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);
    }
```
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package org.apache.dubbo.registry.client.migration;
import org.apache.dubbo.common.URL;
import org.apache.dubbo.common.config.ConfigurationUtils;
import org.apache.dubbo.common.config.configcenter.ConfigChangedEvent;
import org.apache.dubbo.common.config.configcenter.ConfigurationListener;