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操作步骤如下：
1. k8s环境下，consumer端开启mesh模式
2. consumer端需要调用多个provider
3. provider有应用级注册和接口级注册，并且需要的provider不再同一个namespace下
4. 原有项目存在dubbo2.6与3.1多版本间的相互调用，在开启消费端mesh（dubbo3.1）后，无法调用到2.6的服务提供者

**问题：**

1. 如何实现在一个consumer下，调用多个namespace下的provider，并且provider有应用级注册和接口级注册
2. dubbo3.1消费端在开启mesh后，如何调用版本是2.6的服务提供者
import static org.apache.dubbo.common.constants.LoggerCodeConstants.CONFIG_FAILED_LOAD_ENV_VARIABLE;
import static org.apache.dubbo.common.constants.LoggerCodeConstants.CONFIG_PROPERTY_CONFLICT;
import static org.apache.dubbo.common.constants.RegistryConstants.PROVIDED_BY;
import static org.apache.dubbo.common.constants.RegistryConstants.SUBSCRIBED_SERVICE_NAMES_KEY;
import static org.apache.dubbo.common.utils.NetUtils.isInvalidLocalHost;
import static org.apache.dubbo.common.utils.StringUtils.splitToSet;
import static org.apache.dubbo.config.Constants.DUBBO_IP_TO_REGISTRY;
import static org.apache.dubbo.registry.Constants.CONSUMER_PROTOCOL;
import static org.apache.dubbo.registry.Constants.REGISTER_IP_KEY;
import static org.apache.dubbo.rpc.Constants.GENERIC_KEY;
import static org.apache.dubbo.rpc.Constants.LOCAL_PROTOCOL;
import static org.apache.dubbo.rpc.cluster.Constants.PEER_KEY;
import static org.apache.dubbo.rpc.cluster.Constants.REFER_KEY;
/**
 * Please avoid using this class for any new application,
 * use {@link ReferenceConfigBase} instead.
 */
public class ReferenceConfig<T> extends ReferenceConfigBase<T> {
    public static final ErrorTypeAwareLogger logger = LoggerFactory.getErrorTypeAwareLogger(ReferenceConfig.class);
    /**
     * The {@link Protocol} implementation with adaptive functionality,it will be different in different scenarios.
     * A particular {@link Protocol} implementation is determined by the protocol attribute in the {@link URL}.
     * For example:
     *
     * <li>when the url is registry://224.5.6.7:1234/org.apache.dubbo.registry.RegistryService?application=dubbo-sample,
     * then the protocol is <b>RegistryProtocol</b></li>
     *
     * <li>when the url is dubbo://224.5.6.7:1234/org.apache.dubbo.config.api.DemoService?application=dubbo-sample, then
     * the protocol is <b>DubboProtocol</b></li>
     * <p>