为确保AbstractRoutingDataSource中的TransactionSynchronizationManager返回正确的值，可以尝试使用以下示例代码：

public class CustomRoutingDataSource extends AbstractRoutingDataSource {

    @Override
    protected Object determineCurrentLookupKey() {
        Object key = RequestContextHolder.getRequestAttributes().getAttribute("dbKey", RequestAttributes.SCOPE_REQUEST);

        if (key == null) {
            // fall back to a default data source.
            key = "defaultDataSource";
        }

        return key;
    }

    @Override
    public void afterPropertiesSet() {
        super.afterPropertiesSet();
        setTargetDataSources(new HashMap(dataSources));
    }

    @Override
    public void setTargetDataSources(Map dataSources) {
        super.setTargetDataSources(dataSources);
        // create a copy of targetDataSources for the dynamic modification
        this.dataSources = new HashMap(dataSources);
    }

    private Map dataSources;

}

此代码通过覆盖determineCurrentLookupKey()方法，从RequestContextHolder中获取当前请求的数据源标识符。同时，使用afterPropertiesSet()和setTargetDataSources()方法确保返回正确的目标数据源。