微服務生態元件之Spring Cloud LoadBalancer詳解和原始碼分析

Spring Cloud LoadBalancer

概述

Spring Cloud LoadBalancer目前Spring官方是放在spring-cloud-commons裡，Spring Cloud最新版本為2021.0.2

Spring Cloud LoadBalancer 官網文件地址 https://docs.spring.io/spring-cloud-commons/docs/3.1.2/reference/html/#spring-cloud-loadbalancer

Spring Cloud官網文件地址 https://docs.spring.io/spring-cloud/docs/current/reference/html/

一方面Netflix Ribbon停止更新，Spring Cloud LoadBalancer是Spring Cloud官方自己提供的客戶端負載均衡器,抽象和實現，用來替代Ribbon。

• 常見負載均衡器分為服務端負載均衡器(如閘道器層均衡負載)和客戶端層均衡負載。
  • 閘道器層如硬體層面的F5或軟體層面的LVS、或者nginx等。
  • 客戶端層就如Spring Cloud LoadBalancer，作為一個客戶端去發現更新維護服務列表，自定義服務的均衡負載策略（隨機、輪詢、小流量的金絲雀等等）。

Spring Cloud提供了自己的客戶端負載平衡器抽象和實現。對於負載均衡機制，增加了ReactiveLoadBalancer介面，並提供了基於round-robin輪詢和Random隨機的實現。為了從響應式ServiceInstanceListSupplier中選擇例項，需要使用ServiceInstanceListSupplier。目前支援ServiceInstanceListSupplier的基於服務發現的實現，該實現使用類路徑中的發現客戶端從Service Discovery中檢索可用的例項。

可以通過如下配置來禁用Spring Cloud LoadBalance

spring:
  cloud:
    loadbalancer:
      enabled: false

入門示例

前面simple-ecommerce專案建立已在父Pom引入三大父依賴，詳細可以看下前面的文章<<SpringCloudAlibaba註冊中心與配置中心之利器Nacos實戰與原始碼分析>>，其中Spring Cloud的版本為2021.0.1，前面文章也已說過，Spring Cloud Alibaba整合在spring-cloud-starter-alibaba-nacos-discovery本身就依賴spring-cloud-loadbalancer。

注意如果是Hoxton之前的版本，預設負載均衡器為Ribbon，需要移除Ribbon引用和增加配置spring.cloud.loadbalancer.ribbon.enabled: false。

如果是在Spring Boot專案中新增下面的啟動器依賴，該starter也包含了Spring Boot Caching and Evictor.

        <dependency>
            <groupId>org.springframework.cloud</groupId>
            <artifactId>spring-cloud-starter-loadbalancer</artifactId>
        </dependency>

我們使用Spring官方提供了負載均衡的客戶端之一RestTemplate，RestTemplate是Spring提供的用於訪問Rest服務的客戶端，RestTemplate提供了多種便捷訪問遠端Http服務的方法，能夠大大提高客戶端的編寫效率。預設情況下，RestTemplate預設依賴jdk的HTTP連線工具。建立RestTemplateConfig配置類，標註 @LoadBalanced註解，預設使用的ReactiveLoadBalancer實現是RoundRobinLoadBalancer。

package cn.itxs.ecom.order.config;

import org.springframework.cloud.client.loadbalancer.LoadBalanced;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.client.RestTemplate;

@Configuration
public class RestTemplateConfig {
    @LoadBalanced
    @Bean
    public RestTemplate restTemplate() {
        return new RestTemplate() ;
    }
}

訂單微服務中訂單控制器增加deductRest方法

package cn.itxs.ecom.order.controller;

import cn.itxs.ecom.commons.service.OrderService;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
import org.springframework.web.client.RestTemplate;

/**
 * @Name ：OrderController
 * @Description ：訂單控制器
 * @Author ：itxs
 * @Date ：2022/4/10 20:15
 * @Version ：1.0
 * @History ：
 */
@RestController
public class OrderController {

    @Autowired
    OrderService orderService;

    @Autowired
    private RestTemplate restTemplate;

    @RequestMapping("/create/{userId}/{commodityCode}/{count}")
    public String create(@PathVariable("userId") String userId,@PathVariable("commodityCode") String commodityCode, @PathVariable("count") int count){
        return orderService.create(userId,commodityCode,count).toString();
    }

