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 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 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
@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 execute(String serviceId, LoadBalancerRequest 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 serverList, ServerListFilter 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 = 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 chooseRoundRobinAfterFiltering(List servers, Object loadBalancerKey) {
List 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 createRequest(final HttpRequest request,
final byte[] body, final ClientHttpRequestExecution execution) {
return new LoadBalancerRequest() {
@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);
}
};
}