Spring Cloud LoadBalancer服务解析失败问题排查求助

问题场景 #

按照Spring Cloud LoadBalancer指南实现客户端负载均衡，调用http://my-service/endpoint时抛出UnknownHostException，无法解析服务名。相关代码及配置如下：

Controller代码 #

@RestController
class MyController(
    val loadBalancedWebClientBuilder: WebClient.Builder
) { 
    private fun makeRequest() {
        loadBalancedWebClientBuilder.build().post()
                .uri(URI("http://my-service/endpoint"))
                .body(BodyInserters.fromValue("{ \"data\" = \"data\" }"))
                .retrieve()
                .bodyToMono(String::class.java)
                .block()
    }
}

WebClient配置 #

@Configuration
@LoadBalancerClient(name = "my-service", configuration = [LoadBalancerConfig::class])
class WebClientConfig {
    @LoadBalanced
    @Bean
    fun webClientBuilder(): WebClient.Builder {
        return WebClient.builder()
    }
}

负载均衡配置 #

class LoadBalancerConfig {
    @Bean
    @Primary
    fun serviceInstanceListSupplier(): ServiceInstanceListSupplier {
        return ServiceInstanceListSupplerImpl("my-service")
    }
}

class ServiceInstanceListSupplerImpl(private val serviceId: String) : ServiceInstanceListSupplier {
    override fun getServiceId(): String {
        return serviceId
    }

    override fun get(): Flux<List<ServiceInstance>> {
        return Flux.just(listOf("http://localhost").mapIndexed { index, s -
            DefaultServiceInstance(index.toString(), serviceId, s, 8088, true) })
    }

}

配置文件 #

spring:
  cloud:
    discovery:
      client:
        simple:
          instances:
            my-service[0].uri: http://localhost:8088

异常信息 #

org.springframework.web.reactive.function.client.WebClientRequestException: Failed to resolve 'my-service' after 4 queries ; nested exception is java.net.UnknownHostException: Failed to resolve 'my-service' after 4 queries 
    at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.lambda$wrapException$9(ExchangeFunctions.java:141) ~[spring-webflux-5.3.24.jar:5.3.24]
    Suppressed: reactor.core.publisher.FluxOnAssembly$OnAssemblyException: 
Error has been observed at the following site(s):
    *__checkpoint ⇢ Request to POST http://my-service/endpoint [DefaultWebClient]
Original Stack Trace:
        at org.springframework.web.reactive.function.client.ExchangeFunctions$DefaultExchangeFunction.lambda$wrapException$9(ExchangeFunctions.java:141) ~[spring-webflux-5.3.24.jar:5.3.24]
        at reactor.core.publisher.MonoErrorSupplied.subscribe(MonoErrorSupplied.java:55) ~[reactor-core-3.4.25.jar:3.4.25]
        ...

问题原因及解决方法 #

1. WebClient URI 使用方式错误 #

问题：代码中使用URI("http://my-service/endpoint")创建URI对象，会绕过Spring Cloud LoadBalancer的服务名解析逻辑，直接将my-service当作主机名解析，导致DNS查询失败。

解决：改用字符串形式传入URI，让LoadBalanced WebClient能识别并替换服务名：

.uri("http://my-service/endpoint")

2. 自定义服务实例供应商类存在语法错误 #

问题：

• 类名拼写错误：ServiceInstanceListSupplerImpl应为ServiceInstanceListSupplierImpl（Suppler → Supplier）
• Lambda表达式语法错误：mapIndexed { index, s - 应为 mapIndexed { index, s ->

这些错误会导致该类无法正常编译或实例化，LoadBalancer无法获取服务实例列表。

解决：修正类名和语法错误：

class LoadBalancerConfig {
    @Bean
    @Primary
    fun serviceInstanceListSupplier(): ServiceInstanceListSupplier {
        return ServiceInstanceListSupplierImpl("my-service")
    }
}

class ServiceInstanceListSupplierImpl(private val serviceId: String) : ServiceInstanceListSupplier {
    override fun getServiceId(): String {
        return serviceId
    }

    override fun get(): Flux<List<ServiceInstance>> {
        return Flux.just(listOf("localhost").mapIndexed { index, s ->
            DefaultServiceInstance(index.toString(), serviceId, s, 8088, true)
        })
    }
}

注意：DefaultServiceInstance的第三个参数是主机名（不是完整URL），所以传入localhost而非http://localhost

3. 简单发现客户端配置格式错误 #

问题：配置文件中my-service[0].uri的格式不符合Spring Boot YAML数组配置规范，导致SimpleDiscoveryClient无法正确加载服务实例。

解决：修正为正确的数组格式：

spring:
  cloud:
    discovery:
      client:
        simple:
          instances:
            my-service:
              - uri: http://localhost:8088

4. 自定义配置与默认发现客户端冲突 #

问题：LoadBalancerConfig中使用@Primary注解标记自定义ServiceInstanceListSupplier，但如果同时启用了SimpleDiscoveryClient，可能存在Bean加载优先级冲突，导致自定义供应商未被正确使用。

解决：

• 如果不需要自定义供应商，可删除LoadBalancerConfig及相关类，仅依靠SimpleDiscoveryClient配置即可
• 如果需要自定义供应商，确保类无语法错误，且@LoadBalancerClient注解正确关联到该配置类

内容的提问来源于stack exchange，提问作者Vlad