    @RequestMapping("/deductRest/{commodityCode}/{count}")
    public String deductRest(@PathVariable("commodityCode") String commodityCode, @PathVariable("count") int count){
        String url = "http://ecom-storage-service/deduct/"+commodityCode+"/"+count;
        return restTemplate.getForObject(url, String.class);
    }
}

前面server.port我們是放在Nacos配置中心裡，這裡我們註釋Nacos配置中心的配置放在本地配置檔案bootstrap.yml裡，分別配置為4080、4081、4082啟動3個庫存服務例項，並啟動訂單微服務

server:
  port: 4080

檢視nacos服務管理-服務列表裡服務詳情，可以看到3個健康的庫存例項和1個訂單微服務例項

訪問6次訂單dedect介面：http://localhost:4070/deductRest/1001/1 ，從測試的結果也驗證了LoadBalancer預設是輪詢負載均衡策略。

負載均衡演算法切換

建立自定義負載均衡配置類CustomLoadBalancerConfiguration

package cn.itxs.ecom.order.config;

import org.springframework.cloud.client.ServiceInstance;
import org.springframework.cloud.loadbalancer.core.RandomLoadBalancer;
import org.springframework.cloud.loadbalancer.core.ReactorLoadBalancer;
import org.springframework.cloud.loadbalancer.core.ServiceInstanceListSupplier;
import org.springframework.cloud.loadbalancer.support.LoadBalancerClientFactory;
import org.springframework.context.annotation.Bean;
import org.springframework.core.env.Environment;

public class CustomLoadBalancerConfiguration {

    @Bean
    ReactorLoadBalancer<ServiceInstance> randomLoadBalancer(Environment environment,
                                                            LoadBalancerClientFactory loadBalancerClientFactory) {
        String name = environment.getProperty(LoadBalancerClientFactory.PROPERTY_NAME);
        return new RandomLoadBalancer(loadBalancerClientFactory
                .getLazyProvider(name, ServiceInstanceListSupplier.class),
                name);
    }
}

RestTemplateConfig配置類LoadBalancerClient指定隨機的配置類，value的值為提供者也即是庫存微服務名稱。

package cn.itxs.ecom.order.config;

import org.springframework.boot.web.client.RestTemplateBuilder;
import org.springframework.cloud.client.loadbalancer.LoadBalanced;
import org.springframework.cloud.loadbalancer.annotation.LoadBalancerClient;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.client.RestTemplate;

@Configuration
@LoadBalancerClient(value = "ecom-storage-service", configuration = CustomLoadBalancerConfiguration.class)
public class RestTemplateConfig {

    @LoadBalanced
    @Bean
    public RestTemplate restTemplate(RestTemplateBuilder builder) {
        return builder.build() ;
    }
}

多次訪問訂單dedect介面測試確認已切換為隨機負載均衡策略。

整合方式

官網提供3中整合Spring Cloud LoadBalancer的方式，除了第一種上面已使用過，還支援Spring Web Flux響應式程式設計，WebClient是從Spring WebFlux 5.0版本開始提供的一個非阻塞的基於響應式程式設計的進行Http請求的客戶端工具。它的響應式程式設計的基於Reactor的。WebClient中提供了標準Http請求方式對應的get、post、put、delete等方法，可以用來發起相應的請求。

在訂單微服務中引入spring-boot-starter-webflux依賴

        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-starter-webflux</artifactId>
        </dependency>

訂單微服務中增加WebClientConfig配置類

package cn.itxs.ecom.order.config;

import org.springframework.cloud.client.loadbalancer.LoadBalanced;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.web.reactive.function.client.WebClient;

@Configuration
public class WebClientConfig {
    @LoadBalanced
    @Bean
    WebClient.Builder webClientBuilder() {
        return WebClient.builder();
    }

    @Bean
    WebClient webClient() {
        return webClientBuilder().build();
    }
}

訂單微服務訂單控制器中新增WebClient介面實現，程式碼如下

    @Autowired
    private WebClient webClient;   

