环境：Spring Cloud 2021.0.7 + Spring Boot 2.7.12

配置依赖

maven依赖

<dependency><groupId>org.springframework.cloud</groupId><artifactId>spring-cloud-starter-openfeign</artifactId>
</dependency>
<dependency><groupId>org.springframework.cloud</groupId><artifactId>spring-cloud-loadbalancer</artifactId>
</dependency>
<dependencyManagement><dependencies><dependency><groupId>org.springframework.cloud</groupId><artifactId>spring-cloud-dependencies</artifactId><version>${spring-cloud.version}</version><type>pom</type><scope>import</scope></dependency></dependencies>
</dependencyManagement>

开启注解功能

@SpringBootApplication
// 开启Feign功能，在该注解中你还可以配置，如下3个重要的信息：
// 1. 为所有的FeignClient提供统一默认的配置
// 2. 指定扫描那些包写的类
// 3. 指定有哪些@FeignClient类
@EnableFeignClients
public class AppApplication {public static void main(String[] args) {SpringApplication.run(AppApplication.class, args);}}

FeignClient生成Bean原理

容器在启动过程中会找到所有@FeignClient的接口类，然后将这些类注册为容器Bean，而每一个Feign客户端对应的是FactoryBean对象FeignClientFactoryBean。

具体如何找这些带有@FeignClient注解的接口类可以查看FeignClientsRegistrar该类就在@EnableFeignClients中被导入。

FeignClientFactoryBean

public class FeignClientFactoryBean implements FactoryBean {public Object getObject() {return getTarget();}<T> T getTarget() {FeignContext context = beanFactory != null ? beanFactory.getBean(FeignContext.class) : applicationContext.getBean(FeignContext.class);Feign.Builder builder = feign(context);if (!StringUtils.hasText(url)) {if (!name.startsWith("http")) {url = "http://" + name;}else {url = name;}url += cleanPath();// 负载均衡处理return (T) loadBalance(builder, context, new HardCodedTarget<>(type, name, url));}// ...}protected <T> T loadBalance(Feign.Builder builder, FeignContext context, HardCodedTarget<T> target) {// 在OpenFeign中核心实现负载均衡的类就是具体的Client类// Feign负载均衡能力实现通过具体Client实现，每一个FeignClient客户端都会对应一个子容器AnnotationConfigApplicationContext// 根据@FeignClient配置的服务名name或value为key，从一个LoadBalancerClientFactory（父类）中的Map中查找该name对应的容器// 如果不存在则创建一个AnnotationConfigApplicationContext。每个子容器都设置了父容器，如果通过子容器查找不到Client的实现，那么会从父容器中查找Client client = getOptional(context, Client.class);}
}

Client实现

Client的具体实现可以有如下：

apache httpclient
okhttp
default（jdk）

具体使用哪个是根据你环境引入了哪个依赖（httpclient，okhttp）

<!-- httpclient -->
<dependency><groupId>io.github.openfeign</groupId><artifactId>feign-httpclient</artifactId><version>${version}</version>
</dependency>
<!-- okhttp -->
<dependency><groupId>io.github.openfeign</groupId><artifactId>feign-okhttp</artifactId><version>${version}</version>
</dependency>

具体选择通过如下配置

@Import({ HttpClientFeignLoadBalancerConfiguration.class, OkHttpFeignLoadBalancerConfiguration.class, HttpClient5FeignLoadBalancerConfiguration.class, DefaultFeignLoadBalancerConfiguration.class })
public class FeignLoadBalancerAutoConfiguration {
}

如果你的环境有多个实现，那么这里会根据这里的导入顺序加载。这里以最后一个
DefaultFeignLoadBalancerConfiguration为例。

class DefaultFeignLoadBalancerConfiguration {@Bean@ConditionalOnMissingBean// 没有启用spring-retry重试功能@Conditional(OnRetryNotEnabledCondition.class)public Client feignClient(LoadBalancerClient loadBalancerClient, LoadBalancerClientFactory loadBalancerClientFactory) {// 这里构造函数第一个参数将会成为最终执行远程接口调用的实现return new FeignBlockingLoadBalancerClient(new Client.Default(null, null), loadBalancerClient, loadBalancerClientFactory);}
}

在没有导入httpclient或者okhttp情况下，使用的Client实现是
FeignBlockingLoadBalancerClient。

负载均衡实现

构造
FeignBlockingLoadBalancerClient传入了负载均衡客户端LoadBalancerClient及负载均衡客户端工厂LoadBalancerClientFactory该工厂是用来创建每一个Feign客户端对应的子容器
AnnotationConfigApplicationContext及从对应子容器获取相应的Bean实例对象，如：Client，Request.Options，Logger.Level等。

public class FeignBlockingLoadBalancerClient implements Client {// 此Client代理对象是上面的new Client.Default(null, null)private final Client delegate;private final LoadBalancerClient loadBalancerClient;private final LoadBalancerClientFactory loadBalancerClientFactory;public FeignBlockingLoadBalancerClient(Client delegate, LoadBalancerClient loadBalancerClient, LoadBalancerClientFactory loadBalancerClientFactory) {this.delegate = delegate;this.loadBalancerClient = loadBalancerClient;this.loadBalancerClientFactory = loadBalancerClientFactory;}@Overridepublic Response execute(Request request, Request.Options options) throws IOException {final URI originalUri = URI.create(request.url());// 获取服务名serviceIdString serviceId = originalUri.getHost();String hint = getHint(serviceId);DefaultRequest<RequestDataContext> lbRequest = new DefaultRequest<>(new RequestDataContext(buildRequestData(request), hint));// ...// 通过负载均衡客户端获取指定serviceId的服务实例ServiceInstance instance = loadBalancerClient.choose(serviceId, lbRequest);// ...// 通过获取到的ServiceInstance实例，重新构造请求地址String reconstructedUrl = loadBalancerClient.reconstructURI(instance, originalUri).toString();// 重新构建一个新的请求Request newRequest = buildRequest(request, reconstructedUrl);LoadBalancerProperties loadBalancerProperties = loadBalancerClientFactory.getProperties(serviceId);return executeWithLoadBalancerLifecycleProcessing(delegate, options, newRequest, lbRequest, lbResponse, supportedLifecycleProcessors, loadBalancerProperties.isUseRawStatusCodeInResponseData());}protected Request buildRequest(Request request, String reconstructedUrl) {return Request.create(request.httpMethod(), reconstructedUrl, request.headers(), request.body(),request.charset(), request.requestTemplate());}
}

