1：ribbon是提供通过serviceId访问服务的能力，而且提供了负载均衡能力。在springcloud里面，有2个自动配置加载类 LoadBalancerAutoConfiguration 和 RibbonAutoConfiguration。LoadBalancerAutoConfiguration主要完成对 RestTemplate 对象的负载均衡访问，RibbonAutoConfiguration主要是对Ribbon所需组件的实例化。分别在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 主要是对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，也会创建：SpringClientFactory 对象。

3：LoadBalancerAutoConfiguration 主要是对 RestTemplate 进行负载均衡配置。 其主要实现是创建拦截器 LoadBalancerInterceptor。然后对 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);
                }
            };
        }
    }

在上面的代码中，增加的拦截器类是：LoadBalancerInterceptor。 LoadBalancerClient 接口的实现类是：RibbonLoadBalancerClient。

4：在使用 RestTemplate 进行 POST， GET 请求的时候，最终执行的是 方法：LoadBalancerInterceptor#intercept。 在该方法里，调用方法：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 方法里，主要是 获取到负载均衡器，获取服务提供者的访问路径，访问

@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);  //获取服务访问路径，包括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); //访问服务
    }

6：在 SpringClientFactory#getLoadBalancer 里，获取的是接口 ILoadBalancer 的实例。其实现是 构造Spring的上下文对象 AnnotationConfigApplicationContext， 然后加载配置类:RibbonClientConfiguration.
在RibbonClientConfiguration类里，会实例化 ZoneAwareLoadBalancer 对象。获取到 ILoadBalancer 的单例模式后，会缓存在内存里面。

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() 方法。 该方法里面，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() 方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 后面 使用 chooseRoundRobinAfterFiltering() 选择服务，依照代码，使用的是循环算法选择 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，里面有访问的实例的IP和端口信息。会使用 httpClient 对象访问对象的节点。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);
            }

        };
    }