	@RequestMapping(value = "/deductWebClient/{commodityCode}/{count}")
    public Mono<String> deductWebClient(@PathVariable("commodityCode") String commodityCode, @PathVariable("count") int count) {
        String url = "http://ecom-storage-service/deduct/"+commodityCode+"/"+count;
        // 基於WebClient
        Mono<String> result = webClient.get().uri(url)
                .retrieve().bodyToMono(String.class);
        return result;
    }

重新啟動訂單微服務

訪問訂單控制器中的減庫存WebClient介面，http://localhost:4070/deductWebClient/1001/1 ，結果返回成功

我們還可以配置基於過濾器的方式，通過WebClient使用ReactiveLoadBalancer。如果專案中新增了Spring Cloud LoadBalancer starter，並且Spring -webflux在類路徑中，ReactorLoadBalancerExchangeFilterFunction則是自動配置的。

訂單微服務訂單控制器中新增WebClient使用ReactiveLoadBalancer介面實現，程式碼如下

    @Autowired
    private ReactorLoadBalancerExchangeFilterFunction lbFunction;   

    @RequestMapping(value = "/deductWebFluxReactor/{commodityCode}/{count}")
    public Mono<String> deductWebFluxReactor(@PathVariable("commodityCode") String commodityCode, @PathVariable("count") int count) {
        String url = "/deduct/"+commodityCode+"/"+count;
        Mono<String> result = WebClient.builder().baseUrl("http://ecom-storage-service")
                .filter(lbFunction)
                .build()
                .get()
                .uri(url)
                .retrieve()
                .bodyToMono(String.class);
        return result;
    }

重新啟動訂單微服務

訪問訂單控制器中的減庫存WebClient介面，http://localhost:4070/deductWebFluxReactor/1001/1 ，結果返回成功

關於LoadBalancer官網還提供很多其他功能，有興趣可自行詳細查閱和動手實驗

原理

RestTemplate

Spring Cloud LoadBalancer原始碼分析我們先從RestTemplate負載均衡的簡單實現來分析入手，除此之外其支援Spring Web Flux響應式程式設計的實現原理思想也是相同，都是通過客戶端新增攔截器，在攔截器中實現負載均衡。從RestTemplate的原始碼中可以知道其繼承自InterceptingHttpAccessor抽象類

而InterceptingHttpAccessor抽象類則提供了一個方法setInterceptors，用於設定攔截器，攔截器需要實現ClientHttpRequestInterceptor介面即可，在實際遠端請求服務端介面之前會先呼叫攔截器的intercept方法。這裡的攔截器相當於Servlet技術中的Filter功能

	// 程式碼實現在抽象父類InterceptingHttpAccessor裡
	// RestTemplate.InterceptingHttpAccessor#setInterceptors
	public void setInterceptors(List<ClientHttpRequestInterceptor> interceptors) {
		Assert.noNullElements(interceptors, "'interceptors' must not contain null elements");
		// Take getInterceptors() List as-is when passed in here
		if (this.interceptors != interceptors) {
			this.interceptors.clear();
			this.interceptors.addAll(interceptors);
			AnnotationAwareOrderComparator.sort(this.interceptors);
		}
	}

LoadBalancerAutoConfiguration

從官網可以知道Spring Cloud LoadBalancer放在spring-cloud-commons，因此也作為其核心的@LoadBalanced註解也就是由spring-cloud-commons來實現，依據SpringBoot自動裝配的原理先檢視依賴包的實現邏輯，不難發現spring-cloud-commons引入了自動配置類LoadBalancerAutoConfiguration和ReactorLoadBalancerClientAutoConfiguration。

當滿足上述的條件時（@Conditional為條件註解），將自動建立LoadBalancerInterceptor並注入到RestTemplate中。

LoadBalancerLnterceptor

LoadBalancerInterceptor實現了ClientHttpRequestInterceptor介面，因此也實現intercept方法，用於實現負載均衡的攔截處理。

LoadBalancerClient

LoadBalancerClient用於進行負載均衡邏輯，繼承自ServiceInstanceChooser介面，從服務列表中選擇出一個服務地址進行呼叫。在LoadBalancerClient種存在兩個execute()方法，均是用來執行請求的，reconstructURI()是用來重構URL。

對於LoadBalancerClient介面Spring Cloud LoadBalancer的提供預設實現為BlockingLoadBalancerClient

