MigrationRuleListener类中有以下缺陷

1、
```java
    public synchronized void process(ConfigChangedEvent event) {
        // 如果有其他Reference未调用onRefer方法，此时配置中心传入空配置，会导致后续onRefer方法调用失败
        rawRule = event.getContent();
        if (StringUtils.isEmpty(rawRule)) {
            logger.warn("Received empty migration rule, will ignore.");
            return;
        }

        logger.info("Using the following migration rule to migrate:");
        logger.info(rawRule);

        rule = parseRule(rawRule);

        if (CollectionUtils.isNotEmptyMap(handlers)) {
            handlers.forEach((_key, handler) -> handler.doMigrate(rule));
        }
    }

    @Override
    public synchronized void onRefer(RegistryProtocol registryProtocol, ClusterInvoker<?> invoker, URL consumerUrl, URL registryURL) {
        MigrationRuleHandler<?> migrationRuleHandler = handlers.computeIfAbsent((MigrationInvoker<?>) invoker, _key -> {
            ((MigrationInvoker<?>) invoker).setMigrationRuleListener(this);
            return new MigrationRuleHandler<>((MigrationInvoker<?>) invoker, consumerUrl);
        });

        rule = parseRule(rawRule);

        migrationRuleHandler.doMigrate(rule);
    }
```

2、
```java
    public synchronized void process(ConfigChangedEvent event) {
        rawRule = event.getContent();
        if (StringUtils.isEmpty(rawRule)) {
            logger.warn("Received empty migration rule, will ignore.");
            return;
        }

        logger.info("Using the following migration rule to migrate:");
        logger.info(rawRule);

        rule = parseRule(rawRule);

        if (CollectionUtils.isNotEmptyMap(handlers)) {
            handlers.forEach((_key, handler) -> handler.doMigrate(rule));
        }
    }

    @Override
    public synchronized void onRefer(RegistryProtocol registryProtocol, ClusterInvoker<?> invoker, URL consumerUrl, URL registryURL) {
        MigrationRuleHandler<?> migrationRuleHandler = handlers.computeIfAbsent((MigrationInvoker<?>) invoker, _key -> {
            ((MigrationInvoker<?>) invoker).setMigrationRuleListener(this);
            return new MigrationRuleHandler<>((MigrationInvoker<?>) invoker, consumerUrl);
        });

        // 该方法在有大量Reference场景下会调用多次yaml反序列化操作
        rule = parseRule(rawRule);

        migrationRuleHandler.doMigrate(rule);
    }
```
import org.apache.dubbo.common.config.configcenter.DynamicConfiguration;
import org.apache.dubbo.common.extension.Activate;
import org.apache.dubbo.common.logger.Logger;
import org.apache.dubbo.common.logger.LoggerFactory;
import org.apache.dubbo.common.utils.CollectionUtils;
import org.apache.dubbo.common.utils.NamedThreadFactory;
import org.apache.dubbo.common.utils.StringUtils;
import org.apache.dubbo.registry.client.migration.model.MigrationRule;
import org.apache.dubbo.registry.integration.RegistryProtocol;
import org.apache.dubbo.registry.integration.RegistryProtocolListener;
import org.apache.dubbo.rpc.Exporter;
import org.apache.dubbo.rpc.cluster.ClusterInvoker;
import org.apache.dubbo.rpc.model.ApplicationModel;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;
import static org.apache.dubbo.common.constants.RegistryConstants.INIT;
@Activate
public class MigrationRuleListener implements RegistryProtocolListener, ConfigurationListener {
    private static final Logger logger = LoggerFactory.getLogger(MigrationRuleListener.class);
    private static final String DUBBO_SERVICEDISCOVERY_MIGRATION = "DUBBO_SERVICEDISCOVERY_MIGRATION";
    private static final String MIGRATION_DELAY_KEY = "dubbo.application.migration.delay";
    private final String RULE_KEY = ApplicationModel.getName() + ".migration";
    private final Map<MigrationInvoker, MigrationRuleHandler> handlers = new ConcurrentHashMap<>();
    private DynamicConfiguration configuration;
    private volatile String rawRule;
    private volatile MigrationRule rule;
    public MigrationRuleListener() {
        this.configuration = ApplicationModel.getEnvironment().getDynamicConfiguration().orElse(null);