code: protected <RESULT> RESULT doMasterAccessFixedly(SlaveDBCallback<RESULT> noArgLambda) {
          assertCallbackNotNull(noArgLambda);
          final String currentKey = selectableDataSourceHolder.getCurrentSelectableDataSourceKey();
          final String masterDB = MASTER_DB;
          if (logger.isDebugEnabled()) {
              logger.debug(buildMasterAccessFixedlyDebugMessage(masterDB));
          }
          selectableDataSourceHolder.switchSelectableDataSourceKey(masterDB);
          try {
              return noArgLambda.callback();
          } finally {
              selectableDataSourceHolder.switchSelectableDataSourceKey(currentKey);
          }
      }
docs: (it might be available in your UnitTest, so this method was prepared)