@SuppressWarnings({ "unchecked", "rawtypes" })
public class BlockingLoadBalancerClient implements LoadBalancerClient {

	private final ReactiveLoadBalancer.Factory<ServiceInstance> loadBalancerClientFactory;

	/**
	 * @deprecated in favour of
	 * {@link BlockingLoadBalancerClient#BlockingLoadBalancerClient(ReactiveLoadBalancer.Factory)}
	 */
	@Deprecated
	public BlockingLoadBalancerClient(LoadBalancerClientFactory loadBalancerClientFactory,
			LoadBalancerProperties properties) {
		this.loadBalancerClientFactory = loadBalancerClientFactory;
	}

	public BlockingLoadBalancerClient(ReactiveLoadBalancer.Factory<ServiceInstance> loadBalancerClientFactory) {
		this.loadBalancerClientFactory = loadBalancerClientFactory;
	}

	@Override
	public <T> T execute(String serviceId, LoadBalancerRequest<T> request) throws IOException {
		String hint = getHint(serviceId);
		LoadBalancerRequestAdapter<T, DefaultRequestContext> lbRequest = new LoadBalancerRequestAdapter<>(request,
				new DefaultRequestContext(request, hint));
		Set<LoadBalancerLifecycle> supportedLifecycleProcessors = getSupportedLifecycleProcessors(serviceId);
		supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onStart(lbRequest));
		ServiceInstance serviceInstance = choose(serviceId, lbRequest);
        // 選擇服務
		if (serviceInstance == null) {
			supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onComplete(
					new CompletionContext<>(CompletionContext.Status.DISCARD, lbRequest, new EmptyResponse())));
			throw new IllegalStateException("No instances available for " + serviceId);
		}
		return execute(serviceId, serviceInstance, lbRequest);
	}

	@Override
	public <T> T execute(String serviceId, ServiceInstance serviceInstance, LoadBalancerRequest<T> request)
			throws IOException {
		DefaultResponse defaultResponse = new DefaultResponse(serviceInstance);
		Set<LoadBalancerLifecycle> supportedLifecycleProcessors = getSupportedLifecycleProcessors(serviceId);
		Request lbRequest = request instanceof Request ? (Request) request : new DefaultRequest<>();
		supportedLifecycleProcessors
				.forEach(lifecycle -> lifecycle.onStartRequest(lbRequest, new DefaultResponse(serviceInstance)));
		try {
			T response = request.apply(serviceInstance);
			Object clientResponse = getClientResponse(response);
			supportedLifecycleProcessors
					.forEach(lifecycle -> lifecycle.onComplete(new CompletionContext<>(CompletionContext.Status.SUCCESS,
							lbRequest, defaultResponse, clientResponse)));
			return response;
		}
		catch (IOException iOException) {
			supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onComplete(
					new CompletionContext<>(CompletionContext.Status.FAILED, iOException, lbRequest, defaultResponse)));
			throw iOException;
		}
		catch (Exception exception) {
			supportedLifecycleProcessors.forEach(lifecycle -> lifecycle.onComplete(
					new CompletionContext<>(CompletionContext.Status.FAILED, exception, lbRequest, defaultResponse)));
			ReflectionUtils.rethrowRuntimeException(exception);
		}
		return null;
	}

	private <T> Object getClientResponse(T response) {
		ClientHttpResponse clientHttpResponse = null;
		if (response instanceof ClientHttpResponse) {
			clientHttpResponse = (ClientHttpResponse) response;
		}
		if (clientHttpResponse != null) {
			try {
				return new ResponseData(clientHttpResponse, null);
			}
			catch (IOException ignored) {
			}
		}
		return response;
	}

	private Set<LoadBalancerLifecycle> getSupportedLifecycleProcessors(String serviceId) {
		return LoadBalancerLifecycleValidator.getSupportedLifecycleProcessors(
				loadBalancerClientFactory.getInstances(serviceId, LoadBalancerLifecycle.class),
				DefaultRequestContext.class, Object.class, ServiceInstance.class);
	}

	@Override
	public URI reconstructURI(ServiceInstance serviceInstance, URI original) {
		return LoadBalancerUriTools.reconstructURI(serviceInstance, original);
	}

	@Override
	public ServiceInstance choose(String serviceId) {
		return choose(serviceId, REQUEST);
	}