LoadBalancerClient具体实现：BlockingLoadBalancerClient

public class BlockingLoadBalancerClient implements LoadBalancerClient {private final ReactiveLoadBalancer.Factory<ServiceInstance> loadBalancerClientFactory;public BlockingLoadBalancerClient(ReactiveLoadBalancer.Factory<ServiceInstance> loadBalancerClientFactory) {this.loadBalancerClientFactory = loadBalancerClientFactory;}public <T> ServiceInstance choose(String serviceId, Request<T> request) {// 获取一个负载均衡器，默认是轮询策略ReactiveLoadBalancer<ServiceInstance> loadBalancer = loadBalancerClientFactory.getInstance(serviceId);if (loadBalancer == null) {return null;}Response<ServiceInstance> loadBalancerResponse = Mono.from(loadBalancer.choose(request)).block();if (loadBalancerResponse == null) {return null;}return loadBalancerResponse.getServer();}// 重新构造请求的uripublic URI reconstructURI(ServiceInstance serviceInstance, URI original) {return LoadBalancerUriTools.reconstructURI(serviceInstance, original);}
}
public final class LoadBalancerUriTools {public static URI reconstructURI(ServiceInstance serviceInstance, URI original) {// ...return doReconstructURI(serviceInstance, original);}private static URI doReconstructURI(ServiceInstance serviceInstance, URI original) {String host = serviceInstance.getHost();String scheme = Optional.ofNullable(serviceInstance.getScheme()).orElse(computeScheme(original, serviceInstance));int port = computePort(serviceInstance.getPort(), scheme);if (Objects.equals(host, original.getHost()) && port == original.getPort() && Objects.equals(scheme, original.getScheme())) {return original;}boolean encoded = containsEncodedParts(original);return UriComponentsBuilder.fromUri(original).scheme(scheme).host(host).port(port).build(encoded).toUri();}
}

轮询算法

public class RoundRobinLoadBalancer implements ReactorServiceInstanceLoadBalancer {public Mono<Response<ServiceInstance>> choose(Request request) {ServiceInstanceListSupplier supplier = serviceInstanceListSupplierProvider.getIfAvailable(NoopServiceInstanceListSupplier::new);return supplier.get(request).next().map(serviceInstances -> processInstanceResponse(supplier, serviceInstances));}private Response<ServiceInstance> processInstanceResponse(ServiceInstanceListSupplier supplier, List<ServiceInstance> serviceInstances) {Response<ServiceInstance> serviceInstanceResponse = getInstanceResponse(serviceInstances);if (supplier instanceof SelectedInstanceCallback && serviceInstanceResponse.hasServer()) {((SelectedInstanceCallback) supplier).selectedServiceInstance(serviceInstanceResponse.getServer());}return serviceInstanceResponse;}private Response<ServiceInstance> getInstanceResponse(List<ServiceInstance> instances) {// ...// 如果只有一个实例if (instances.size() == 1) {return new DefaultResponse(instances.get(0));}// Ignore the sign bit, this allows pos to loop sequentially from 0 to// Integer.MAX_VALUEint pos = this.position.incrementAndGet() & Integer.MAX_VALUE;ServiceInstance instance = instances.get(pos % instances.size());return new DefaultResponse(instance);}
}

执行远程调用

接着上面
FeignBlockingLoadBalancerClient#execute方法最终的返回方法执行

final class LoadBalancerUtils {static Response executeWithLoadBalancerLifecycleProcessing(Client feignClient, Request.Options options,Request feignRequest, org.springframework.cloud.client.loadbalancer.Request lbRequest,org.springframework.cloud.client.loadbalancer.Response<ServiceInstance> lbResponse,Set<LoadBalancerLifecycle> supportedLifecycleProcessors, boolean useRawStatusCodes) throws IOException {return executeWithLoadBalancerLifecycleProcessing(feignClient, options, feignRequest, lbRequest, lbResponse, supportedLifecycleProcessors, true, useRawStatusCodes);}static Response executeWithLoadBalancerLifecycleProcessing(Client feignClient, Request.Options options,Request feignRequest, org.springframework.cloud.client.loadbalancer.Request lbRequest,org.springframework.cloud.client.loadbalancer.Response<ServiceInstance> lbResponse,Set<LoadBalancerLifecycle> supportedLifecycleProcessors, boolean loadBalanced, boolean useRawStatusCodes) throws IOException {// 这里执行生命周期实际调用前动作try {// 执行时间的调用，而这里的feignClient就是在FeignBlockingLoadBalancerClient传递过来的，new Client.Default(null, null)Response response = feignClient.execute(feignRequest, options);// 这里执行生命周期回调，省略return response;}// ...}
}

Client.Default

public interface Client {public Response execute(Request request, Options options) throws IOException {// 通过JDK自带的网络连接进行处理HttpURLConnection connection = convertAndSend(request, options);return convertResponse(connection, request);}
}

以上就是一个Feign客户端请求的负载均衡实现原理

完毕~！~！

求助三连~~~