SpringCloud升级之路-2020.0.x - 6.使用 Spring Cloud LoadBalancer(1)

public static final String NAMESPACE = "loadbalancer";
public static final String PROPERTY_NAME = NAMESPACE + ".client.name";
public LoadBalancerClientFactory() {
	super(LoadBalancerClientConfiguration.class, NAMESPACE, PROPERTY_NAME);
}

Mono> choose(Request request);

@Bean
@ConditionalOnMissingBean
public ReactorLoadBalancer reactorServiceInstanceLoadBalancer(
		Environment environment,
		LoadBalancerClientFactory loadBalancerClientFactory) {
	//获取微服务名称
	String name = environment.getProperty(LoadBalancerClientFactory.PROPERTY_NAME);
	//创建 RoundRobinLoadBalancer 
	//注意这里注入的是 LazyProvider，这主要因为在注册这个 Bean 的时候相关的 Bean 可能还没有被加载注册，利用 LazyProvider 而不是直接注入所需的 Bean 防止报找不到 Bean 注入的错误。
	return new RoundRobinLoadBalancer(loadBalancerClientFactory.getLazyProvider(name,
			ServiceInstanceListSupplier.class), name);
}

public Mono> choose(Request request) {
    //注入的时候注入的是 Lazy Provider，这里取出真正的 Bean，也就是 ServiceInstanceListSupplier
	ServiceInstanceListSupplier supplier = serviceInstanceListSupplierProvider
			.getIfAvailable(NoopServiceInstanceListSupplier::new);
			//获取实例列表
	return supplier.get(request)
	        .next()
	        //从列表中选择一个实例
			.map(serviceInstances -> processInstanceResponse(supplier, serviceInstances));
}

private Response processInstanceResponse(ServiceInstanceListSupplier supplier,
		List serviceInstances) {
	Response serviceInstanceResponse = getInstanceResponse(serviceInstances);
	// 如果 ServiceInstanceListSupplier 也实现了 SelectedInstanceCallback，则执行下面的逻辑进行回调。SelectedInstanceCallback 就是每次负载均衡器选择实例之后进行的回调
	if (supplier instanceof SelectedInstanceCallback && serviceInstanceResponse.hasServer()) {
		((SelectedInstanceCallback) supplier).selectedServiceInstance(serviceInstanceResponse.getServer());
	}
	return serviceInstanceResponse;
}

private Response getInstanceResponse(List instances) {
	if (instances.isEmpty()) {
		return new EmptyResponse();
	}
	//postion 原子 +1 并取绝对值
	int pos = Math.abs(this.position.incrementAndGet());
    //返回对应下标的实例
	ServiceInstance instance = instances.get(pos % instances.size());
	return new DefaultResponse(instance);
}

public interface ServiceInstanceListSupplier extends Supplier>> {
	String getServiceId();
	default Flux> get(Request request) {
		return get();
	}
	static ServiceInstanceListSupplierBuilder builder() {
		return new ServiceInstanceListSupplierBuilder();
	}
}

@Bean
@ConditionalOnBean(ReactiveDiscoveryClient.class)
@ConditionalOnMissingBean
//spring.cloud.loadbalancer.configurations 未指定或者为 default
@ConditionalOnProperty(value = "spring.cloud.loadbalancer.configurations", havingValue = "default",
		matchIfMissing = true)
public ServiceInstanceListSupplier discoveryClientServiceInstanceListSupplier(
		ConfigurableApplicationContext context) {
	return ServiceInstanceListSupplier.builder()
	//通过 DiscoveryClient 提供实例
	.withDiscoveryClient()
	//开启缓存
	.withCaching()
	.build(context);
}