    // 通過不同的負載均衡客戶端實現選擇不同的服務
	@Override
	public <T> ServiceInstance choose(String serviceId, Request<T> request) {
		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();
	}

	private String getHint(String serviceId) {
		LoadBalancerProperties properties = loadBalancerClientFactory.getProperties(serviceId);
		String defaultHint = properties.getHint().getOrDefault("default", "default");
		String hintPropertyValue = properties.getHint().get(serviceId);
		return hintPropertyValue != null ? hintPropertyValue : defaultHint;
	}

}

LoadBalancerClientFactory

BlockingLoadBalancerClient中持有LoadBalancerClientFactory通過呼叫其getInstance方法獲取具體的負載均衡客戶端。通過工廠類LoadBalancerClientFactory獲取具體的負載均衡器例項，後面的loadBalancer.choose(request)呼叫其介面choose()方法實現根據負載均衡演算法選擇下一個伺服器完成負載均衡，而ReactiveLoadBalancer getInstance(String serviceId) 有預設實現LoadBalancerClientFactory

LoadBalancerClientFactory客戶端實現了不同的負載均衡演算法，比如輪詢、隨機等。LoadBalancerClientFactory繼承自NamedContextFactory，NamedContextFactory繼承ApplicationContextAware，實現Spring ApplicationContext容器操作。

ReactiveLoadBalancer

ReactiveLoadBalancer負載均衡器實現服務選擇，Spring Cloud Balancer中實現了輪詢RoundRobinLoadBalancer、隨機RandomLoadBalancer、NacosLoadBalancer演算法。

LoadBalancerClientConfiguration

如果沒有顯式指定負載均衡演算法，預設預設值為RoundRobinLoadBalancer

	@Bean
	@ConditionalOnMissingBean
	public ReactorLoadBalancer<ServiceInstance> reactorServiceInstanceLoadBalancer(Environment environment,
			LoadBalancerClientFactory loadBalancerClientFactory) {
		String name = environment.getProperty(LoadBalancerClientFactory.PROPERTY_NAME);
		return new RoundRobinLoadBalancer(
				loadBalancerClientFactory.getLazyProvider(name, ServiceInstanceListSupplier.class), name);
	}

LoadBalancerRequestFactory

LoadBalancerRequest工廠類呼叫createRequest方法用於建立LoadBalancerRequest。其內部持有LoadBalancerClient物件也即持有BlockingLoadBalancerClient。

在日常專案中，一般負載均衡都是結合Feign使用，後續我們有時間再來分析Feign整合LoadBalancer的自動配置類FeignLoadBalancerAutoConfiguration的實現

ReactorLoadBalancerClientAutoConfiguration

我們也拋一下基於WebClient的@Loadbalanced的流程的引入，首先宣告負載均衡過濾器ReactorLoadBalancerClientAutoConfiguration是一個自動裝配器類，在專案中引入了 WebClient 和 ReactiveLoadBalancer 類之後，自動裝配流程就開始執行，它會初始化一個實現了 ExchangeFilterFunction 的例項，在後面該例項將作為過濾器被注入到WebClient。後續流程有興趣再自行研究

自定義負載均衡器

從上面可以知道LoadBalancerClientFactory是建立客戶機、負載均衡器和客戶機配置例項的工廠。它根據客戶端名稱建立一個Spring ApplicationContext，並從中提取所需的bean。因此進入到LoadBalancerClientFactory類中，需要去實現它的子介面ReactorServiceInstanceLoadBalancer，因為去獲取負載均衡器例項的時候，是通過去容器中查詢ReactorServiceInstanceLoadBalancer型別的bean來實現的，可以參照RandomLoadBalancer實現程式碼

package org.springframework.cloud.loadbalancer.core;

import java.util.List;
import java.util.concurrent.ThreadLocalRandom;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import reactor.core.publisher.Mono;

import org.springframework.beans.factory.ObjectProvider;
import org.springframework.cloud.client.ServiceInstance;
import org.springframework.cloud.client.loadbalancer.DefaultResponse;
import org.springframework.cloud.client.loadbalancer.EmptyResponse;
import org.springframework.cloud.client.loadbalancer.Request;
import org.springframework.cloud.client.loadbalancer.Response;

public class RandomLoadBalancer implements ReactorServiceInstanceLoadBalancer {

	private static final Log log = LogFactory.getLog(RandomLoadBalancer.class);

	private final String serviceId;

	private ObjectProvider<ServiceInstanceListSupplier> serviceInstanceListSupplierProvider;

