polaris load balancer: 提取负载均衡器公共抽象基类，实现加权轮训负载均衡

--issue=968

spring-cloud-starter-tencent-polaris-discovery/src/main/java/com/tencent/cloud/polaris/loadbalancer/PolarisAbstractLoadBalancer.java

@@ -0,0 +1,82 @@
+package com.tencent.cloud.polaris.loadbalancer;
+
+import com.tencent.cloud.common.metadata.MetadataContext;
+import com.tencent.cloud.common.pojo.PolarisServiceInstance;
+import com.tencent.polaris.api.pojo.DefaultServiceInstances;
+import com.tencent.polaris.api.pojo.Instance;
+import com.tencent.polaris.api.pojo.ServiceInstances;
+import com.tencent.polaris.api.pojo.ServiceKey;
+import com.tencent.polaris.router.api.core.RouterAPI;
+import com.tencent.polaris.router.api.rpc.ProcessLoadBalanceRequest;
+import com.tencent.polaris.router.api.rpc.ProcessLoadBalanceResponse;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+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;
+import org.springframework.cloud.loadbalancer.core.NoopServiceInstanceListSupplier;
+import org.springframework.cloud.loadbalancer.core.ReactorServiceInstanceLoadBalancer;
+import org.springframework.cloud.loadbalancer.core.ServiceInstanceListSupplier;
+import reactor.core.publisher.Mono;
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+/**
+ * Abstract Loadbalancer of Polaris.
+ *
+ * @author <a href="mailto:veteranchen@tencent.com">veteranchen</a>
+ */
+public abstract class PolarisAbstractLoadBalancer implements ReactorServiceInstanceLoadBalancer {
+
+	private static final Logger log = LoggerFactory.getLogger(PolarisAbstractLoadBalancer.class);
+
+	private final String serviceId;
+
+	private final RouterAPI routerAPI;
+
+	private ObjectProvider<ServiceInstanceListSupplier> supplierObjectProvider;
+
+	public PolarisAbstractLoadBalancer(String serviceId, ObjectProvider<ServiceInstanceListSupplier> supplierObjectProvider, RouterAPI routerAPI) {
+		this.serviceId = serviceId;
+		this.supplierObjectProvider = supplierObjectProvider;
+		this.routerAPI = routerAPI;
+	}
+
+	private static ServiceInstances convertToPolarisServiceInstances(List<ServiceInstance> serviceInstances) {
+		ServiceKey serviceKey = new ServiceKey(MetadataContext.LOCAL_NAMESPACE, serviceInstances.get(0).getServiceId());
+		List<Instance> polarisInstances = serviceInstances.stream()
+				.map(serviceInstance -> ((PolarisServiceInstance) serviceInstance).getPolarisInstance())
+				.collect(Collectors.toList());
+		return new DefaultServiceInstances(serviceKey, polarisInstances);
+	}
+
+	@Override
+	public Mono<Response<ServiceInstance>> choose(Request request) {
+		ServiceInstanceListSupplier supplier = supplierObjectProvider
+				.getIfAvailable(NoopServiceInstanceListSupplier::new);
+		return supplier.get(request).next().map(serviceInstances -> {
+			if (serviceInstances.isEmpty()) {
+				log.warn("No servers available for service: " + this.serviceId);
+				return new EmptyResponse();
+			}
+
+			ProcessLoadBalanceRequest req = new ProcessLoadBalanceRequest();
+			req.setDstInstances(convertToPolarisServiceInstances(serviceInstances));
+			req = setProcessLoadBalanceRequest(req);
+
+			try {
+				ProcessLoadBalanceResponse response = routerAPI.processLoadBalance(req);
+				return new DefaultResponse(new PolarisServiceInstance(response.getTargetInstance()));
+			} catch (Exception e) {
+				log.warn("PolarisRoutingLoadbalancer error", e);
+				return new EmptyResponse();
+			}
+		});
+	}
+
+	protected abstract ProcessLoadBalanceRequest setProcessLoadBalanceRequest(ProcessLoadBalanceRequest req);
+}