ribbon是一个负载均衡组件，它可以将请求分散到多个服务提供者实例中，提高系统的性能和可用性。本章分析ribbon是如何实现负载均衡的
1、@LoadBalanced
消费者在引入ribbon组件后，给http客户端添加@LoadBalanced注解就可以启用负载均衡功能。@LoadBalanced注解比较简单，本身没有包含什么业务逻辑，值得一提的是@Qualifier注解。@Qualifier通常配合@Autowired一起使用，当有多个同类型的bean时可以根据@Qualifier指定的bean名称完成注入；此外在bean的入口和出口使用@Qualifier修饰，也能建立稳定的供应关系，ribbon收集所有被@LoadBalanced修饰的restTemplate，用的就是这种方法
@Target({ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Qualifier
public @interface LoadBalanced {
}
@Component
@Qualifier
public class A {}
public class B {
// 精准注入A
@Autowired
@Qualifier
private A a;
}2、LoadBalancerAutoConfiguration
和负载均衡相关的配置在LoadBalancerAutoConfiguration内，查看spring-cloud-common包的spring.factories文件，我们知道这是一个自动配置类
# AutoConfiguration
...
org.springframework.cloud.client.loadbalancer.LoadBalancerAutoConfiguration,\
...LoadBalancerAutoConfiguration注入了所有使用@LoadBalanced修饰的restTemplate，为什么加了@LoadBalanced就能引入其他其他被@LoadBalanced修饰的bean，原因就是1中提到的@Qualifier。
LoadBalancerAutoConfiguration内部还有一个loadBalancedRestTemplateInitializerDeprecated方法，这个方法会对restTemplate做定制化处理
@LoadBalanced
@Autowired(
required = false
)
private List<RestTemplate> restTemplates = Collections.emptyList();
@Bean
public SmartInitializingSingleton loadBalancedRestTemplateInitializerDeprecated(final ObjectProvider<List<RestTemplateCustomizer>> restTemplateCustomizers) {
return () -> {
restTemplateCustomizers.ifAvailable((customizers) -> {
Iterator var2 = this.restTemplates.iterator();
while(var2.hasNext()) {
RestTemplate restTemplate = (RestTemplate)var2.next();
Iterator var4 = customizers.iterator();
while(var4.hasNext()) {
RestTemplateCustomizer customizer = (RestTemplateCustomizer)var4.next();
// 定制化处理
customizer.customize(restTemplate);
}
}
});
};
}RestTemplateCustomizer接口的实现类在LoadBalancerAutoConfiguration内，它们的定制化处理其实就是给restTemplate添加RetryLoadBalancerInterceptor拦截器
@Bean
@ConditionalOnMissingBean
public RestTemplateCustomizer restTemplateCustomizer(final RetryLoadBalancerInterceptor loadBalancerInterceptor) {
return (restTemplate) -> {
List<ClientHttpRequestInterceptor> list = new ArrayList(restTemplate.getInterceptors());
list.add(loadBalancerInterceptor);
// 客户端添加拦截器
restTemplate.setInterceptors(list);
};
}3、RetryLoadBalancerInterceptor
查看RetryLoadBalancerInterceptor的拦截方法，核心语句是loadBalancer.choose，也就是说服务实例的选择功能其实是由LoadBalancerClient接口实现类完成的
private final LoadBalancerClient loadBalancer;
public ClientHttpResponse intercept(final HttpRequest request, final byte[] body, final ClientHttpRequestExecution execution) throws IOException {
...
if (serviceInstance == null) {
if (LOG.isDebugEnabled()) {
LOG.debug("Service instance retrieved from LoadBalancedRetryContext: was null. Reattempting service instance selection");
}
// 选择服务实例
serviceInstance = this.loadBalancer.choose(serviceName);
if (LOG.isDebugEnabled()) {
LOG.debug(String.format("Selected service instance: %s", serviceInstance));
}
}
...
ClientHttpResponse response = (ClientHttpResponse)
this.loadBalancer.execute(serviceName, serviceInstance, this.requestFactory.createRequest(request, body, execution));
...
}RibbonLoadBalancerClient是LoadBalancerClient接口实现类之一，查看它的choose方法
// 选择合适的服务实例
public ServiceInstance choose(String serviceId, Object hint) {
Server server = this.getServer(this.getLoadBalancer(serviceId), hint);
return server == null ? null : new RibbonServer(serviceId, server, this.isSecure(server, serviceId), this.serverIntrospector(serviceId).getMetadata(server));
}
// 选择合适的服务器
protected Server getServer(ILoadBalancer loadBalancer, Object hint) {
return loadBalancer == null ? null : loadBalancer.chooseServer(hint != null ? hint : "default");
}
选择服务器的任务移交给了ILoadBalancer的实现类，如ZoneAwareLoadBalancer。ZoneAwareLoadBalancer会调用父类BaseLoadBalancer的choose方法，按照指定的策略选取服务，如轮询、加权等
public Server chooseServer(Object key) {
if (this.counter == null) {
this.counter = this.createCounter();
}
this.counter.increment();
if (this.rule == null) {
return null;
} else {
try {
// 按规则选择服务
return this.rule.choose(key);
} catch (Exception var3) {
logger.warn("LoadBalancer [{}]: Error choosing server for key {}", new Object[]{this.name, key, var3});
return null;
}
}
}再聊聊服务的来源，LoadBalancer是从哪里挑选的服务？
ZoneAwareLoadBalancer在创建时会调用父类DynamicServerListLoadBalancer的构造方法，然后调用updateListOfServers方法获取服务列表
public DynamicServerListLoadBalancer(IClientConfig clientConfig, IRule rule, IPing ping, ServerList<T> serverList, ServerListFilter<T> filter, ServerListUpdater serverListUpdater) {
...
this.restOfInit(clientConfig);
}
void restOfInit(IClientConfig clientConfig) {
...
this.updateListOfServers();
...
}
@VisibleForTesting
public void updateListOfServers() {
List<T> servers = new ArrayList();
if (this.serverListImpl != null) {
// 获取服务列表
servers = this.serverListImpl.getUpdatedListOfServers();
LOGGER.debug("List of Servers for {} obtained from Discovery client: {}", this.getIdentifier(), servers);
if (this.filter != null) {
servers = this.filter.getFilteredListOfServers((List)servers);
LOGGER.debug("Filtered List of Servers for {} obtained from Discovery client: {}", this.getIdentifier(), servers);
}
}
this.updateAllServerList((List)servers);
}看到服务实例自然联想到eureka，跟踪getUpdatedListOfServers方法
调用链：
-> DynamicServerListLoadBalancer.updateListOfServers
-> this.serverListImpl.getUpdatedListOfServers;
-> DiscoveryEnabledNIWSServerList.obtainServersViaDiscovery
-> eurekaClient.getInstancesByVipAddressprivate List<DiscoveryEnabledServer> obtainServersViaDiscovery() {
...
// eureka客户端拉取服务
List<InstanceInfo> listOfInstanceInfo = eurekaClient.getInstancesByVipAddress(vipAddress, this.isSecure, this.targetRegion);
Iterator var8 = listOfInstanceInfo.iterator();
...
}eureka客户端从远程拉取服务实例，然后ribbon从服务实例中挑选服务
3、总结
ribbon组件向restTemplate中添加拦截器，实现负载均衡功能增强
