apollo-common/src/main/java/com/ctrip/framework/apollo/common/utils/GrayReleaseRuleItemTransformer.java/GrayReleaseRuleItemTransformer/batchTransformToJSON
class GrayReleaseRuleItemTransformer:  public static String batchTransformToJSON(Set<GrayReleaseRuleItemDTO> ruleItems) {
    return gson.toJson(ruleItems);
  }