关于springboot:Did-you-forget-to-include-springcloudstarterloadbalancer原因探究

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前提：本案例是基于 spring boot 2.7.5,spring cloud 3.1.6，其余版本可能会有些差异！

对于“Did you forget to include spring-cloud-starter-loadbalancer？”报错的起因，网上有一大堆博客供大家参考，这里我给大家提供一种新的参考。

先说论断：

 报“Did you forget to include spring-cloud-starter-loadbalancer”的起因之一是可能没有引入 "spring-cloud-loadbalancer" pom 依赖。

spring boot 启动时执行 org.springframework.cloud.openfeign.FeignClientFactoryBean.loadBalance() 时，getOptional() 范畴为 null，而后抛出异样“Caused by: java.lang.IllegalStateException: No Feign Client for loadBalancing defined. Did you forget to include spring-cloud-starter-loadbalancer?”;

protected <T> T loadBalance(Feign.Builder builder, FeignContext context, HardCodedTarget<T> target) {
        // 返回为 null
        Client client = getOptional(context, Client.class);
        if (client != null) {builder.client(client);
            applyBuildCustomizers(context, builder);
            Targeter targeter = get(context, Targeter.class);
            return targeter.target(this, builder, context, target);
        }
        // 到这里抛出异样
        throw new IllegalStateException("No Feign Client for loadBalancing defined. Did you forget to include spring-cloud-starter-loadbalancer?");
    }

也就是说，DefaultListableBeanFactory 外面没有类型为“feign.Client”的 bean; 上面的图是以后我的项目里所有“feign.Client”的实现类：

图一

好的，我就拿 Default 类做为入口剖析一波：

@Configuration(proxyBeanMethods = false)
@EnableConfigurationProperties(LoadBalancerClientsProperties.class)
class DefaultFeignLoadBalancerConfiguration {

    @Bean
    @ConditionalOnMissingBean
    @Conditional(OnRetryNotEnabledCondition.class)
    public Client feignClient(LoadBalancerClient loadBalancerClient,
            LoadBalancerClientFactory loadBalancerClientFactory) {return new FeignBlockingLoadBalancerClient(new Client.Default(null, null), loadBalancerClient,
                loadBalancerClientFactory);
    }
}

也就是说，在 DefaultFeignLoadBalancerConfiguration 类中会创立 FeignBlockingLoadBalancerClient 实例，而这个 FeignBlockingLoadBalancerClient

public class FeignBlockingLoadBalancerClient implements Client {private static final Log LOG = LogFactory.getLog(FeignBlockingLoadBalancerClient.class);

    private final Client delegate;

    private final LoadBalancerClient loadBalancerClient;

    private final LoadBalancerClientFactory loadBalancerClientFactory;
}

同样也是 FeignClient 的实现类。而后咱们接着往下看，发现

@ConditionalOnClass(Feign.class)
@ConditionalOnBean({LoadBalancerClient.class, LoadBalancerClientFactory.class})
@AutoConfigureBefore(FeignAutoConfiguration.class)
@AutoConfigureAfter({BlockingLoadBalancerClientAutoConfiguration.class, LoadBalancerAutoConfiguration.class})
@EnableConfigurationProperties(FeignHttpClientProperties.class)
@Configuration(proxyBeanMethods = false)
// Order is important here, last should be the default, first should be optional
// see
// https://github.com/spring-cloud/spring-cloud-netflix/issues/2086#issuecomment-316281653
@Import({ HttpClientFeignLoadBalancerConfiguration.class, OkHttpFeignLoadBalancerConfiguration.class,
        HttpClient5FeignLoadBalancerConfiguration.class, DefaultFeignLoadBalancerConfiguration.class })
public class FeignLoadBalancerAutoConfiguration {}

FeignLoadBalancerAutoConfiguration 主动配置类，引入了 DefaultFeignLoadBalancerConfiguration 主动配置类，失常状况下，我的项目启动时，FeignBlockingLoadBalancerClient 会被创立，然而因为 FeignBlockingLoadBalancerClient 的构造方法会依赖 LoadBalancerClientFactory，而 LoadBalancerClientFactory 存在“spring-cloud-loadbalancer”jar 包中，如果没有引入这个 jar, 我的项目启动时，会创立 FeignBlockingLoadBalancerClient 失败，最终会导致 DefaultListableBeanFactory 外面没有类型为“feign.Client”的 bean 实例，所以咱们启动我的项目时会报错“No Feign Client for loadBalancing defined. Did you forget to include spring-cloud-starter-loadbalancer?”！

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