code: private final int getDefaultIsolationLevel() {
          int defaultIsolationLevel = resRefInfo == null ? Connection.TRANSACTION_NONE : resRefInfo.getIsolationLevel();
          if (defaultIsolationLevel == Connection.TRANSACTION_NONE)
              defaultIsolationLevel = dsConfig.get().isolationLevel;
          if (defaultIsolationLevel == -1)
              defaultIsolationLevel = mcf.getHelper().getDefaultIsolationLevel();
          return defaultIsolationLevel;
      }
docs: Determine the default isolation level for this data source.
  
  @return the default isolation level for this data source.