【SpringCloud】Ribbon源码解析

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注解就能启用负载均衡功能。@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。我们知道@Qualifier通常和@Autowired一起使用，用来指明注入bean的名称，这样就能在众多同类型的bean中选择真正需要的bean，除此之外@Qualifier也可以不指定名称，只需要在bean的入口和出口都用@Qualifier修饰，就能建立起对应关系

java 复制代码

@Target({ElementType.FIELD, ElementType.PARAMETER, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Qualifier
public @interface LoadBalanced {
}

java 复制代码

@Component
@Qualifier
public class A {}


public class B {
  
   // 精准注入A
   @Autowired
   @Qualifier
   private A a;
}

2、LoadBalancerAutoConfiguration

和负载均衡真正相关内容在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

java 复制代码

# 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进行了定制化处理

java 复制代码

@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接口的实现类也在LoadBalancerAutoConfiguration内，对restTemplate的定制化其实就是给它添加了RetryLoadBalancerInterceptor拦截器

java 复制代码

@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

java 复制代码

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

java 复制代码

// 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

java 复制代码

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，然后通过updateListOfServers方法获取服务列表

java 复制代码

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，继续跟踪getUpdatedListOfServers方法

java 复制代码

调用链：
-> DynamicServerListLoadBalancer.updateListOfServers
-> this.serverListImpl.getUpdatedListOfServers;
-> DiscoveryEnabledNIWSServerList.obtainServersViaDiscovery
-> eurekaClient.getInstancesByVipAddress

java 复制代码

private 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();
	...
}

方法体中出现eureka客户端，也就是说ribbon选择的服务实例其实来自于eurka服务端，是通过eureka客户端拉取到本地的

3、总结

ribbon组件向restTemplate中添加拦截器，实现负载均衡功能增强