会调用 ZoneAvoidanceRule 里的 choose() 办法。该办法外面，通过负载均衡器里，获取所有的服务节点（IRule 实例里的负载均衡器是在 ZoneAwareLoadBalancer 实例化的时候设置的）。lb.getAllServers() 办法获取的是获取的缓存对象 allServerList。
@Override
public Server choose(Object key) {ILoadBalancer lb = getLoadBalancer();
Optional<Server> server = getPredicate().chooseRoundRobinAfterFiltering(lb.getAllServers(), key);
if (server.isPresent()) {return server.get();
} else {return null;}
} 8：获取到服务列表后，ZoneAvoidanceRule 前面 应用 chooseRoundRobinAfterFiltering() 抉择服务，按照代码，应用的是循环算法抉择 Server
public Optional<Server> chooseRoundRobinAfterFiltering(List<Server> servers, Object loadBalancerKey) {List<Server> eligible = getEligibleServers(servers, loadBalancerKey);
if (eligible.size() == 0) {return Optional.absent();
}
return Optional.of(eligible.get(nextIndex.getAndIncrement() % eligible.size()));
} 9：选定 Server 对象后，外面有拜访的实例的 IP 和端口信息。会应用 httpClient 对象拜访对象的节点。代码如下：
public LoadBalancerRequest<ClientHttpResponse> createRequest(final HttpRequest request,
final byte[] body, final ClientHttpRequestExecution execution) {return new LoadBalancerRequest<ClientHttpResponse>() {
@Override
public ClientHttpResponse apply(final ServiceInstance instance)
throws Exception {HttpRequest serviceRequest = new ServiceRequestWrapper(request, instance, loadBalancer);
if (transformers != null) {for (LoadBalancerRequestTransformer transformer : transformers) {serviceRequest = transformer.transformRequest(serviceRequest, instance);
}
}
return execution.execute(serviceRequest, body);
}
};
}