@Bean
@ConditionalOnBean(ReactiveDiscoveryClient.class)
@ConditionalOnMissingBean
//如果 spring.cloud.loadbalancer.configurations 指定为 zone-preference
@ConditionalOnProperty(value = "spring.cloud.loadbalancer.configurations", havingValue = "zone-preference")
public ServiceInstanceListSupplier zonePreferenceDiscoveryClientServiceInstanceListSupplier(
		ConfigurableApplicationContext context) {
	return ServiceInstanceListSupplier.builder()
	//通过 DiscoveryClient 提供实例
	.withDiscoveryClient()
	//启用更倾向于同一个 zone 下实例的特性
	.withZonePreference()
	//开启缓存
	.withCaching()
			.build(context);
}

return  //开启服务实例缓存
        new CachingServiceInstanceListSupplier(
                        //启用通过 discoveryClient 的服务发现
                        new DiscoveryClientServiceInstanceListSupplier(
                                discoveryClient, env
                        )
                , cacheManagerProvider.getIfAvailable()
        );

private final ObjectProvider> configurations;

public LoadBalancerAutoConfiguration(ObjectProvider> configurations) {
    //注入 LoadBalancerClientSpecification List 的 provider
    //在 Bean 创建的时候，进行载入，而不是注册的时候
	this.configurations = configurations;
}

@ConditionalOnMissingBean
@Bean
public LoadBalancerClientFactory loadBalancerClientFactory() {
    //创建 LoadBalancerClientFactory
	LoadBalancerClientFactory clientFactory = new LoadBalancerClientFactory();
    //读取所有的 LoadBalancerClientSpecification，设置为 LoadBalancerClientFactory 的配置
	clientFactory.setConfigurations(this.configurations.getIfAvailable(Collections::emptyList));
	return clientFactory;
}

@Configuration(proxyBeanMethods = false)
@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE })
@Documented
@Import(LoadBalancerClientConfigurationRegistrar.class)
public @interface LoadBalancerClients {
    //可以指定多个 LoadBalancerClient
	LoadBalancerClient[] value() default {};
	//指定所有的负载均衡配置的默认配置
	Class[] defaultConfiguration() default {};
}

@Configuration(proxyBeanMethods = false)
@Import(LoadBalancerClientConfigurationRegistrar.class)
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface LoadBalancerClient {
    //name 和 value 都是微服务名称
	@AliasFor("name")
	String value() default "";
	@AliasFor("value")
	String name() default "";
	//这个微服务的配置
	Class[] configuration() default {};
}

@Override
public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
    //获取 LoadBalancerClients 注解的元数据
	Map attrs = metadata.getAnnotationAttributes(LoadBalancerClients.class.getName(), true);
	if (attrs != null && attrs.containsKey("value")) {
		AnnotationAttributes[] clients = (AnnotationAttributes[]) attrs.get("value");
		//对于 value 属性，其实就是一个 LoadBalancerClient 列表，对于每个生成一个特定微服务名字的  LoadBalancerClientSpecification
		for (AnnotationAttributes client : clients) {
			registerClientConfiguration(registry, getClientName(client), client.get("configuration"));
		}
	}
	//如果指定了 defaultConfiguration，则注册为 default 的配置
	if (attrs != null && attrs.containsKey("defaultConfiguration")) {
		String name;
		if (metadata.hasEnclosingClass()) {
			name = "default." + metadata.getEnclosingClassName();
		}
		else {
			name = "default." + metadata.getClassName();
		}
		registerClientConfiguration(registry, name, attrs.get("defaultConfiguration"));
	}
	//获取 LoadBalancerClient 注解的元数据
	Map client = metadata.getAnnotationAttributes(LoadBalancerClient.class.getName(), true);
	String name = getClientName(client);
	if (name != null) {
		registerClientConfiguration(registry, name, client.get("configuration"));
	}
}

private static void registerClientConfiguration(BeanDefinitionRegistry registry, Object name, Object configuration) {
    //初始化 LoadBalancerClientSpecification 的 BeanDefinition，用于注册一个 LoadBalancerClientSpecification Bean
	BeanDefinitionBuilder builder = BeanDefinitionBuilder
			.genericBeanDefinition(LoadBalancerClientSpecification.class);
	//构造器参数
	builder.addConstructorArgValue(name);
	builder.addConstructorArgValue(configuration);
	//注册 Bean
	registry.registerBeanDefinition(name + ".LoadBalancerClientSpecification", builder.getBeanDefinition());
}