CDN的JAVA SDK刷新缓存，regionid可以不填吗？怎么刷新所有城市的

import com.qcloud.cdn.CdnClient;
import com.qcloud.cdn.CdnRequest;
import com.qcloud.cdn.CdnResponse;
import com.qcloud.cdn.model.Cache;
import com.qcloud.cdn.model.CacheOptResult;
import com.qcloud.cdn.model.DescribePurgeTasksRequest;
import com.qcloud.cdn.model.DescribePurgeTasksResponse;
import com.qcloud.cdn.model.PurgeTask;
import com.qcloud.cdn.model.PurgeUrl;
import com.qcloud.cdn.model.Task;

import java.util.ArrayList;
import java.util.List;

public class CdnPurgeExample {

    public static void main(String[] args) {
        // 创建CDN客户端
        CdnClient cdnClient = new CdnClient(
                "YOUR_SECRET_ID",
                "YOUR_SECRET_KEY");

        // 查询所有可用区域
        List<String> regions = queryAvailableRegions(cdnClient);

        // 遍历所有可用区域，并刷新缓存
        for (String region : regions) {
            purgeCache(cdnClient, region);
        }
    }

    private static List<String> queryAvailableRegions(CdnClient cdnClient) {
        // 查询所有可用区域
        DescribePurgeTasksRequest request = new DescribePurgeTasksRequest();
        request.setPageSize(100);
        DescribePurgeTasksResponse response = cdnClient.describePurgeTasks(request);
        List<PurgeTask> purgeTasks = response.getPurgeTasks();

        // 提取所有可用区域
        List<String> regions = new ArrayList<>();
        for (PurgeTask purgeTask : purgeTasks) {
            regions.add(purgeTask.getRegion());
        }
        return regions;
    }

    private static void purgeCache(CdnClient cdnClient, String region) {
        // 构造刷新缓存请求
        Cache cache = new Cache();
        cache.setUrls(new String[]{"http://example.com/file.txt"});

        CdnRequest cdnRequest = new CdnRequest();
        cdnRequest.setCache(cache);
        cdnRequest.setRegion(region);

        // 发送刷新缓存请求
        CdnResponse cdnResponse = cdnClient.purgeUrl(cdnRequest);
        CacheOptResult result = cdnResponse.getCacheOptResult();

        // 等待刷新任务完成
        while (!"success".equals(result.getCode())) {
            DescribePurgeTasksRequest describeRequest = new DescribePurgeTasksRequest();
            describeRequest.setTaskId(result.getTaskId());
            DescribePurgeTasksResponse describeResponse = cdnClient.describePurgeTasks(describeRequest);
            Task task = describeResponse.getTask();
            result = task.getStatus();
        }
    }
}