	/**
	 * @param serviceInstanceListSupplierProvider a provider of
	 * {@link ServiceInstanceListSupplier} that will be used to get available instances
	 * @param serviceId id of the service for which to choose an instance
	 */
	public RandomLoadBalancer(ObjectProvider<ServiceInstanceListSupplier> serviceInstanceListSupplierProvider,
			String serviceId) {
		this.serviceId = serviceId;
		this.serviceInstanceListSupplierProvider = serviceInstanceListSupplierProvider;
	}

	@SuppressWarnings("rawtypes")
	@Override
	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) {
		if (instances.isEmpty()) {
			if (log.isWarnEnabled()) {
				log.warn("No servers available for service: " + serviceId);
			}
			return new EmptyResponse();
		}
		int index = ThreadLocalRandom.current().nextInt(instances.size());

		ServiceInstance instance = instances.get(index);

		return new DefaultResponse(instance);
	}

}

保留核心實現進行簡單仿寫如下

package cn.itxs.ecom.order.config;

import java.util.List;
import java.util.Random;
import org.springframework.beans.factory.ObjectProvider;
import org.springframework.cloud.client.ServiceInstance;
import org.springframework.cloud.loadbalancer.core.ReactorServiceInstanceLoadBalancer;
import org.springframework.cloud.loadbalancer.core.ServiceInstanceListSupplier;
import org.springframework.cloud.client.loadbalancer.DefaultResponse;
import org.springframework.cloud.client.loadbalancer.EmptyResponse;
import org.springframework.cloud.client.loadbalancer.Request;
import org.springframework.cloud.client.loadbalancer.Response;
import reactor.core.publisher.Mono;

public class ItxsRandomLoadBalancerClient implements ReactorServiceInstanceLoadBalancer {
    // 服務列表
    private ObjectProvider<ServiceInstanceListSupplier> serviceInstanceListSupplierProvider;

    public ItxsRandomLoadBalancerClient(ObjectProvider<ServiceInstanceListSupplier> serviceInstanceListSupplierProvider) {
        this.serviceInstanceListSupplierProvider = serviceInstanceListSupplierProvider;
    }

    @Override
    public Mono<Response<ServiceInstance>> choose(Request request) {
        ServiceInstanceListSupplier supplier = serviceInstanceListSupplierProvider.getIfAvailable();
        return supplier.get().next().map(this::getInstanceResponse);
    }

    /**
     * 使用隨機數獲取服務
     * @param instances
     * @return
     */
    private Response<ServiceInstance> getInstanceResponse(
            List<ServiceInstance> instances) {
        System.out.println("ItxsRandomLoadBalancerClient start");
        if (instances.isEmpty()) {
            return new EmptyResponse();
        }

        System.out.println("ItxsRandomLoadBalancerClient random");
        // 隨機演算法
        int size = instances.size();
        Random random = new Random();
        ServiceInstance instance = instances.get(random.nextInt(size));

        return new DefaultResponse(instance);
    }
}

將上面CustomLoadBalancerConfiguration替換為如下內容

package cn.itxs.ecom.order.config;

import org.springframework.beans.factory.ObjectProvider;
import org.springframework.cloud.loadbalancer.core.ReactorServiceInstanceLoadBalancer;
import org.springframework.cloud.loadbalancer.core.ServiceInstanceListSupplier;
import org.springframework.context.annotation.Bean;

public class CustomLoadBalancerConfiguration {

    @Bean
    public ReactorServiceInstanceLoadBalancer customLoadBalancer(ObjectProvider<ServiceInstanceListSupplier> serviceInstanceListSupplierProvider) {
        return new ItxsRandomLoadBalancerClient(serviceInstanceListSupplierProvider);
    }
}

啟動庫存微服務和訂單微服務，訪問http://localhost:4070/deductRest/1001/1 ，控制檯已列印自定義ItxsRandomLoadBalancerClient中的日誌和成功訪問結果

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