SpringCloud升级之路2020.0.x版-22.Spring Cloud LoadBalancer核心源码

经过上一节的详细分析，我们知道可以通过

LoadBalancerClientFactory

LoadBalancerClientConfiguration

environment.getProperty(LoadBalancerClientFactory.PROPERTY_NAME);

LoadBalancerClientFactory

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

查看配置类

LoadBalancerClientConfiguration

ReactorLoadBalancer<ServiceInstance>

ServiceInstanceListSupplier

ReactorLoadBalancer

ReactorLoadBalancer

Mono<Response<T>> choose(Request request);

在默认配置中的实现是：

LoadBalancerClientConfiguration

@Bean
@ConditionalOnMissingBean
public ReactorLoadBalancer<ServiceInstance> 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);
}

可以看出，默认配置的

ReactorLoadBalancer

RoundRobinLoadBalancer

AtomicInteger position

ServiceInstanceListSupplier

position

ServiceInstance

RoundRobinLoadBalancer

public Mono<Response<ServiceInstance>> 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<ServiceInstance> processInstanceResponse(ServiceInstanceListSupplier supplier,
List<ServiceInstance> serviceInstances) {
Response<ServiceInstance> 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<ServiceInstance> getInstanceResponse(List<ServiceInstance> 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);
}

ServiceInstanceListSupplier

ServiceInstanceListSupplier

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

spring-cloud-loadbalancer 中有很多

ServiceInstanceListSupplier

spring.cloud.loadbalancer.configurations

LoadBalancerClientConfiguration

@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);
}

可以看到，可以通过

ServiceInstanceListSupplier.builder()

ServiceInstanceListSupplier

ServiceInstanceListSupplier

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()
);

除了默认配置

LoadBalancerClientConfiguration

@LoadBalancerClients

@LoadBalancerClient

LoadBalancerClientConfigurationRegistrar

LoadBalancerClientFactory

NamedContextFactory

[

LoadBalancerAutoConfiguration

private final ObjectProvider<List<LoadBalancerClientSpecification>> configurations;

public LoadBalancerAutoConfiguration(ObjectProvider<List<LoadBalancerClientSpecification>> 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;
}

那么，

LoadBalancerClientSpecification

@LoadBalancerClients

@LoadBalancerClient

@Import(LoadBalancerClientConfigurationRegistrar.class)

@Import

ImportBeanDefinitionRegistrar

LoadBalancerClientConfigurationRegistrar

ImportBeanDefinitionRegistrar

BeanDefinitionRegistry

BeanDefinitionRegistry

[

LoadBalancerClients

@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 {};
}

[

LoadBalancerClient

@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 {};
}

[

LoadBalancerClientConfigurationRegistrar

@Override
public void registerBeanDefinitions(AnnotationMetadata metadata, BeanDefinitionRegistry registry) {
//获取 LoadBalancerClients 注解的元数据
Map<String, Object> 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<String, Object> 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());
}

从代码中我们可以看出，通过使用

@LoadBalancerClients

@LoadBalancerClient

LoadBalancerClientSpecification

我们这一节详细分析 Spring Cloud LoadBalancer 的源代码来理解其中的原理，下一节我们将介绍在我们项目中使用 Spring Cloud LoadBalancer 要实现的功能。