customize getting service instance in service subscribe.

spring-cloud-alibaba-dubbo/src/main/java/com/alibaba/cloud/dubbo/autoconfigure/DubboMetadataAutoConfiguration.java

@@ -16,8 +16,10 @@
 package com.alibaba.cloud.dubbo.autoconfigure;
 
 import java.util.Collection;
+import java.util.Optional;
 import java.util.function.Supplier;
 
+import com.alibaba.cloud.dubbo.metadata.repository.MetadataServiceInstanceSelector;
 import org.apache.dubbo.config.ProtocolConfig;
 import org.apache.dubbo.config.spring.ServiceBean;
 import org.apache.dubbo.config.spring.context.event.ServiceBeanExportedEvent;
@@ -42,6 +44,7 @@ import com.alibaba.cloud.dubbo.service.IntrospectiveDubboMetadataService;
 import com.alibaba.cloud.dubbo.util.JSONUtils;
 
 import feign.Contract;
+import org.springframework.util.CollectionUtils;
 
 /**
  * Spring Boot Auto-Configuration class for Dubbo Metadata
@@ -68,6 +71,15 @@ public class DubboMetadataAutoConfiguration {
 		return new DubboServiceBeanMetadataResolver(contract);
 	}
 
+	@Bean
+	@ConditionalOnMissingBean
+	public MetadataServiceInstanceSelector metadataServiceInstanceSelector() {
+		return serviceInstances ->
+				CollectionUtils.isEmpty(serviceInstances)
+						? Optional.empty()
+						: serviceInstances.stream().findAny();
+	}
+
 	@Bean
 	public Supplier<ProtocolConfig> dubboProtocolConfigSupplier(
 			ObjectProvider<Collection<ProtocolConfig>> protocols) {

spring-cloud-alibaba-dubbo/src/main/java/com/alibaba/cloud/dubbo/metadata/repository/DubboServiceMetadataRepository.java

@@ -165,6 +165,9 @@ public class DubboServiceMetadataRepository
 	@Autowired
 	private DiscoveryClient discoveryClient;
 
+	@Autowired
+	private MetadataServiceInstanceSelector metadataServiceInstanceSelector;
+
 	@Autowired
 	private JSONUtils jsonUtils;
 
@@ -618,7 +621,7 @@ public class DubboServiceMetadataRepository
 	}
 
 	protected void initSubscribedDubboMetadataService(String serviceName) {
-		discoveryClient.getInstances(serviceName).stream().findAny()
+		metadataServiceInstanceSelector.choose(discoveryClient.getInstances(serviceName))
 				.map(this::getDubboMetadataServiceURLs)
 				.ifPresent(dubboMetadataServiceURLs -> {
 					dubboMetadataServiceURLs.forEach(dubboMetadataServiceURL -> {

spring-cloud-alibaba-dubbo/src/main/java/com/alibaba/cloud/dubbo/metadata/repository/MetadataServiceInstanceSelector.java

@@ -0,0 +1,23 @@
+package com.alibaba.cloud.dubbo.metadata.repository;
+
+import org.springframework.cloud.client.ServiceInstance;
+
+import java.util.List;
+import java.util.Optional;
+
+/**
+ * metadata service instance selector
+ *
+ * @author liuxx
+ * @date 2019/9/4
+ */
+public interface MetadataServiceInstanceSelector {
+
+
+    /**
+     * choose a service instance to get metadata
+     * @param serviceInstances all service instance
+     * @return the service instance to get metadata
+     */
+    Optional<ServiceInstance> choose(List<ServiceInstance> serviceInstances);
+}